bpo-33201: Modernize "Extension types" doc (GH-6337)

* bpo-33201: Modernize "Extension types" doc
* Split tutorial and other topics
* Some small fixes
* Address some review comments
* Rename noddy* to custom* and shoddy to sublist
* Fix markup

15 files changed, 1733 insertions, 1917 deletions

diff --git a/Doc/extending/index.rst b/Doc/extending/index.rst
index 80594e3..0994e3e 100644
--- a/Doc/extending/index.rst
+++ b/Doc/extending/index.rst
@@ -26,9 +26,11 @@ Recommended third party tools
 =============================
 
 This guide only covers the basic tools for creating extensions provided
-as part of this version of CPython. Third party tools like Cython,
-``cffi``, SWIG and Numba offer both simpler and more sophisticated
-approaches to creating C and C++ extensions for Python.
+as part of this version of CPython. Third party tools like
+`Cython <http://cython.org/>`_, `cffi <https://cffi.readthedocs.io>`_,
+`SWIG <http://www.swig.org>`_ and `Numba <https://numba.pydata.org/>`_
+offer both simpler and more sophisticated approaches to creating C and C++
+extensions for Python.
 
 .. seealso::
 
@@ -52,6 +54,7 @@ C extensions.
    :numbered:
 
    extending.rst
+   newtypes_tutorial.rst
    newtypes.rst
    building.rst
    windows.rst
diff --git a/Doc/extending/newtypes.rst b/Doc/extending/newtypes.rst
index d5eb7fb..d0d2ec1 100644
--- a/Doc/extending/newtypes.rst
+++ b/Doc/extending/newtypes.rst
@@ -1,889 +1,11 @@
 .. highlightlang:: c
 
-
-.. _defining-new-types:
-
-******************
-Defining New Types
-******************
-
-.. sectionauthor:: Michael Hudson <mwh@python.net>
-.. sectionauthor:: Dave Kuhlman <dkuhlman@rexx.com>
-.. sectionauthor:: Jim Fulton <jim@zope.com>
-
-
-As mentioned in the last chapter, Python allows the writer of an extension
-module to define new types that can be manipulated from Python code, much like
-strings and lists in core Python.
-
-This is not hard; the code for all extension types follows a pattern, but there
-are some details that you need to understand before you can get started.
-
-
-.. _dnt-basics:
-
-The Basics
-==========
-
-The Python runtime sees all Python objects as variables of type
-:c:type:`PyObject\*`, which serves as a "base type" for all Python objects.
-:c:type:`PyObject` itself only contains the refcount and a pointer to the
-object's "type object".  This is where the action is; the type object determines
-which (C) functions get called when, for instance, an attribute gets looked
-up on an object or it is multiplied by another object.  These C functions
-are called "type methods".
-
-So, if you want to define a new object type, you need to create a new type
-object.
-
-This sort of thing can only be explained by example, so here's a minimal, but
-complete, module that defines a new type:
-
-.. literalinclude:: ../includes/noddy.c
-
-
-Now that's quite a bit to take in at once, but hopefully bits will seem familiar
-from the last chapter.
-
-The first bit that will be new is::
-
-   typedef struct {
-       PyObject_HEAD
-   } noddy_NoddyObject;
-
-This is what a Noddy object will contain---in this case, nothing more than what
-every Python object contains---a field called ``ob_base`` of type
-:c:type:`PyObject`.  :c:type:`PyObject` in turn, contains an ``ob_refcnt``
-field and a pointer to a type object.  These can be accessed using the macros
-:c:macro:`Py_REFCNT` and :c:macro:`Py_TYPE` respectively.  These are the fields
-the :c:macro:`PyObject_HEAD` macro brings in.  The reason for the macro is to
-standardize the layout and to enable special debugging fields in debug builds.
-
-Note that there is no semicolon after the :c:macro:`PyObject_HEAD` macro;
-one is included in the macro definition.  Be wary of adding one by
-accident; it's easy to do from habit, and your compiler might not complain,
-but someone else's probably will!  (On Windows, MSVC is known to call this an
-error and refuse to compile the code.)
-
-For contrast, let's take a look at the corresponding definition for standard
-Python floats::
-
-   typedef struct {
-       PyObject_HEAD
-       double ob_fval;
-   } PyFloatObject;
-
-Moving on, we come to the crunch --- the type object. ::
-
-   static PyTypeObject noddy_NoddyType = {
-       PyVarObject_HEAD_INIT(NULL, 0)
-       "noddy.Noddy",             /* tp_name */
-       sizeof(noddy_NoddyObject), /* tp_basicsize */
-       0,                         /* tp_itemsize */
-       0,                         /* tp_dealloc */
-       0,                         /* tp_print */
-       0,                         /* tp_getattr */
-       0,                         /* tp_setattr */
-       0,                         /* tp_as_async */
-       0,                         /* tp_repr */
-       0,                         /* tp_as_number */
-       0,                         /* tp_as_sequence */
-       0,                         /* tp_as_mapping */
-       0,                         /* tp_hash  */
-       0,                         /* tp_call */
-       0,                         /* tp_str */
-       0,                         /* tp_getattro */
-       0,                         /* tp_setattro */
-       0,                         /* tp_as_buffer */
-       Py_TPFLAGS_DEFAULT,        /* tp_flags */
-       "Noddy objects",           /* tp_doc */
-   };
-
-Now if you go and look up the definition of :c:type:`PyTypeObject` in
-:file:`object.h` you'll see that it has many more fields that the definition
-above.  The remaining fields will be filled with zeros by the C compiler, and
-it's common practice to not specify them explicitly unless you need them.
-
-This is so important that we're going to pick the top of it apart still
-further::
-
-   PyVarObject_HEAD_INIT(NULL, 0)
-
-This line is a bit of a wart; what we'd like to write is::
-
-   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-
-as the type of a type object is "type", but this isn't strictly conforming C and
-some compilers complain.  Fortunately, this member will be filled in for us by
-:c:func:`PyType_Ready`. ::
-
-   "noddy.Noddy",              /* tp_name */
-
-The name of our type.  This will appear in the default textual representation of
-our objects and in some error messages, for example::
-
-   >>> "" + noddy.new_noddy()
-   Traceback (most recent call last):
-     File "<stdin>", line 1, in <module>
-   TypeError: cannot add type "noddy.Noddy" to string
-
-Note that the name is a dotted name that includes both the module name and the
-name of the type within the module. The module in this case is :mod:`noddy` and
-the type is :class:`Noddy`, so we set the type name to :class:`noddy.Noddy`.
-One side effect of using an undotted name is that the pydoc documentation tool
-will not list the new type in the module documentation. ::
-
-   sizeof(noddy_NoddyObject),  /* tp_basicsize */
-
-This is so that Python knows how much memory to allocate when you call
-:c:func:`PyObject_New`.
-
-.. note::
-
-   If you want your type to be subclassable from Python, and your type has the same
-   :c:member:`~PyTypeObject.tp_basicsize` as its base type, you may have problems with multiple
-   inheritance.  A Python subclass of your type will have to list your type first
-   in its :attr:`~class.__bases__`, or else it will not be able to call your type's
-   :meth:`__new__` method without getting an error.  You can avoid this problem by
-   ensuring that your type has a larger value for :c:member:`~PyTypeObject.tp_basicsize` than its
-   base type does.  Most of the time, this will be true anyway, because either your
-   base type will be :class:`object`, or else you will be adding data members to
-   your base type, and therefore increasing its size.
-
-::
-
-   0,                          /* tp_itemsize */
-
-This has to do with variable length objects like lists and strings. Ignore this
-for now.
-
-Skipping a number of type methods that we don't provide, we set the class flags
-to :const:`Py_TPFLAGS_DEFAULT`. ::
-
-   Py_TPFLAGS_DEFAULT,        /* tp_flags */
-
-All types should include this constant in their flags.  It enables all of the
-members defined until at least Python 3.3.  If you need further members,
-you will need to OR the corresponding flags.
-
-We provide a doc string for the type in :c:member:`~PyTypeObject.tp_doc`. ::
-
-   "Noddy objects",           /* tp_doc */
-
-Now we get into the type methods, the things that make your objects different
-from the others.  We aren't going to implement any of these in this version of
-the module.  We'll expand this example later to have more interesting behavior.
-
-For now, all we want to be able to do is to create new :class:`Noddy` objects.
-To enable object creation, we have to provide a :c:member:`~PyTypeObject.tp_new` implementation.
-In this case, we can just use the default implementation provided by the API
-function :c:func:`PyType_GenericNew`. ::
-
-   PyType_GenericNew,         /* tp_new */
-
-All the other type methods are *NULL*, so we'll go over them later --- that's
-for a later section!
-
-Everything else in the file should be familiar, except for some code in
-:c:func:`PyInit_noddy`::
-
-   if (PyType_Ready(&noddy_NoddyType) < 0)
-       return;
-
-This initializes the :class:`Noddy` type, filing in a number of members,
-including :attr:`ob_type` that we initially set to *NULL*. ::
-
-   PyModule_AddObject(m, "Noddy", (PyObject *)&noddy_NoddyType);
-
-This adds the type to the module dictionary.  This allows us to create
-:class:`Noddy` instances by calling the :class:`Noddy` class::
-
-   >>> import noddy
-   >>> mynoddy = noddy.Noddy()
-
-That's it!  All that remains is to build it; put the above code in a file called
-:file:`noddy.c` and ::
-
-   from distutils.core import setup, Extension
-   setup(name="noddy", version="1.0",
-         ext_modules=[Extension("noddy", ["noddy.c"])])
-
-in a file called :file:`setup.py`; then typing
-
-.. code-block:: shell-session
-
-   $ python setup.py build
-
-at a shell should produce a file :file:`noddy.so` in a subdirectory; move to
-that directory and fire up Python --- you should be able to ``import noddy`` and
-play around with Noddy objects.
-
-That wasn't so hard, was it?
-
-Of course, the current Noddy type is pretty uninteresting. It has no data and
-doesn't do anything. It can't even be subclassed.
-
-
-Adding data and methods to the Basic example
---------------------------------------------
-
-Let's extend the basic example to add some data and methods.  Let's also make
-the type usable as a base class. We'll create a new module, :mod:`noddy2` that
-adds these capabilities:
-
-.. literalinclude:: ../includes/noddy2.c
-
-
-This version of the module has a number of changes.
-
-We've added an extra include::
-
-   #include <structmember.h>
-
-This include provides declarations that we use to handle attributes, as
-described a bit later.
-
-The name of the :class:`Noddy` object structure has been shortened to
-:class:`Noddy`.  The type object name has been shortened to :class:`NoddyType`.
-
-The  :class:`Noddy` type now has three data attributes, *first*, *last*, and
-*number*.  The *first* and *last* variables are Python strings containing first
-and last names. The *number* attribute is an integer.
-
-The object structure is updated accordingly::
-
-   typedef struct {
-       PyObject_HEAD
-       PyObject *first;
-       PyObject *last;
-       int number;
-   } Noddy;
-
-Because we now have data to manage, we have to be more careful about object
-allocation and deallocation.  At a minimum, we need a deallocation method::
-
-   static void
-   Noddy_dealloc(Noddy* self)
-   {
-       Py_XDECREF(self->first);
-       Py_XDECREF(self->last);
-       Py_TYPE(self)->tp_free((PyObject*)self);
-   }
-
-which is assigned to the :c:member:`~PyTypeObject.tp_dealloc` member::
-
-   (destructor)Noddy_dealloc, /*tp_dealloc*/
-
-This method decrements the reference counts of the two Python attributes. We use
-:c:func:`Py_XDECREF` here because the :attr:`first` and :attr:`last` members
-could be *NULL*.  It then calls the :c:member:`~PyTypeObject.tp_free` member of the object's type
-to free the object's memory.  Note that the object's type might not be
-:class:`NoddyType`, because the object may be an instance of a subclass.
-
-We want to make sure that the first and last names are initialized to empty
-strings, so we provide a new method::
-
-   static PyObject *
-   Noddy_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
-   {
-       Noddy *self;
-
-       self = (Noddy *)type->tp_alloc(type, 0);
-       if (self != NULL) {
-           self->first = PyUnicode_FromString("");
-           if (self->first == NULL) {
-               Py_DECREF(self);
-               return NULL;
-           }
-
-           self->last = PyUnicode_FromString("");
-           if (self->last == NULL) {
-               Py_DECREF(self);
-               return NULL;
-           }
-
-           self->number = 0;
-       }
-
-       return (PyObject *)self;
-   }
-
-and install it in the :c:member:`~PyTypeObject.tp_new` member::
-
-   Noddy_new,                 /* tp_new */
-
-The new member is responsible for creating (as opposed to initializing) objects
-of the type.  It is exposed in Python as the :meth:`__new__` method.  See the
-paper titled "Unifying types and classes in Python" for a detailed discussion of
-the :meth:`__new__` method.  One reason to implement a new method is to assure
-the initial values of instance variables.  In this case, we use the new method
-to make sure that the initial values of the members :attr:`first` and
-:attr:`last` are not *NULL*. If we didn't care whether the initial values were
-*NULL*, we could have used :c:func:`PyType_GenericNew` as our new method, as we
-did before.  :c:func:`PyType_GenericNew` initializes all of the instance variable
-members to *NULL*.
-
-The new method is a static method that is passed the type being instantiated and
-any arguments passed when the type was called, and that returns the new object
-created. New methods always accept positional and keyword arguments, but they
-often ignore the arguments, leaving the argument handling to initializer
-methods. Note that if the type supports subclassing, the type passed may not be
-the type being defined.  The new method calls the :c:member:`~PyTypeObject.tp_alloc` slot to
-allocate memory. We don't fill the :c:member:`~PyTypeObject.tp_alloc` slot ourselves. Rather
-:c:func:`PyType_Ready` fills it for us by inheriting it from our base class,
-which is :class:`object` by default.  Most types use the default allocation.
-
-.. note::
-
-   If you are creating a co-operative :c:member:`~PyTypeObject.tp_new` (one that calls a base type's
-   :c:member:`~PyTypeObject.tp_new` or :meth:`__new__`), you must *not* try to determine what method
-   to call using method resolution order at runtime.  Always statically determine
-   what type you are going to call, and call its :c:member:`~PyTypeObject.tp_new` directly, or via
-   ``type->tp_base->tp_new``.  If you do not do this, Python subclasses of your
-   type that also inherit from other Python-defined classes may not work correctly.
-   (Specifically, you may not be able to create instances of such subclasses
-   without getting a :exc:`TypeError`.)
-
-We provide an initialization function::
-
-   static int
-   Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
-   {
-       PyObject *first=NULL, *last=NULL, *tmp;
-
-       static char *kwlist[] = {"first", "last", "number", NULL};
-
-       if (! PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
-                                         &first, &last,
-                                         &self->number))
-           return -1;
-
-       if (first) {
-           tmp = self->first;
-           Py_INCREF(first);
-           self->first = first;
-           Py_XDECREF(tmp);
-       }
-
-       if (last) {
-           tmp = self->last;
-           Py_INCREF(last);
-           self->last = last;
-           Py_XDECREF(tmp);
-       }
-
-       return 0;
-   }
-
-by filling the :c:member:`~PyTypeObject.tp_init` slot. ::
-
-   (initproc)Noddy_init,         /* tp_init */
-
-The :c:member:`~PyTypeObject.tp_init` slot is exposed in Python as the :meth:`__init__` method. It
-is used to initialize an object after it's created. Unlike the new method, we
-can't guarantee that the initializer is called.  The initializer isn't called
-when unpickling objects and it can be overridden.  Our initializer accepts
-arguments to provide initial values for our instance. Initializers always accept
-positional and keyword arguments. Initializers should return either ``0`` on
-success or ``-1`` on error.
-
-Initializers can be called multiple times.  Anyone can call the :meth:`__init__`
-method on our objects.  For this reason, we have to be extra careful when
-assigning the new values.  We might be tempted, for example to assign the
-:attr:`first` member like this::
-
-   if (first) {
-       Py_XDECREF(self->first);
-       Py_INCREF(first);
-       self->first = first;
-   }
-
-But this would be risky.  Our type doesn't restrict the type of the
-:attr:`first` member, so it could be any kind of object.  It could have a
-destructor that causes code to be executed that tries to access the
-:attr:`first` member.  To be paranoid and protect ourselves against this
-possibility, we almost always reassign members before decrementing their
-reference counts.  When don't we have to do this?
-
-* when we absolutely know that the reference count is greater than 1
-
-* when we know that deallocation of the object [#]_ will not cause any calls
-  back into our type's code
-
-* when decrementing a reference count in a :c:member:`~PyTypeObject.tp_dealloc` handler when
-  garbage-collections is not supported [#]_
-
-We want to expose our instance variables as attributes. There are a
-number of ways to do that. The simplest way is to define member definitions::
-
-   static PyMemberDef Noddy_members[] = {
-       {"first", T_OBJECT_EX, offsetof(Noddy, first), 0,
-        "first name"},
-       {"last", T_OBJECT_EX, offsetof(Noddy, last), 0,
-        "last name"},
-       {"number", T_INT, offsetof(Noddy, number), 0,
-        "noddy number"},
-       {NULL}  /* Sentinel */
-   };
-
-and put the definitions in the :c:member:`~PyTypeObject.tp_members` slot::
-
-   Noddy_members,             /* tp_members */
-
-Each member definition has a member name, type, offset, access flags and
-documentation string. See the :ref:`Generic-Attribute-Management` section below for
-details.
-
-A disadvantage of this approach is that it doesn't provide a way to restrict the
-types of objects that can be assigned to the Python attributes.  We expect the
-first and last names to be strings, but any Python objects can be assigned.
-Further, the attributes can be deleted, setting the C pointers to *NULL*.  Even
-though we can make sure the members are initialized to non-*NULL* values, the
-members can be set to *NULL* if the attributes are deleted.
-
-We define a single method, :meth:`name`, that outputs the objects name as the
-concatenation of the first and last names. ::
-
-   static PyObject *
-   Noddy_name(Noddy* self)
-   {
-       if (self->first == NULL) {
-           PyErr_SetString(PyExc_AttributeError, "first");
-           return NULL;
-       }
-
-       if (self->last == NULL) {
-           PyErr_SetString(PyExc_AttributeError, "last");
-           return NULL;
-       }
-
-       return PyUnicode_FromFormat("%S %S", self->first, self->last);
-   }
-
-The method is implemented as a C function that takes a :class:`Noddy` (or
-:class:`Noddy` subclass) instance as the first argument.  Methods always take an
-instance as the first argument. Methods often take positional and keyword
-arguments as well, but in this case we don't take any and don't need to accept
-a positional argument tuple or keyword argument dictionary. This method is
-equivalent to the Python method::
-
-   def name(self):
-      return "%s %s" % (self.first, self.last)
-
-Note that we have to check for the possibility that our :attr:`first` and
-:attr:`last` members are *NULL*.  This is because they can be deleted, in which
-case they are set to *NULL*.  It would be better to prevent deletion of these
-attributes and to restrict the attribute values to be strings.  We'll see how to
-do that in the next section.
-
-Now that we've defined the method, we need to create an array of method
-definitions::
-
-   static PyMethodDef Noddy_methods[] = {
-       {"name", (PyCFunction)Noddy_name, METH_NOARGS,
-        "Return the name, combining the first and last name"
-       },
-       {NULL}  /* Sentinel */
-   };
-
-and assign them to the :c:member:`~PyTypeObject.tp_methods` slot::
-
-   Noddy_methods,             /* tp_methods */
-
-Note that we used the :const:`METH_NOARGS` flag to indicate that the method is
-passed no arguments.
-
-Finally, we'll make our type usable as a base class.  We've written our methods
-carefully so far so that they don't make any assumptions about the type of the
-object being created or used, so all we need to do is to add the
-:const:`Py_TPFLAGS_BASETYPE` to our class flag definition::
-
-   Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /*tp_flags*/
-
-We rename :c:func:`PyInit_noddy` to :c:func:`PyInit_noddy2` and update the module
-name in the :c:type:`PyModuleDef` struct.
-
-Finally, we update our :file:`setup.py` file to build the new module::
-
-   from distutils.core import setup, Extension
-   setup(name="noddy", version="1.0",
-         ext_modules=[
-            Extension("noddy", ["noddy.c"]),
-            Extension("noddy2", ["noddy2.c"]),
-            ])
-
-
-Providing finer control over data attributes
---------------------------------------------
-
-In this section, we'll provide finer control over how the :attr:`first` and
-:attr:`last` attributes are set in the :class:`Noddy` example. In the previous
-version of our module, the instance variables :attr:`first` and :attr:`last`
-could be set to non-string values or even deleted. We want to make sure that
-these attributes always contain strings.
-
-.. literalinclude:: ../includes/noddy3.c
-
-
-To provide greater control, over the :attr:`first` and :attr:`last` attributes,
-we'll use custom getter and setter functions.  Here are the functions for
-getting and setting the :attr:`first` attribute::
-
-   Noddy_getfirst(Noddy *self, void *closure)
-   {
-       Py_INCREF(self->first);
-       return self->first;
-   }
-
-   static int
-   Noddy_setfirst(Noddy *self, PyObject *value, void *closure)
-   {
-     if (value == NULL) {
-       PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
-       return -1;
-     }
-
-     if (! PyUnicode_Check(value)) {
-       PyErr_SetString(PyExc_TypeError,
-                       "The first attribute value must be a str");
-       return -1;
-     }
-
-     Py_DECREF(self->first);
-     Py_INCREF(value);
-     self->first = value;
-
-     return 0;
-   }
-
-The getter function is passed a :class:`Noddy` object and a "closure", which is
-void pointer. In this case, the closure is ignored. (The closure supports an
-advanced usage in which definition data is passed to the getter and setter. This
-could, for example, be used to allow a single set of getter and setter functions
-that decide the attribute to get or set based on data in the closure.)
-
-The setter function is passed the :class:`Noddy` object, the new value, and the
-closure. The new value may be *NULL*, in which case the attribute is being
-deleted.  In our setter, we raise an error if the attribute is deleted or if the
-attribute value is not a string.
-
-We create an array of :c:type:`PyGetSetDef` structures::
-
-   static PyGetSetDef Noddy_getseters[] = {
-       {"first",
-        (getter)Noddy_getfirst, (setter)Noddy_setfirst,
-        "first name",
-        NULL},
-       {"last",
-        (getter)Noddy_getlast, (setter)Noddy_setlast,
-        "last name",
-        NULL},
-       {NULL}  /* Sentinel */
-   };
-
-and register it in the :c:member:`~PyTypeObject.tp_getset` slot::
-
-   Noddy_getseters,           /* tp_getset */
-
-to register our attribute getters and setters.
-
-The last item in a :c:type:`PyGetSetDef` structure is the closure mentioned
-above. In this case, we aren't using the closure, so we just pass *NULL*.
-
-We also remove the member definitions for these attributes::
-
-   static PyMemberDef Noddy_members[] = {
-       {"number", T_INT, offsetof(Noddy, number), 0,
-        "noddy number"},
-       {NULL}  /* Sentinel */
-   };
-
-We also need to update the :c:member:`~PyTypeObject.tp_init` handler to only allow strings [#]_ to
-be passed::
-
-   static int
-   Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
-   {
-       PyObject *first=NULL, *last=NULL, *tmp;
-
-       static char *kwlist[] = {"first", "last", "number", NULL};
-
-       if (! PyArg_ParseTupleAndKeywords(args, kwds, "|SSi", kwlist,
-                                         &first, &last,
-                                         &self->number))
-           return -1;
-
-       if (first) {
-           tmp = self->first;
-           Py_INCREF(first);
-           self->first = first;
-           Py_DECREF(tmp);
-       }
-
-       if (last) {
-           tmp = self->last;
-           Py_INCREF(last);
-           self->last = last;
-           Py_DECREF(tmp);
-       }
-
-       return 0;
-   }
-
-With these changes, we can assure that the :attr:`first` and :attr:`last`
-members are never *NULL* so we can remove checks for *NULL* values in almost all
-cases. This means that most of the :c:func:`Py_XDECREF` calls can be converted to
-:c:func:`Py_DECREF` calls. The only place we can't change these calls is in the
-deallocator, where there is the possibility that the initialization of these
-members failed in the constructor.
-
-We also rename the module initialization function and module name in the
-initialization function, as we did before, and we add an extra definition to the
-:file:`setup.py` file.
-
-
-Supporting cyclic garbage collection
-------------------------------------
-
-Python has a cyclic-garbage collector that can identify unneeded objects even
-when their reference counts are not zero. This can happen when objects are
-involved in cycles.  For example, consider::
-
-   >>> l = []
-   >>> l.append(l)
-   >>> del l
-
-In this example, we create a list that contains itself. When we delete it, it
-still has a reference from itself. Its reference count doesn't drop to zero.
-Fortunately, Python's cyclic-garbage collector will eventually figure out that
-the list is garbage and free it.
-
-In the second version of the :class:`Noddy` example, we allowed any kind of
-object to be stored in the :attr:`first` or :attr:`last` attributes [#]_. This
-means that :class:`Noddy` objects can participate in cycles::
-
-   >>> import noddy2
-   >>> n = noddy2.Noddy()
-   >>> l = [n]
-   >>> n.first = l
-
-This is pretty silly, but it gives us an excuse to add support for the
-cyclic-garbage collector to the :class:`Noddy` example.  To support cyclic
-garbage collection, types need to fill two slots and set a class flag that
-enables these slots:
-
-.. literalinclude:: ../includes/noddy4.c
-
-
-The traversal method provides access to subobjects that could participate in
-cycles::
-
-   static int
-   Noddy_traverse(Noddy *self, visitproc visit, void *arg)
-   {
-       int vret;
-
-       if (self->first) {
-           vret = visit(self->first, arg);
-           if (vret != 0)
-               return vret;
-       }
-       if (self->last) {
-           vret = visit(self->last, arg);
-           if (vret != 0)
-               return vret;
-       }
-
-       return 0;
-   }
-
-For each subobject that can participate in cycles, we need to call the
-:c:func:`visit` function, which is passed to the traversal method. The
-:c:func:`visit` function takes as arguments the subobject and the extra argument
-*arg* passed to the traversal method.  It returns an integer value that must be
-returned if it is non-zero.
-
-Python provides a :c:func:`Py_VISIT` macro that automates calling visit
-functions.  With :c:func:`Py_VISIT`, :c:func:`Noddy_traverse` can be simplified::
-
-   static int
-   Noddy_traverse(Noddy *self, visitproc visit, void *arg)
-   {
-       Py_VISIT(self->first);
-       Py_VISIT(self->last);
-       return 0;
-   }
-
-.. note::
-
-   Note that the :c:member:`~PyTypeObject.tp_traverse` implementation must name its arguments exactly
-   *visit* and *arg* in order to use :c:func:`Py_VISIT`.  This is to encourage
-   uniformity across these boring implementations.
-
-We also need to provide a method for clearing any subobjects that can
-participate in cycles.
-
-::
-
-   static int
-   Noddy_clear(Noddy *self)
-   {
-       PyObject *tmp;
-
-       tmp = self->first;
-       self->first = NULL;
-       Py_XDECREF(tmp);
-
-       tmp = self->last;
-       self->last = NULL;
-       Py_XDECREF(tmp);
-
-       return 0;
-   }
-
-Notice the use of a temporary variable in :c:func:`Noddy_clear`. We use the
-temporary variable so that we can set each member to *NULL* before decrementing
-its reference count.  We do this because, as was discussed earlier, if the
-reference count drops to zero, we might cause code to run that calls back into
-the object.  In addition, because we now support garbage collection, we also
-have to worry about code being run that triggers garbage collection.  If garbage
-collection is run, our :c:member:`~PyTypeObject.tp_traverse` handler could get called. We can't
-take a chance of having :c:func:`Noddy_traverse` called when a member's reference
-count has dropped to zero and its value hasn't been set to *NULL*.
-
-Python provides a :c:func:`Py_CLEAR` that automates the careful decrementing of
-reference counts.  With :c:func:`Py_CLEAR`, the :c:func:`Noddy_clear` function can
-be simplified::
-
-   static int
-   Noddy_clear(Noddy *self)
-   {
-       Py_CLEAR(self->first);
-       Py_CLEAR(self->last);
-       return 0;
-   }
-
-Note that :c:func:`Noddy_dealloc` may call arbitrary functions through
-``__del__`` method or weakref callback. It means circular GC can be
-triggered inside the function.  Since GC assumes reference count is not zero,
-we need to untrack the object from GC by calling :c:func:`PyObject_GC_UnTrack`
-before clearing members. Here is reimplemented deallocator which uses
-:c:func:`PyObject_GC_UnTrack` and :c:func:`Noddy_clear`.
-
-::
-
-   static void
-   Noddy_dealloc(Noddy* self)
-   {
-       PyObject_GC_UnTrack(self);
-       Noddy_clear(self);
-       Py_TYPE(self)->tp_free((PyObject*)self);
-   }
-
-Finally, we add the :const:`Py_TPFLAGS_HAVE_GC` flag to the class flags::
-
-   Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC, /* tp_flags */
-
-That's pretty much it.  If we had written custom :c:member:`~PyTypeObject.tp_alloc` or
-:c:member:`~PyTypeObject.tp_free` slots, we'd need to modify them for cyclic-garbage collection.
-Most extensions will use the versions automatically provided.
-
-
-Subclassing other types
------------------------
-
-It is possible to create new extension types that are derived from existing
-types. It is easiest to inherit from the built in types, since an extension can
-easily use the :class:`PyTypeObject` it needs. It can be difficult to share
-these :class:`PyTypeObject` structures between extension modules.
-
-In this example we will create a :class:`Shoddy` type that inherits from the
-built-in :class:`list` type. The new type will be completely compatible with
-regular lists, but will have an additional :meth:`increment` method that
-increases an internal counter. ::
-
-   >>> import shoddy
-   >>> s = shoddy.Shoddy(range(3))
-   >>> s.extend(s)
-   >>> print(len(s))
-   6
-   >>> print(s.increment())
-   1
-   >>> print(s.increment())
-   2
-
-.. literalinclude:: ../includes/shoddy.c
-
-
-As you can see, the source code closely resembles the :class:`Noddy` examples in
-previous sections. We will break down the main differences between them. ::
-
-   typedef struct {
-       PyListObject list;
-       int state;
-   } Shoddy;
-
-The primary difference for derived type objects is that the base type's object
-structure must be the first value. The base type will already include the
-:c:func:`PyObject_HEAD` at the beginning of its structure.
-
-When a Python object is a :class:`Shoddy` instance, its *PyObject\** pointer can
-be safely cast to both *PyListObject\** and *Shoddy\**. ::
-
-   static int
-   Shoddy_init(Shoddy *self, PyObject *args, PyObject *kwds)
-   {
-       if (PyList_Type.tp_init((PyObject *)self, args, kwds) < 0)
-          return -1;
-       self->state = 0;
-       return 0;
-   }
-
-In the :attr:`__init__` method for our type, we can see how to call through to
-the :attr:`__init__` method of the base type.
-
-This pattern is important when writing a type with custom :attr:`new` and
-:attr:`dealloc` methods. The :attr:`new` method should not actually create the
-memory for the object with :c:member:`~PyTypeObject.tp_alloc`, that will be handled by the base
-class when calling its :c:member:`~PyTypeObject.tp_new`.
-
-When filling out the :c:func:`PyTypeObject` for the :class:`Shoddy` type, you see
-a slot for :c:func:`tp_base`. Due to cross platform compiler issues, you can't
-fill that field directly with the :c:func:`PyList_Type`; it can be done later in
-the module's :c:func:`init` function. ::
-
-   PyMODINIT_FUNC
-   PyInit_shoddy(void)
-   {
-       PyObject *m;
-
-       ShoddyType.tp_base = &PyList_Type;
-       if (PyType_Ready(&ShoddyType) < 0)
-           return NULL;
-
-       m = PyModule_Create(&shoddymodule);
-       if (m == NULL)
-           return NULL;
-
-       Py_INCREF(&ShoddyType);
-       PyModule_AddObject(m, "Shoddy", (PyObject *) &ShoddyType);
-       return m;
-   }
-
-Before calling :c:func:`PyType_Ready`, the type structure must have the
-:c:member:`~PyTypeObject.tp_base` slot filled in. When we are deriving a new type, it is not
-necessary to fill out the :c:member:`~PyTypeObject.tp_alloc` slot with :c:func:`PyType_GenericNew`
--- the allocate function from the base type will be inherited.
-
-After that, calling :c:func:`PyType_Ready` and adding the type object to the
-module is the same as with the basic :class:`Noddy` examples.
-
+*****************************************
+Defining Extension Types: Assorted Topics
+*****************************************
 
 .. _dnt-type-methods:
 
-Type Methods
-============
-
 This section aims to give a quick fly-by on the various type methods you can
 implement and what they do.
 
@@ -893,21 +15,20 @@ debug builds omitted:
 .. literalinclude:: ../includes/typestruct.h
 
 
-Now that's a *lot* of methods.  Don't worry too much though - if you have a type
-you want to define, the chances are very good that you will only implement a
-handful of these.
+Now that's a *lot* of methods.  Don't worry too much though -- if you have
+a type you want to define, the chances are very good that you will only
+implement a handful of these.
 
 As you probably expect by now, we're going to go over this and give more
 information about the various handlers.  We won't go in the order they are
 defined in the structure, because there is a lot of historical baggage that
-impacts the ordering of the fields; be sure your type initialization keeps the
-fields in the right order!  It's often easiest to find an example that includes
-all the fields you need (even if they're initialized to ``0``) and then change
-the values to suit your new type. ::
+impacts the ordering of the fields.  It's often easiest to find an example
+that includes the fields you need and then change the values to suit your new
+type. ::
 
    const char *tp_name; /* For printing */
 
-The name of the type - as mentioned in the last section, this will appear in
+The name of the type -- as mentioned in the previous chapter, this will appear in
 various places, almost entirely for diagnostic purposes. Try to choose something
 that will be helpful in such a situation! ::
 
@@ -915,7 +36,7 @@ that will be helpful in such a situation! ::
 
 These fields tell the runtime how much memory to allocate when new objects of
 this type are created.  Python has some built-in support for variable length
-structures (think: strings, lists) which is where the :c:member:`~PyTypeObject.tp_itemsize` field
+structures (think: strings, tuples) which is where the :c:member:`~PyTypeObject.tp_itemsize` field
 comes in.  This will be dealt with later. ::
 
    const char *tp_doc;
@@ -923,7 +44,7 @@ comes in.  This will be dealt with later. ::
 Here you can put a string (or its address) that you want returned when the
 Python script references ``obj.__doc__`` to retrieve the doc string.
 
-Now we come to the basic type methods---the ones most extension types will
+Now we come to the basic type methods -- the ones most extension types will
 implement.
 
 
@@ -947,7 +68,7 @@ object itself needs to be freed here as well.  Here is an example of this
 function::
 
    static void
-   newdatatype_dealloc(newdatatypeobject * obj)
+   newdatatype_dealloc(newdatatypeobject *obj)
    {
        free(obj->obj_UnderlyingDatatypePtr);
        Py_TYPE(obj)->tp_free(obj);
@@ -1035,7 +156,7 @@ example::
    static PyObject *
    newdatatype_repr(newdatatypeobject * obj)
    {
-       return PyUnicode_FromFormat("Repr-ified_newdatatype{{size:\%d}}",
+       return PyUnicode_FromFormat("Repr-ified_newdatatype{{size:%d}}",
                                    obj->obj_UnderlyingDatatypePtr->size);
    }
 
@@ -1055,7 +176,7 @@ Here is a simple example::
    static PyObject *
    newdatatype_str(newdatatypeobject * obj)
    {
-       return PyUnicode_FromFormat("Stringified_newdatatype{{size:\%d}}",
+       return PyUnicode_FromFormat("Stringified_newdatatype{{size:%d}}",
                                    obj->obj_UnderlyingDatatypePtr->size);
    }
 
@@ -1236,7 +357,7 @@ example that simply raises an exception; if this were really all you wanted, the
    static int
    newdatatype_setattr(newdatatypeobject *obj, char *name, PyObject *v)
    {
-       (void)PyErr_Format(PyExc_RuntimeError, "Read-only attribute: \%s", name);
+       PyErr_Format(PyExc_RuntimeError, "Read-only attribute: %s", name);
        return -1;
    }
 
@@ -1321,17 +442,23 @@ these in the :file:`Objects` directory of the Python source distribution. ::
    hashfunc tp_hash;
 
 This function, if you choose to provide it, should return a hash number for an
-instance of your data type. Here is a moderately pointless example::
+instance of your data type. Here is a simple example::
 
-   static long
+   static Py_hash_t
    newdatatype_hash(newdatatypeobject *obj)
    {
-       long result;
-       result = obj->obj_UnderlyingDatatypePtr->size;
-       result = result * 3;
+       Py_hash_t result;
+       result = obj->some_size + 32767 * obj->some_number;
+       if (result == -1)
+          result = -2;
        return result;
    }
 
+:c:type:`Py_hash_t` is a signed integer type with a platform-varying width.
+Returning ``-1`` from :c:member:`~PyTypeObject.tp_hash` indicates an error,
+which is why you should be careful to avoid returning it when hash computation
+is successful, as seen above.
+
 ::
 
    ternaryfunc tp_call;
@@ -1342,27 +469,22 @@ contains ``obj1('hello')``, the :c:member:`~PyTypeObject.tp_call` handler is inv
 
 This function takes three arguments:
 
-#. *arg1* is the instance of the data type which is the subject of the call. If
-   the call is ``obj1('hello')``, then *arg1* is ``obj1``.
+#. *self* is the instance of the data type which is the subject of the call.
+   If the call is ``obj1('hello')``, then *self* is ``obj1``.
 
-#. *arg2* is a tuple containing the arguments to the call.  You can use
+#. *args* is a tuple containing the arguments to the call.  You can use
    :c:func:`PyArg_ParseTuple` to extract the arguments.
 
-#. *arg3* is a dictionary of keyword arguments that were passed. If this is
+#. *kwds* is a dictionary of keyword arguments that were passed. If this is
    non-*NULL* and you support keyword arguments, use
-   :c:func:`PyArg_ParseTupleAndKeywords` to extract the arguments.  If you do not
-   want to support keyword arguments and this is non-*NULL*, raise a
+   :c:func:`PyArg_ParseTupleAndKeywords` to extract the arguments.  If you
+   do not want to support keyword arguments and this is non-*NULL*, raise a
    :exc:`TypeError` with a message saying that keyword arguments are not supported.
 
-Here is a desultory example of the implementation of the call function. ::
+Here is a toy ``tp_call`` implementation::
 
-   /* Implement the call function.
-    *    obj1 is the instance receiving the call.
-    *    obj2 is a tuple containing the arguments to the call, in this
-    *         case 3 strings.
-    */
    static PyObject *
-   newdatatype_call(newdatatypeobject *obj, PyObject *args, PyObject *other)
+   newdatatype_call(newdatatypeobject *self, PyObject *args, PyObject *kwds)
    {
        PyObject *result;
        const char *arg1;
@@ -1373,7 +495,7 @@ Here is a desultory example of the implementation of the call function. ::
            return NULL;
        }
        result = PyUnicode_FromFormat(
-           "Returning -- value: [\%d] arg1: [\%s] arg2: [\%s] arg3: [\%s]\n",
+           "Returning -- value: [%d] arg1: [%s] arg2: [%s] arg3: [%s]\n",
            obj->obj_UnderlyingDatatypePtr->size,
            arg1, arg2, arg3);
        return result;
@@ -1385,32 +507,36 @@ Here is a desultory example of the implementation of the call function. ::
    getiterfunc tp_iter;
    iternextfunc tp_iternext;
 
-These functions provide support for the iterator protocol.  Any object which
-wishes to support iteration over its contents (which may be generated during
-iteration) must implement the ``tp_iter`` handler.  Objects which are returned
-by a ``tp_iter`` handler must implement both the ``tp_iter`` and ``tp_iternext``
-handlers. Both handlers take exactly one parameter, the instance for which they
-are being called, and return a new reference.  In the case of an error, they
-should set an exception and return *NULL*.
-
-For an object which represents an iterable collection, the ``tp_iter`` handler
-must return an iterator object.  The iterator object is responsible for
-maintaining the state of the iteration.  For collections which can support
-multiple iterators which do not interfere with each other (as lists and tuples
-do), a new iterator should be created and returned.  Objects which can only be
-iterated over once (usually due to side effects of iteration) should implement
-this handler by returning a new reference to themselves, and should also
-implement the ``tp_iternext`` handler.  File objects are an example of such an
-iterator.
-
-Iterator objects should implement both handlers.  The ``tp_iter`` handler should
-return a new reference to the iterator (this is the same as the ``tp_iter``
-handler for objects which can only be iterated over destructively).  The
-``tp_iternext`` handler should return a new reference to the next object in the
-iteration if there is one.  If the iteration has reached the end, it may return
-*NULL* without setting an exception or it may set :exc:`StopIteration`; avoiding
-the exception can yield slightly better performance.  If an actual error occurs,
-it should set an exception and return *NULL*.
+These functions provide support for the iterator protocol.  Both handlers
+take exactly one parameter, the instance for which they are being called,
+and return a new reference.  In the case of an error, they should set an
+exception and return *NULL*.  :c:member:`~PyTypeObject.tp_iter` corresponds
+to the Python :meth:`__iter__` method, while :c:member:`~PyTypeObject.tp_iternext`
+corresponds to the Python :meth:`~iterator.__next__` method.
+
+Any :term:`iterable` object must implement the :c:member:`~PyTypeObject.tp_iter`
+handler, which must return an :term:`iterator` object.  Here the same guidelines
+apply as for Python classes:
+
+* For collections (such as lists and tuples) which can support multiple
+  independent iterators, a new iterator should be created and returned by
+  each call to :c:member:`~PyTypeObject.tp_iter`.
+* Objects which can only be iterated over once (usually due to side effects of
+  iteration, such as file objects) can implement :c:member:`~PyTypeObject.tp_iter`
+  by returning a new reference to themselves -- and should also therefore
+  implement the :c:member:`~PyTypeObject.tp_iternext`  handler.
+
+Any :term:`iterator` object should implement both :c:member:`~PyTypeObject.tp_iter`
+and :c:member:`~PyTypeObject.tp_iternext`.  An iterator's
+:c:member:`~PyTypeObject.tp_iter` handler should return a new reference
+to the iterator.  Its :c:member:`~PyTypeObject.tp_iternext` handler should
+return a new reference to the next object in the iteration, if there is one.
+If the iteration has reached the end, :c:member:`~PyTypeObject.tp_iternext`
+may return *NULL* without setting an exception, or it may set
+:exc:`StopIteration` *in addition* to returning *NULL*; avoiding
+the exception can yield slightly better performance.  If an actual error
+occurs, :c:member:`~PyTypeObject.tp_iternext` should always set an exception
+and return *NULL*.
 
 
 .. _weakref-support:
@@ -1418,110 +544,76 @@ it should set an exception and return *NULL*.
 Weak Reference Support
 ----------------------
 
-One of the goals of Python's weak-reference implementation is to allow any type
+One of the goals of Python's weak reference implementation is to allow any type
 to participate in the weak reference mechanism without incurring the overhead on
-those objects which do not benefit by weak referencing (such as numbers).
+performance-critical objects (such as numbers).
 
-For an object to be weakly referencable, the extension must include a
-:c:type:`PyObject\*` field in the instance structure for the use of the weak
-reference mechanism; it must be initialized to *NULL* by the object's
-constructor.  It must also set the :c:member:`~PyTypeObject.tp_weaklistoffset` field of the
-corresponding type object to the offset of the field. For example, the instance
-type is defined with the following structure::
+.. seealso::
+   Documentation for the :mod:`weakref` module.
 
-   typedef struct {
-       PyObject_HEAD
-       PyClassObject *in_class;       /* The class object */
-       PyObject      *in_dict;        /* A dictionary */
-       PyObject      *in_weakreflist; /* List of weak references */
-   } PyInstanceObject;
-
-The statically-declared type object for instances is defined this way::
-
-   PyTypeObject PyInstance_Type = {
-       PyVarObject_HEAD_INIT(&PyType_Type, 0)
-       0,
-       "module.instance",
-
-       /* Lots of stuff omitted for brevity... */
-
-       Py_TPFLAGS_DEFAULT,                         /* tp_flags */
-       0,                                          /* tp_doc */
-       0,                                          /* tp_traverse */
-       0,                                          /* tp_clear */
-       0,                                          /* tp_richcompare */
-       offsetof(PyInstanceObject, in_weakreflist), /* tp_weaklistoffset */
-   };
+For an object to be weakly referencable, the extension type must do two things:
 
-The type constructor is responsible for initializing the weak reference list to
-*NULL*::
+#. Include a :c:type:`PyObject\*` field in the C object structure dedicated to
+   the weak reference mechanism.  The object's constructor should leave it
+   *NULL* (which is automatic when using the default
+   :c:member:`~PyTypeObject.tp_alloc`).
 
-   static PyObject *
-   instance_new() {
-       /* Other initialization stuff omitted for brevity */
+#. Set the :c:member:`~PyTypeObject.tp_weaklistoffset` type member
+   to the offset of the aforementioned field in the C object structure,
+   so that the interpreter knows how to access and modify that field.
 
-       self->in_weakreflist = NULL;
+Concretely, here is how a trivial object structure would be augmented
+with the required field::
 
-       return (PyObject *) self;
-   }
+   typedef struct {
+       PyObject_HEAD
+       PyObject *weakreflist;  /* List of weak references */
+   } TrivialObject;
 
-The only further addition is that the destructor needs to call the weak
-reference manager to clear any weak references.  This is only required if the
-weak reference list is non-*NULL*::
+And the corresponding member in the statically-declared type object::
 
-   static void
-   instance_dealloc(PyInstanceObject *inst)
-   {
-       /* Allocate temporaries if needed, but do not begin
-          destruction just yet.
-        */
+   static PyTypeObject TrivialType = {
+       PyVarObject_HEAD_INIT(NULL, 0)
+       /* ... other members omitted for brevity ... */
+       .tp_weaklistoffset = offsetof(TrivialObject, weakreflist),
+   };
 
-       if (inst->in_weakreflist != NULL)
-           PyObject_ClearWeakRefs((PyObject *) inst);
+The only further addition is that ``tp_dealloc`` needs to clear any weak
+references (by calling :c:func:`PyObject_ClearWeakRefs`) if the field is
+non-*NULL*::
 
-       /* Proceed with object destruction normally. */
+   static void
+   Trivial_dealloc(TrivialObject *self)
+   {
+       /* Clear weakrefs first before calling any destructors */
+       if (self->weakreflist != NULL)
+           PyObject_ClearWeakRefs((PyObject *) self);
+       /* ... remainder of destruction code omitted for brevity ... */
+       Py_TYPE(self)->tp_free((PyObject *) self);
    }
 
 
 More Suggestions
 ----------------
 
-Remember that you can omit most of these functions, in which case you provide
-``0`` as a value.  There are type definitions for each of the functions you must
-provide.  They are in :file:`object.h` in the Python include directory that
-comes with the source distribution of Python.
-
 In order to learn how to implement any specific method for your new data type,
-do the following: Download and unpack the Python source distribution.  Go to
-the :file:`Objects` directory, then search the C source files for ``tp_`` plus
-the function you want (for example, ``tp_richcompare``).  You will find examples
-of the function you want to implement.
+get the :term:`CPython` source code.  Go to the :file:`Objects` directory,
+then search the C source files for ``tp_`` plus the function you want
+(for example, ``tp_richcompare``).  You will find examples of the function
+you want to implement.
 
-When you need to verify that an object is an instance of the type you are
-implementing, use the :c:func:`PyObject_TypeCheck` function. A sample of its use
-might be something like the following::
+When you need to verify that an object is a concrete instance of the type you
+are implementing, use the :c:func:`PyObject_TypeCheck` function.  A sample of
+its use might be something like the following::
 
-   if (! PyObject_TypeCheck(some_object, &MyType)) {
+   if (!PyObject_TypeCheck(some_object, &MyType)) {
        PyErr_SetString(PyExc_TypeError, "arg #1 not a mything");
        return NULL;
    }
 
-.. rubric:: Footnotes
-
-.. [#] This is true when we know that the object is a basic type, like a string or a
-   float.
-
-.. [#] We relied on this in the :c:member:`~PyTypeObject.tp_dealloc` handler in this example, because our
-   type doesn't support garbage collection. Even if a type supports garbage
-   collection, there are calls that can be made to "untrack" the object from
-   garbage collection, however, these calls are advanced and not covered here.
-
-.. [#] We now know that the first and last members are strings, so perhaps we could be
-   less careful about decrementing their reference counts, however, we accept
-   instances of string subclasses. Even though deallocating normal strings won't
-   call back into our objects, we can't guarantee that deallocating an instance of
-   a string subclass won't call back into our objects.
+.. seealso::
+   Download CPython source releases.
+      https://www.python.org/downloads/source/
 
-.. [#] Even in the third version, we aren't guaranteed to avoid cycles.  Instances of
-   string subclasses are allowed and string subclasses could allow cycles even if
-   normal strings don't.
+   The CPython project on GitHub, where the CPython source code is developed.
+      https://github.com/python/cpython
diff --git a/Doc/extending/newtypes_tutorial.rst b/Doc/extending/newtypes_tutorial.rst
new file mode 100644
index 0000000..5e05cf6
--- /dev/null
+++ b/Doc/extending/newtypes_tutorial.rst
@@ -0,0 +1,896 @@
+.. highlightlang:: c
+
+.. _defining-new-types:
+
+**********************************
+Defining Extension Types: Tutorial
+**********************************
+
+.. sectionauthor:: Michael Hudson <mwh@python.net>
+.. sectionauthor:: Dave Kuhlman <dkuhlman@rexx.com>
+.. sectionauthor:: Jim Fulton <jim@zope.com>
+
+
+Python allows the writer of a C extension module to define new types that
+can be manipulated from Python code, much like the built-in :class:`str`
+and :class:`list` types.  The code for all extension types follows a
+pattern, but there are some details that you need to understand before you
+can get started.  This document is a gentle introduction to the topic.
+
+
+.. _dnt-basics:
+
+The Basics
+==========
+
+The :term:`CPython` runtime sees all Python objects as variables of type
+:c:type:`PyObject\*`, which serves as a "base type" for all Python objects.
+The :c:type:`PyObject` structure itself only contains the object's
+:term:`reference count` and a pointer to the object's "type object".
+This is where the action is; the type object determines which (C) functions
+get called by the interpreter when, for instance, an attribute gets looked up
+on an object, a method called, or it is multiplied by another object.  These
+C functions are called "type methods".
+
+So, if you want to define a new extension type, you need to create a new type
+object.
+
+This sort of thing can only be explained by example, so here's a minimal, but
+complete, module that defines a new type named :class:`Custom` inside a C
+extension module :mod:`custom`:
+
+.. note::
+   What we're showing here is the traditional way of defining *static*
+   extension types.  It should be adequate for most uses.  The C API also
+   allows defining heap-allocated extension types using the
+   :c:func:`PyType_FromSpec` function, which isn't covered in this tutorial.
+
+.. literalinclude:: ../includes/custom.c
+
+Now that's quite a bit to take in at once, but hopefully bits will seem familiar
+from the previous chapter.  This file defines three things:
+
+#. What a :class:`Custom` **object** contains: this is the ``CustomObject``
+   struct, which is allocated once for each :class:`Custom` instance.
+#. How the :class:`Custom` **type** behaves: this is the ``CustomType`` struct,
+   which defines a set of flags and function pointers that the interpreter
+   inspects when specific operations are requested.
+#. How to initialize the :mod:`custom` module: this is the ``PyInit_custom``
+   function and the associated ``custommodule`` struct.
+
+The first bit is::
+
+   typedef struct {
+       PyObject_HEAD
+   } CustomObject;
+
+This is what a Custom object will contain.  ``PyObject_HEAD`` is mandatory
+at the start of each object struct and defines a field called ``ob_base``
+of type :c:type:`PyObject`, containing a pointer to a type object and a
+reference count (these can be accessed using the macros :c:macro:`Py_REFCNT`
+and :c:macro:`Py_TYPE` respectively).  The reason for the macro is to
+abstract away the layout and to enable additional fields in debug builds.
+
+.. note::
+   There is no semicolon above after the :c:macro:`PyObject_HEAD` macro.
+   Be wary of adding one by accident: some compilers will complain.
+
+Of course, objects generally store additional data besides the standard
+``PyObject_HEAD`` boilerplate; for example, here is the definition for
+standard Python floats::
+
+   typedef struct {
+       PyObject_HEAD
+       double ob_fval;
+   } PyFloatObject;
+
+The second bit is the definition of the type object. ::
+
+   static PyTypeObject CustomType = {
+       PyVarObject_HEAD_INIT(NULL, 0)
+       .tp_name = "custom.Custom",
+       .tp_doc = "Custom objects",
+       .tp_basicsize = sizeof(CustomObject),
+       .tp_itemsize = 0,
+       .tp_new = PyType_GenericNew,
+   };
+
+.. note::
+   We recommend using C99-style designated initializers as above, to
+   avoid listing all the :c:type:`PyTypeObject` fields that you don't care
+   about and also to avoid caring about the fields' declaration order.
+
+The actual definition of :c:type:`PyTypeObject` in :file:`object.h` has
+many more :ref:`fields <type-structs>` than the definition above.  The
+remaining fields will be filled with zeros by the C compiler, and it's
+common practice to not specify them explicitly unless you need them.
+
+We're going to pick it apart, one field at a time::
+
+   PyVarObject_HEAD_INIT(NULL, 0)
+
+This line is mandatory boilerplate to initialize the ``ob_base``
+field mentioned above. ::
+
+   .tp_name = "custom.Custom",
+
+The name of our type.  This will appear in the default textual representation of
+our objects and in some error messages, for example:
+
+.. code-block:: python
+
+   >>> "" + custom.Custom()
+   Traceback (most recent call last):
+     File "<stdin>", line 1, in <module>
+   TypeError: can only concatenate str (not "custom.Custom") to str
+
+Note that the name is a dotted name that includes both the module name and the
+name of the type within the module. The module in this case is :mod:`custom` and
+the type is :class:`Custom`, so we set the type name to :class:`custom.Custom`.
+Using the real dotted import path is important to make your type compatible
+with the :mod:`pydoc` and :mod:`pickle` modules. ::
+
+   .tp_basicsize = sizeof(CustomObject),
+   .tp_itemsize = 0,
+
+This is so that Python knows how much memory to allocate when creating
+new :class:`Custom` instances.  :c:member:`~PyTypeObject.tp_itemsize` is
+only used for variable-sized objects and should otherwise be zero.
+
+.. note::
+
+   If you want your type to be subclassable from Python, and your type has the same
+   :c:member:`~PyTypeObject.tp_basicsize` as its base type, you may have problems with multiple
+   inheritance.  A Python subclass of your type will have to list your type first
+   in its :attr:`~class.__bases__`, or else it will not be able to call your type's
+   :meth:`__new__` method without getting an error.  You can avoid this problem by
+   ensuring that your type has a larger value for :c:member:`~PyTypeObject.tp_basicsize` than its
+   base type does.  Most of the time, this will be true anyway, because either your
+   base type will be :class:`object`, or else you will be adding data members to
+   your base type, and therefore increasing its size.
+
+We set the class flags to :const:`Py_TPFLAGS_DEFAULT`. ::
+
+   .tp_flags = Py_TPFLAGS_DEFAULT,
+
+All types should include this constant in their flags.  It enables all of the
+members defined until at least Python 3.3.  If you need further members,
+you will need to OR the corresponding flags.
+
+We provide a doc string for the type in :c:member:`~PyTypeObject.tp_doc`. ::
+
+   .tp_doc = "Custom objects",
+
+To enable object creation, we have to provide a :c:member:`~PyTypeObject.tp_new`
+handler.  This is the equivalent of the Python method :meth:`__new__`, but
+has to be specified explicitly.  In this case, we can just use the default
+implementation provided by the API function :c:func:`PyType_GenericNew`. ::
+
+   .tp_new = PyType_GenericNew,
+
+Everything else in the file should be familiar, except for some code in
+:c:func:`PyInit_custom`::
+
+   if (PyType_Ready(&CustomType) < 0)
+       return;
+
+This initializes the :class:`Custom` type, filling in a number of members
+to the appropriate default values, including :attr:`ob_type` that we initially
+set to *NULL*. ::
+
+   PyModule_AddObject(m, "Custom", (PyObject *) &CustomType);
+
+This adds the type to the module dictionary.  This allows us to create
+:class:`Custom` instances by calling the :class:`Custom` class:
+
+.. code-block:: python
+
+   >>> import custom
+   >>> mycustom = custom.Custom()
+
+That's it!  All that remains is to build it; put the above code in a file called
+:file:`custom.c` and:
+
+.. code-block:: python
+
+   from distutils.core import setup, Extension
+   setup(name="custom", version="1.0",
+         ext_modules=[Extension("custom", ["custom.c"])])
+
+in a file called :file:`setup.py`; then typing
+
+.. code-block:: shell-session
+
+   $ python setup.py build
+
+at a shell should produce a file :file:`custom.so` in a subdirectory; move to
+that directory and fire up Python --- you should be able to ``import custom`` and
+play around with Custom objects.
+
+That wasn't so hard, was it?
+
+Of course, the current Custom type is pretty uninteresting. It has no data and
+doesn't do anything. It can't even be subclassed.
+
+.. note::
+   While this documentation showcases the standard :mod:`distutils` module
+   for building C extensions, it is recommended in real-world use cases to
+   use the newer and better-maintained ``setuptools`` library.  Documentation
+   on how to do this is out of scope for this document and can be found in
+   the `Python Packaging User's Guide <https://packaging.python.org/tutorials/distributing-packages/>`_.
+
+
+Adding data and methods to the Basic example
+============================================
+
+Let's extend the basic example to add some data and methods.  Let's also make
+the type usable as a base class. We'll create a new module, :mod:`custom2` that
+adds these capabilities:
+
+.. literalinclude:: ../includes/custom2.c
+
+
+This version of the module has a number of changes.
+
+We've added an extra include::
+
+   #include <structmember.h>
+
+This include provides declarations that we use to handle attributes, as
+described a bit later.
+
+The  :class:`Custom` type now has three data attributes in its C struct,
+*first*, *last*, and *number*.  The *first* and *last* variables are Python
+strings containing first and last names.  The *number* attribute is a C integer.
+
+The object structure is updated accordingly::
+
+   typedef struct {
+       PyObject_HEAD
+       PyObject *first; /* first name */
+       PyObject *last;  /* last name */
+       int number;
+   } CustomObject;
+
+Because we now have data to manage, we have to be more careful about object
+allocation and deallocation.  At a minimum, we need a deallocation method::
+
+   static void
+   Custom_dealloc(CustomObject *self)
+   {
+       Py_XDECREF(self->first);
+       Py_XDECREF(self->last);
+       Py_TYPE(self)->tp_free((PyObject *) self);
+   }
+
+which is assigned to the :c:member:`~PyTypeObject.tp_dealloc` member::
+
+   .tp_dealloc = (destructor) Custom_dealloc,
+
+This method first clears the reference counts of the two Python attributes.
+:c:func:`Py_XDECREF` correctly handles the case where its argument is
+*NULL* (which might happen here if ``tp_new`` failed midway).  It then
+calls the :c:member:`~PyTypeObject.tp_free` member of the object's type
+(computed by ``Py_TYPE(self)``) to free the object's memory.  Note that
+the object's type might not be :class:`CustomType`, because the object may
+be an instance of a subclass.
+
+.. note::
+   The explicit cast to ``destructor`` above is needed because we defined
+   ``Custom_dealloc`` to take a ``CustomObject *`` argument, but the ``tp_dealloc``
+   function pointer expects to receive a ``PyObject *`` argument.  Otherwise,
+   the compiler will emit a warning.  This is object-oriented polymorphism,
+   in C!
+
+We want to make sure that the first and last names are initialized to empty
+strings, so we provide a ``tp_new`` implementation::
+
+   static PyObject *
+   Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+   {
+       CustomObject *self;
+       self = (CustomObject *) type->tp_alloc(type, 0);
+       if (self != NULL) {
+           self->first = PyUnicode_FromString("");
+           if (self->first == NULL) {
+               Py_DECREF(self);
+               return NULL;
+           }
+           self->last = PyUnicode_FromString("");
+           if (self->last == NULL) {
+               Py_DECREF(self);
+               return NULL;
+           }
+           self->number = 0;
+       }
+       return (PyObject *) self;
+   }
+
+and install it in the :c:member:`~PyTypeObject.tp_new` member::
+
+   .tp_new = Custom_new,
+
+The ``tp_new`` handler is responsible for creating (as opposed to initializing)
+objects of the type.  It is exposed in Python as the :meth:`__new__` method.
+It is not required to define a ``tp_new`` member, and indeed many extension
+types will simply reuse :c:func:`PyType_GenericNew` as done in the first
+version of the ``Custom`` type above.  In this case, we use the ``tp_new``
+handler to initialize the ``first`` and ``last`` attributes to non-*NULL*
+default values.
+
+``tp_new`` is passed the type being instantiated (not necessarily ``CustomType``,
+if a subclass is instantiated) and any arguments passed when the type was
+called, and is expected to return the instance created.  ``tp_new`` handlers
+always accept positional and keyword arguments, but they often ignore the
+arguments, leaving the argument handling to initializer (a.k.a. ``tp_init``
+in C or ``__init__`` in Python) methods.
+
+.. note::
+   ``tp_new`` shouldn't call ``tp_init`` explicitly, as the interpreter
+   will do it itself.
+
+The ``tp_new`` implementation calls the :c:member:`~PyTypeObject.tp_alloc`
+slot to allocate memory::
+
+   self = (CustomObject *) type->tp_alloc(type, 0);
+
+Since memory allocation may fail, we must check the :c:member:`~PyTypeObject.tp_alloc`
+result against *NULL* before proceeding.
+
+.. note::
+   We didn't fill the :c:member:`~PyTypeObject.tp_alloc` slot ourselves. Rather
+   :c:func:`PyType_Ready` fills it for us by inheriting it from our base class,
+   which is :class:`object` by default.  Most types use the default allocation
+   strategy.
+
+.. note::
+   If you are creating a co-operative :c:member:`~PyTypeObject.tp_new` (one
+   that calls a base type's :c:member:`~PyTypeObject.tp_new` or :meth:`__new__`),
+   you must *not* try to determine what method to call using method resolution
+   order at runtime.  Always statically determine what type you are going to
+   call, and call its :c:member:`~PyTypeObject.tp_new` directly, or via
+   ``type->tp_base->tp_new``.  If you do not do this, Python subclasses of your
+   type that also inherit from other Python-defined classes may not work correctly.
+   (Specifically, you may not be able to create instances of such subclasses
+   without getting a :exc:`TypeError`.)
+
+We also define an initialization function which accepts arguments to provide
+initial values for our instance::
+
+   static int
+   Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
+   {
+       static char *kwlist[] = {"first", "last", "number", NULL};
+       PyObject *first = NULL, *last = NULL, *tmp;
+
+       if (!PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
+                                        &first, &last,
+                                        &self->number))
+           return -1;
+
+       if (first) {
+           tmp = self->first;
+           Py_INCREF(first);
+           self->first = first;
+           Py_XDECREF(tmp);
+       }
+       if (last) {
+           tmp = self->last;
+           Py_INCREF(last);
+           self->last = last;
+           Py_XDECREF(tmp);
+       }
+       return 0;
+   }
+
+by filling the :c:member:`~PyTypeObject.tp_init` slot. ::
+
+   .tp_init = (initproc) Custom_init,
+
+The :c:member:`~PyTypeObject.tp_init` slot is exposed in Python as the
+:meth:`__init__` method.  It is used to initialize an object after it's
+created.  Initializers always accept positional and keyword arguments,
+and they should return either ``0`` on success or ``-1`` on error.
+
+Unlike the ``tp_new`` handler, there is no guarantee that ``tp_init``
+is called at all (for example, the :mod:`pickle` module by default
+doesn't call :meth:`__init__` on unpickled instances).  It can also be
+called multiple times.  Anyone can call the :meth:`__init__` method on
+our objects.  For this reason, we have to be extra careful when assigning
+the new attribute values.  We might be tempted, for example to assign the
+``first`` member like this::
+
+   if (first) {
+       Py_XDECREF(self->first);
+       Py_INCREF(first);
+       self->first = first;
+   }
+
+But this would be risky.  Our type doesn't restrict the type of the
+``first`` member, so it could be any kind of object.  It could have a
+destructor that causes code to be executed that tries to access the
+``first`` member; or that destructor could release the
+:term:`Global interpreter Lock` and let arbitrary code run in other
+threads that accesses and modifies our object.
+
+To be paranoid and protect ourselves against this possibility, we almost
+always reassign members before decrementing their reference counts.  When
+don't we have to do this?
+
+* when we absolutely know that the reference count is greater than 1;
+
+* when we know that deallocation of the object [#]_ will neither release
+  the :term:`GIL` nor cause any calls back into our type's code;
+
+* when decrementing a reference count in a :c:member:`~PyTypeObject.tp_dealloc`
+  handler on a type which doesn't support cyclic garbage collection [#]_.
+
+We want to expose our instance variables as attributes. There are a
+number of ways to do that. The simplest way is to define member definitions::
+
+   static PyMemberDef Custom_members[] = {
+       {"first", T_OBJECT_EX, offsetof(CustomObject, first), 0,
+        "first name"},
+       {"last", T_OBJECT_EX, offsetof(CustomObject, last), 0,
+        "last name"},
+       {"number", T_INT, offsetof(CustomObject, number), 0,
+        "custom number"},
+       {NULL}  /* Sentinel */
+   };
+
+and put the definitions in the :c:member:`~PyTypeObject.tp_members` slot::
+
+   .tp_members = Custom_members,
+
+Each member definition has a member name, type, offset, access flags and
+documentation string.  See the :ref:`Generic-Attribute-Management` section
+below for details.
+
+A disadvantage of this approach is that it doesn't provide a way to restrict the
+types of objects that can be assigned to the Python attributes.  We expect the
+first and last names to be strings, but any Python objects can be assigned.
+Further, the attributes can be deleted, setting the C pointers to *NULL*.  Even
+though we can make sure the members are initialized to non-*NULL* values, the
+members can be set to *NULL* if the attributes are deleted.
+
+We define a single method, :meth:`Custom.name()`, that outputs the objects name as the
+concatenation of the first and last names. ::
+
+   static PyObject *
+   Custom_name(CustomObject *self)
+   {
+       if (self->first == NULL) {
+           PyErr_SetString(PyExc_AttributeError, "first");
+           return NULL;
+       }
+       if (self->last == NULL) {
+           PyErr_SetString(PyExc_AttributeError, "last");
+           return NULL;
+       }
+       return PyUnicode_FromFormat("%S %S", self->first, self->last);
+   }
+
+The method is implemented as a C function that takes a :class:`Custom` (or
+:class:`Custom` subclass) instance as the first argument.  Methods always take an
+instance as the first argument. Methods often take positional and keyword
+arguments as well, but in this case we don't take any and don't need to accept
+a positional argument tuple or keyword argument dictionary. This method is
+equivalent to the Python method:
+
+.. code-block:: python
+
+   def name(self):
+       return "%s %s" % (self.first, self.last)
+
+Note that we have to check for the possibility that our :attr:`first` and
+:attr:`last` members are *NULL*.  This is because they can be deleted, in which
+case they are set to *NULL*.  It would be better to prevent deletion of these
+attributes and to restrict the attribute values to be strings.  We'll see how to
+do that in the next section.
+
+Now that we've defined the method, we need to create an array of method
+definitions::
+
+   static PyMethodDef Custom_methods[] = {
+       {"name", (PyCFunction) Custom_name, METH_NOARGS,
+        "Return the name, combining the first and last name"
+       },
+       {NULL}  /* Sentinel */
+   };
+
+(note that we used the :const:`METH_NOARGS` flag to indicate that the method
+is expecting no arguments other than *self*)
+
+and assign it to the :c:member:`~PyTypeObject.tp_methods` slot::
+
+   .tp_methods = Custom_methods,
+
+Finally, we'll make our type usable as a base class for subclassing.  We've
+written our methods carefully so far so that they don't make any assumptions
+about the type of the object being created or used, so all we need to do is
+to add the :const:`Py_TPFLAGS_BASETYPE` to our class flag definition::
+
+   .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+
+We rename :c:func:`PyInit_custom` to :c:func:`PyInit_custom2`, update the
+module name in the :c:type:`PyModuleDef` struct, and update the full class
+name in the :c:type:`PyTypeObject` struct.
+
+Finally, we update our :file:`setup.py` file to build the new module:
+
+.. code-block:: python
+
+   from distutils.core import setup, Extension
+   setup(name="custom", version="1.0",
+         ext_modules=[
+            Extension("custom", ["custom.c"]),
+            Extension("custom2", ["custom2.c"]),
+            ])
+
+
+Providing finer control over data attributes
+============================================
+
+In this section, we'll provide finer control over how the :attr:`first` and
+:attr:`last` attributes are set in the :class:`Custom` example. In the previous
+version of our module, the instance variables :attr:`first` and :attr:`last`
+could be set to non-string values or even deleted. We want to make sure that
+these attributes always contain strings.
+
+.. literalinclude:: ../includes/custom3.c
+
+
+To provide greater control, over the :attr:`first` and :attr:`last` attributes,
+we'll use custom getter and setter functions.  Here are the functions for
+getting and setting the :attr:`first` attribute::
+
+   static PyObject *
+   Custom_getfirst(CustomObject *self, void *closure)
+   {
+       Py_INCREF(self->first);
+       return self->first;
+   }
+
+   static int
+   Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
+   {
+       PyObject *tmp;
+       if (value == NULL) {
+           PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
+           return -1;
+       }
+       if (!PyUnicode_Check(value)) {
+           PyErr_SetString(PyExc_TypeError,
+                           "The first attribute value must be a string");
+           return -1;
+       }
+       tmp = self->first;
+       Py_INCREF(value);
+       self->first = value;
+       Py_DECREF(tmp);
+       return 0;
+   }
+
+The getter function is passed a :class:`Custom` object and a "closure", which is
+a void pointer.  In this case, the closure is ignored.  (The closure supports an
+advanced usage in which definition data is passed to the getter and setter. This
+could, for example, be used to allow a single set of getter and setter functions
+that decide the attribute to get or set based on data in the closure.)
+
+The setter function is passed the :class:`Custom` object, the new value, and the
+closure.  The new value may be *NULL*, in which case the attribute is being
+deleted.  In our setter, we raise an error if the attribute is deleted or if its
+new value is not a string.
+
+We create an array of :c:type:`PyGetSetDef` structures::
+
+   static PyGetSetDef Custom_getsetters[] = {
+       {"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
+        "first name", NULL},
+       {"last", (getter) Custom_getlast, (setter) Custom_setlast,
+        "last name", NULL},
+       {NULL}  /* Sentinel */
+   };
+
+and register it in the :c:member:`~PyTypeObject.tp_getset` slot::
+
+   .tp_getset = Custom_getsetters,
+
+The last item in a :c:type:`PyGetSetDef` structure is the "closure" mentioned
+above.  In this case, we aren't using a closure, so we just pass *NULL*.
+
+We also remove the member definitions for these attributes::
+
+   static PyMemberDef Custom_members[] = {
+       {"number", T_INT, offsetof(CustomObject, number), 0,
+        "custom number"},
+       {NULL}  /* Sentinel */
+   };
+
+We also need to update the :c:member:`~PyTypeObject.tp_init` handler to only
+allow strings [#]_ to be passed::
+
+   static int
+   Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
+   {
+       static char *kwlist[] = {"first", "last", "number", NULL};
+       PyObject *first = NULL, *last = NULL, *tmp;
+
+       if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
+                                        &first, &last,
+                                        &self->number))
+           return -1;
+
+       if (first) {
+           tmp = self->first;
+           Py_INCREF(first);
+           self->first = first;
+           Py_DECREF(tmp);
+       }
+       if (last) {
+           tmp = self->last;
+           Py_INCREF(last);
+           self->last = last;
+           Py_DECREF(tmp);
+       }
+       return 0;
+   }
+
+With these changes, we can assure that the ``first`` and ``last`` members are
+never *NULL* so we can remove checks for *NULL* values in almost all cases.
+This means that most of the :c:func:`Py_XDECREF` calls can be converted to
+:c:func:`Py_DECREF` calls.  The only place we can't change these calls is in
+the ``tp_dealloc`` implementation, where there is the possibility that the
+initialization of these members failed in ``tp_new``.
+
+We also rename the module initialization function and module name in the
+initialization function, as we did before, and we add an extra definition to the
+:file:`setup.py` file.
+
+
+Supporting cyclic garbage collection
+====================================
+
+Python has a :term:`cyclic garbage collector (GC) <garbage collection>` that
+can identify unneeded objects even when their reference counts are not zero.
+This can happen when objects are involved in cycles.  For example, consider:
+
+.. code-block:: python
+
+   >>> l = []
+   >>> l.append(l)
+   >>> del l
+
+In this example, we create a list that contains itself. When we delete it, it
+still has a reference from itself. Its reference count doesn't drop to zero.
+Fortunately, Python's cyclic garbage collector will eventually figure out that
+the list is garbage and free it.
+
+In the second version of the :class:`Custom` example, we allowed any kind of
+object to be stored in the :attr:`first` or :attr:`last` attributes [#]_.
+Besides, in the second and third versions, we allowed subclassing
+:class:`Custom`, and subclasses may add arbitrary attributes.  For any of
+those two reasons, :class:`Custom` objects can participate in cycles:
+
+.. code-block:: python
+
+   >>> import custom3
+   >>> class Derived(custom3.Custom): pass
+   ...
+   >>> n = Derived()
+   >>> n.some_attribute = n
+
+To allow a :class:`Custom` instance participating in a reference cycle to
+be properly detected and collected by the cyclic GC, our :class:`Custom` type
+needs to fill two additional slots and to enable a flag that enables these slots:
+
+.. literalinclude:: ../includes/custom4.c
+
+
+First, the traversal method lets the cyclic GC know about subobjects that could
+participate in cycles::
+
+   static int
+   Custom_traverse(CustomObject *self, visitproc visit, void *arg)
+   {
+       int vret;
+       if (self->first) {
+           vret = visit(self->first, arg);
+           if (vret != 0)
+               return vret;
+       }
+       if (self->last) {
+           vret = visit(self->last, arg);
+           if (vret != 0)
+               return vret;
+       }
+       return 0;
+   }
+
+For each subobject that can participate in cycles, we need to call the
+:c:func:`visit` function, which is passed to the traversal method. The
+:c:func:`visit` function takes as arguments the subobject and the extra argument
+*arg* passed to the traversal method.  It returns an integer value that must be
+returned if it is non-zero.
+
+Python provides a :c:func:`Py_VISIT` macro that automates calling visit
+functions.  With :c:func:`Py_VISIT`, we can minimize the amount of boilerplate
+in ``Custom_traverse``::
+
+   static int
+   Custom_traverse(CustomObject *self, visitproc visit, void *arg)
+   {
+       Py_VISIT(self->first);
+       Py_VISIT(self->last);
+       return 0;
+   }
+
+.. note::
+   The :c:member:`~PyTypeObject.tp_traverse` implementation must name its
+   arguments exactly *visit* and *arg* in order to use :c:func:`Py_VISIT`.
+
+Second, we need to provide a method for clearing any subobjects that can
+participate in cycles::
+
+   static int
+   Custom_clear(CustomObject *self)
+   {
+       Py_CLEAR(self->first);
+       Py_CLEAR(self->last);
+       return 0;
+   }
+
+Notice the use of the :c:func:`Py_CLEAR` macro.  It is the recommended and safe
+way to clear data attributes of arbitrary types while decrementing
+their reference counts.  If you were to call :c:func:`Py_XDECREF` instead
+on the attribute before setting it to *NULL*, there is a possibility
+that the attribute's destructor would call back into code that reads the
+attribute again (*especially* if there is a reference cycle).
+
+.. note::
+   You could emulate :c:func:`Py_CLEAR` by writing::
+
+      PyObject *tmp;
+      tmp = self->first;
+      self->first = NULL;
+      Py_XDECREF(tmp);
+
+   Nevertheless, it is much easier and less error-prone to always
+   use :c:func:`Py_CLEAR` when deleting an attribute.  Don't
+   try to micro-optimize at the expense of robustness!
+
+The deallocator ``Custom_dealloc`` may call arbitrary code when clearing
+attributes.  It means the circular GC can be triggered inside the function.
+Since the GC assumes reference count is not zero, we need to untrack the object
+from the GC by calling :c:func:`PyObject_GC_UnTrack` before clearing members.
+Here is our reimplemented deallocator using :c:func:`PyObject_GC_UnTrack`
+and ``Custom_clear``::
+
+   static void
+   Custom_dealloc(CustomObject *self)
+   {
+       PyObject_GC_UnTrack(self);
+       Custom_clear(self);
+       Py_TYPE(self)->tp_free((PyObject *) self);
+   }
+
+Finally, we add the :const:`Py_TPFLAGS_HAVE_GC` flag to the class flags::
+
+   .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
+
+That's pretty much it.  If we had written custom :c:member:`~PyTypeObject.tp_alloc` or
+:c:member:`~PyTypeObject.tp_free` handlers, we'd need to modify them for cyclic
+garbage collection.  Most extensions will use the versions automatically provided.
+
+
+Subclassing other types
+=======================
+
+It is possible to create new extension types that are derived from existing
+types. It is easiest to inherit from the built in types, since an extension can
+easily use the :c:type:`PyTypeObject` it needs. It can be difficult to share
+these :c:type:`PyTypeObject` structures between extension modules.
+
+In this example we will create a :class:`SubList` type that inherits from the
+built-in :class:`list` type. The new type will be completely compatible with
+regular lists, but will have an additional :meth:`increment` method that
+increases an internal counter:
+
+.. code-block:: python
+
+   >>> import sublist
+   >>> s = sublist.SubList(range(3))
+   >>> s.extend(s)
+   >>> print(len(s))
+   6
+   >>> print(s.increment())
+   1
+   >>> print(s.increment())
+   2
+
+.. literalinclude:: ../includes/sublist.c
+
+
+As you can see, the source code closely resembles the :class:`Custom` examples in
+previous sections. We will break down the main differences between them. ::
+
+   typedef struct {
+       PyListObject list;
+       int state;
+   } SubListObject;
+
+The primary difference for derived type objects is that the base type's
+object structure must be the first value.  The base type will already include
+the :c:func:`PyObject_HEAD` at the beginning of its structure.
+
+When a Python object is a :class:`SubList` instance, its ``PyObject *`` pointer
+can be safely cast to both ``PyListObject *`` and ``SubListObject *``::
+
+   static int
+   SubList_init(SubListObject *self, PyObject *args, PyObject *kwds)
+   {
+       if (PyList_Type.tp_init((PyObject *) self, args, kwds) < 0)
+           return -1;
+       self->state = 0;
+       return 0;
+   }
+
+We see above how to call through to the :attr:`__init__` method of the base
+type.
+
+This pattern is important when writing a type with custom
+:c:member:`~PyTypeObject.tp_new` and :c:member:`~PyTypeObject.tp_dealloc`
+members.  The :c:member:`~PyTypeObject.tp_new` handler should not actually
+create the memory for the object with its :c:member:`~PyTypeObject.tp_alloc`,
+but let the base class handle it by calling its own :c:member:`~PyTypeObject.tp_new`.
+
+The :c:type:`PyTypeObject` struct supports a :c:member:`~PyTypeObject.tp_base`
+specifying the type's concrete base class.  Due to cross-platform compiler
+issues, you can't fill that field directly with a reference to
+:c:type:`PyList_Type`; it should be done later in the module initialization
+function::
+
+   PyMODINIT_FUNC
+   PyInit_sublist(void)
+   {
+       PyObject* m;
+       SubListType.tp_base = &PyList_Type;
+       if (PyType_Ready(&SubListType) < 0)
+           return NULL;
+
+       m = PyModule_Create(&sublistmodule);
+       if (m == NULL)
+           return NULL;
+
+       Py_INCREF(&SubListType);
+       PyModule_AddObject(m, "SubList", (PyObject *) &SubListType);
+       return m;
+   }
+
+Before calling :c:func:`PyType_Ready`, the type structure must have the
+:c:member:`~PyTypeObject.tp_base` slot filled in.  When we are deriving an
+existing type, it is not necessary to fill out the :c:member:`~PyTypeObject.tp_alloc`
+slot with :c:func:`PyType_GenericNew` -- the allocation function from the base
+type will be inherited.
+
+After that, calling :c:func:`PyType_Ready` and adding the type object to the
+module is the same as with the basic :class:`Custom` examples.
+
+
+.. rubric:: Footnotes
+
+.. [#] This is true when we know that the object is a basic type, like a string or a
+   float.
+
+.. [#] We relied on this in the :c:member:`~PyTypeObject.tp_dealloc` handler
+   in this example, because our type doesn't support garbage collection.
+
+.. [#] We now know that the first and last members are strings, so perhaps we
+   could be less careful about decrementing their reference counts, however,
+   we accept instances of string subclasses.  Even though deallocating normal
+   strings won't call back into our objects, we can't guarantee that deallocating
+   an instance of a string subclass won't call back into our objects.
+
+.. [#] Also, even with our attributes restricted to strings instances, the user
+   could pass arbitrary :class:`str` subclasses and therefore still create
+   reference cycles.
diff --git a/Doc/includes/custom.c b/Doc/includes/custom.c
new file mode 100644
index 0000000..fb2c7b2
--- /dev/null
+++ b/Doc/includes/custom.c
@@ -0,0 +1,39 @@
+#include <Python.h>
+
+typedef struct {
+    PyObject_HEAD
+    /* Type-specific fields go here. */
+} CustomObject;
+
+static PyTypeObject CustomType = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "custom.Custom",
+    .tp_doc = "Custom objects",
+    .tp_basicsize = sizeof(CustomObject),
+    .tp_itemsize = 0,
+    .tp_flags = Py_TPFLAGS_DEFAULT,
+    .tp_new = PyType_GenericNew,
+};
+
+static PyModuleDef custommodule = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "custom",
+    .m_doc = "Example module that creates an extension type.",
+    .m_size = -1,
+};
+
+PyMODINIT_FUNC
+PyInit_custom(void)
+{
+    PyObject *m;
+    if (PyType_Ready(&CustomType) < 0)
+        return NULL;
+
+    m = PyModule_Create(&custommodule);
+    if (m == NULL)
+        return NULL;
+
+    Py_INCREF(&CustomType);
+    PyModule_AddObject(m, "Custom", (PyObject *) &CustomType);
+    return m;
+}
diff --git a/Doc/includes/custom2.c b/Doc/includes/custom2.c
new file mode 100644
index 0000000..51ab4b8
--- /dev/null
+++ b/Doc/includes/custom2.c
@@ -0,0 +1,132 @@
+#include <Python.h>
+#include "structmember.h"
+
+typedef struct {
+    PyObject_HEAD
+    PyObject *first; /* first name */
+    PyObject *last;  /* last name */
+    int number;
+} CustomObject;
+
+static void
+Custom_dealloc(CustomObject *self)
+{
+    Py_XDECREF(self->first);
+    Py_XDECREF(self->last);
+    Py_TYPE(self)->tp_free((PyObject *) self);
+}
+
+static PyObject *
+Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    CustomObject *self;
+    self = (CustomObject *) type->tp_alloc(type, 0);
+    if (self != NULL) {
+        self->first = PyUnicode_FromString("");
+        if (self->first == NULL) {
+            Py_DECREF(self);
+            return NULL;
+        }
+        self->last = PyUnicode_FromString("");
+        if (self->last == NULL) {
+            Py_DECREF(self);
+            return NULL;
+        }
+        self->number = 0;
+    }
+    return (PyObject *) self;
+}
+
+static int
+Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"first", "last", "number", NULL};
+    PyObject *first = NULL, *last = NULL, *tmp;
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
+                                     &first, &last,
+                                     &self->number))
+        return -1;
+
+    if (first) {
+        tmp = self->first;
+        Py_INCREF(first);
+        self->first = first;
+        Py_XDECREF(tmp);
+    }
+    if (last) {
+        tmp = self->last;
+        Py_INCREF(last);
+        self->last = last;
+        Py_XDECREF(tmp);
+    }
+    return 0;
+}
+
+static PyMemberDef Custom_members[] = {
+    {"first", T_OBJECT_EX, offsetof(CustomObject, first), 0,
+     "first name"},
+    {"last", T_OBJECT_EX, offsetof(CustomObject, last), 0,
+     "last name"},
+    {"number", T_INT, offsetof(CustomObject, number), 0,
+     "custom number"},
+    {NULL}  /* Sentinel */
+};
+
+static PyObject *
+Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
+{
+    if (self->first == NULL) {
+        PyErr_SetString(PyExc_AttributeError, "first");
+        return NULL;
+    }
+    if (self->last == NULL) {
+        PyErr_SetString(PyExc_AttributeError, "last");
+        return NULL;
+    }
+    return PyUnicode_FromFormat("%S %S", self->first, self->last);
+}
+
+static PyMethodDef Custom_methods[] = {
+    {"name", (PyCFunction) Custom_name, METH_NOARGS,
+     "Return the name, combining the first and last name"
+    },
+    {NULL}  /* Sentinel */
+};
+
+static PyTypeObject CustomType = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "custom2.Custom",
+    .tp_doc = "Custom objects",
+    .tp_basicsize = sizeof(CustomObject),
+    .tp_itemsize = 0,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .tp_new = Custom_new,
+    .tp_init = (initproc) Custom_init,
+    .tp_dealloc = (destructor) Custom_dealloc,
+    .tp_members = Custom_members,
+    .tp_methods = Custom_methods,
+};
+
+static PyModuleDef custommodule = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "custom2",
+    .m_doc = "Example module that creates an extension type.",
+    .m_size = -1,
+};
+
+PyMODINIT_FUNC
+PyInit_custom2(void)
+{
+    PyObject *m;
+    if (PyType_Ready(&CustomType) < 0)
+        return NULL;
+
+    m = PyModule_Create(&custommodule);
+    if (m == NULL)
+        return NULL;
+
+    Py_INCREF(&CustomType);
+    PyModule_AddObject(m, "Custom", (PyObject *) &CustomType);
+    return m;
+}
diff --git a/Doc/includes/custom3.c b/Doc/includes/custom3.c
new file mode 100644
index 0000000..09e8735
--- /dev/null
+++ b/Doc/includes/custom3.c
@@ -0,0 +1,183 @@
+#include <Python.h>
+#include "structmember.h"
+
+typedef struct {
+    PyObject_HEAD
+    PyObject *first; /* first name */
+    PyObject *last;  /* last name */
+    int number;
+} CustomObject;
+
+static void
+Custom_dealloc(CustomObject *self)
+{
+    Py_XDECREF(self->first);
+    Py_XDECREF(self->last);
+    Py_TYPE(self)->tp_free((PyObject *) self);
+}
+
+static PyObject *
+Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    CustomObject *self;
+    self = (CustomObject *) type->tp_alloc(type, 0);
+    if (self != NULL) {
+        self->first = PyUnicode_FromString("");
+        if (self->first == NULL) {
+            Py_DECREF(self);
+            return NULL;
+        }
+        self->last = PyUnicode_FromString("");
+        if (self->last == NULL) {
+            Py_DECREF(self);
+            return NULL;
+        }
+        self->number = 0;
+    }
+    return (PyObject *) self;
+}
+
+static int
+Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"first", "last", "number", NULL};
+    PyObject *first = NULL, *last = NULL, *tmp;
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
+                                     &first, &last,
+                                     &self->number))
+        return -1;
+
+    if (first) {
+        tmp = self->first;
+        Py_INCREF(first);
+        self->first = first;
+        Py_DECREF(tmp);
+    }
+    if (last) {
+        tmp = self->last;
+        Py_INCREF(last);
+        self->last = last;
+        Py_DECREF(tmp);
+    }
+    return 0;
+}
+
+static PyMemberDef Custom_members[] = {
+    {"number", T_INT, offsetof(CustomObject, number), 0,
+     "custom number"},
+    {NULL}  /* Sentinel */
+};
+
+static PyObject *
+Custom_getfirst(CustomObject *self, void *closure)
+{
+    Py_INCREF(self->first);
+    return self->first;
+}
+
+static int
+Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
+{
+    PyObject *tmp;
+    if (value == NULL) {
+        PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
+        return -1;
+    }
+    if (!PyUnicode_Check(value)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "The first attribute value must be a string");
+        return -1;
+    }
+    tmp = self->first;
+    Py_INCREF(value);
+    self->first = value;
+    Py_DECREF(tmp);
+    return 0;
+}
+
+static PyObject *
+Custom_getlast(CustomObject *self, void *closure)
+{
+    Py_INCREF(self->last);
+    return self->last;
+}
+
+static int
+Custom_setlast(CustomObject *self, PyObject *value, void *closure)
+{
+    PyObject *tmp;
+    if (value == NULL) {
+        PyErr_SetString(PyExc_TypeError, "Cannot delete the last attribute");
+        return -1;
+    }
+    if (!PyUnicode_Check(value)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "The last attribute value must be a string");
+        return -1;
+    }
+    tmp = self->last;
+    Py_INCREF(value);
+    self->last = value;
+    Py_DECREF(tmp);
+    return 0;
+}
+
+static PyGetSetDef Custom_getsetters[] = {
+    {"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
+     "first name", NULL},
+    {"last", (getter) Custom_getlast, (setter) Custom_setlast,
+     "last name", NULL},
+    {NULL}  /* Sentinel */
+};
+
+static PyObject *
+Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
+{
+    return PyUnicode_FromFormat("%S %S", self->first, self->last);
+}
+
+static PyMethodDef Custom_methods[] = {
+    {"name", (PyCFunction) Custom_name, METH_NOARGS,
+     "Return the name, combining the first and last name"
+    },
+    {NULL}  /* Sentinel */
+};
+
+static PyTypeObject CustomType = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "custom3.Custom",
+    .tp_doc = "Custom objects",
+    .tp_basicsize = sizeof(CustomObject),
+    .tp_itemsize = 0,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .tp_new = Custom_new,
+    .tp_init = (initproc) Custom_init,
+    .tp_dealloc = (destructor) Custom_dealloc,
+    .tp_members = Custom_members,
+    .tp_methods = Custom_methods,
+    .tp_getset = Custom_getsetters,
+};
+
+static PyModuleDef custommodule = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "custom3",
+    .m_doc = "Example module that creates an extension type.",
+    .m_size = -1,
+};
+
+PyMODINIT_FUNC
+PyInit_custom3(void)
+{
+    PyObject *m;
+    if (PyType_Ready(&CustomType) < 0)
+        return NULL;
+
+    m = PyModule_Create(&custommodule);
+    if (m == NULL)
+        return NULL;
+
+    Py_INCREF(&CustomType);
+    PyModule_AddObject(m, "Custom", (PyObject *) &CustomType);
+    return m;
+}
diff --git a/Doc/includes/custom4.c b/Doc/includes/custom4.c
new file mode 100644
index 0000000..0994d8f
--- /dev/null
+++ b/Doc/includes/custom4.c
@@ -0,0 +1,197 @@
+#include <Python.h>
+#include "structmember.h"
+
+typedef struct {
+    PyObject_HEAD
+    PyObject *first; /* first name */
+    PyObject *last;  /* last name */
+    int number;
+} CustomObject;
+
+static int
+Custom_traverse(CustomObject *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->first);
+    Py_VISIT(self->last);
+    return 0;
+}
+
+static int
+Custom_clear(CustomObject *self)
+{
+    Py_CLEAR(self->first);
+    Py_CLEAR(self->last);
+    return 0;
+}
+
+static void
+Custom_dealloc(CustomObject *self)
+{
+    PyObject_GC_UnTrack(self);
+    Custom_clear(self);
+    Py_TYPE(self)->tp_free((PyObject *) self);
+}
+
+static PyObject *
+Custom_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    CustomObject *self;
+    self = (CustomObject *) type->tp_alloc(type, 0);
+    if (self != NULL) {
+        self->first = PyUnicode_FromString("");
+        if (self->first == NULL) {
+            Py_DECREF(self);
+            return NULL;
+        }
+        self->last = PyUnicode_FromString("");
+        if (self->last == NULL) {
+            Py_DECREF(self);
+            return NULL;
+        }
+        self->number = 0;
+    }
+    return (PyObject *) self;
+}
+
+static int
+Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"first", "last", "number", NULL};
+    PyObject *first = NULL, *last = NULL, *tmp;
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
+                                     &first, &last,
+                                     &self->number))
+        return -1;
+
+    if (first) {
+        tmp = self->first;
+        Py_INCREF(first);
+        self->first = first;
+        Py_DECREF(tmp);
+    }
+    if (last) {
+        tmp = self->last;
+        Py_INCREF(last);
+        self->last = last;
+        Py_DECREF(tmp);
+    }
+    return 0;
+}
+
+static PyMemberDef Custom_members[] = {
+    {"number", T_INT, offsetof(CustomObject, number), 0,
+     "custom number"},
+    {NULL}  /* Sentinel */
+};
+
+static PyObject *
+Custom_getfirst(CustomObject *self, void *closure)
+{
+    Py_INCREF(self->first);
+    return self->first;
+}
+
+static int
+Custom_setfirst(CustomObject *self, PyObject *value, void *closure)
+{
+    if (value == NULL) {
+        PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
+        return -1;
+    }
+    if (!PyUnicode_Check(value)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "The first attribute value must be a string");
+        return -1;
+    }
+    Py_INCREF(value);
+    Py_CLEAR(self->first);
+    self->first = value;
+    return 0;
+}
+
+static PyObject *
+Custom_getlast(CustomObject *self, void *closure)
+{
+    Py_INCREF(self->last);
+    return self->last;
+}
+
+static int
+Custom_setlast(CustomObject *self, PyObject *value, void *closure)
+{
+    if (value == NULL) {
+        PyErr_SetString(PyExc_TypeError, "Cannot delete the last attribute");
+        return -1;
+    }
+    if (!PyUnicode_Check(value)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "The last attribute value must be a string");
+        return -1;
+    }
+    Py_INCREF(value);
+    Py_CLEAR(self->last);
+    self->last = value;
+    return 0;
+}
+
+static PyGetSetDef Custom_getsetters[] = {
+    {"first", (getter) Custom_getfirst, (setter) Custom_setfirst,
+     "first name", NULL},
+    {"last", (getter) Custom_getlast, (setter) Custom_setlast,
+     "last name", NULL},
+    {NULL}  /* Sentinel */
+};
+
+static PyObject *
+Custom_name(CustomObject *self, PyObject *Py_UNUSED(ignored))
+{
+    return PyUnicode_FromFormat("%S %S", self->first, self->last);
+}
+
+static PyMethodDef Custom_methods[] = {
+    {"name", (PyCFunction) Custom_name, METH_NOARGS,
+     "Return the name, combining the first and last name"
+    },
+    {NULL}  /* Sentinel */
+};
+
+static PyTypeObject CustomType = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "custom4.Custom",
+    .tp_doc = "Custom objects",
+    .tp_basicsize = sizeof(CustomObject),
+    .tp_itemsize = 0,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
+    .tp_new = Custom_new,
+    .tp_init = (initproc) Custom_init,
+    .tp_dealloc = (destructor) Custom_dealloc,
+    .tp_traverse = (traverseproc) Custom_traverse,
+    .tp_clear = (inquiry) Custom_clear,
+    .tp_members = Custom_members,
+    .tp_methods = Custom_methods,
+    .tp_getset = Custom_getsetters,
+};
+
+static PyModuleDef custommodule = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "custom4",
+    .m_doc = "Example module that creates an extension type.",
+    .m_size = -1,
+};
+
+PyMODINIT_FUNC
+PyInit_custom4(void)
+{
+    PyObject *m;
+    if (PyType_Ready(&CustomType) < 0)
+        return NULL;
+
+    m = PyModule_Create(&custommodule);
+    if (m == NULL)
+        return NULL;
+
+    Py_INCREF(&CustomType);
+    PyModule_AddObject(m, "Custom", (PyObject *) &CustomType);
+    return m;
+}
diff --git a/Doc/includes/noddy.c b/Doc/includes/noddy.c
deleted file mode 100644
index 07b5d5a..0000000
--- a/Doc/includes/noddy.c
+++ /dev/null
@@ -1,72 +0,0 @@
-#include <Python.h>
-
-typedef struct {
-    PyObject_HEAD
-    /* Type-specific fields go here. */
-} noddy_NoddyObject;
-
-static PyTypeObject noddy_NoddyType = {
-    PyVarObject_HEAD_INIT(NULL, 0)
-    "noddy.Noddy",             /* tp_name */
-    sizeof(noddy_NoddyObject), /* tp_basicsize */
-    0,                         /* tp_itemsize */
-    0,                         /* tp_dealloc */
-    0,                         /* tp_print */
-    0,                         /* tp_getattr */
-    0,                         /* tp_setattr */
-    0,                         /* tp_reserved */
-    0,                         /* tp_repr */
-    0,                         /* tp_as_number */
-    0,                         /* tp_as_sequence */
-    0,                         /* tp_as_mapping */
-    0,                         /* tp_hash  */
-    0,                         /* tp_call */
-    0,                         /* tp_str */
-    0,                         /* tp_getattro */
-    0,                         /* tp_setattro */
-    0,                         /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT,        /* tp_flags */
-    "Noddy objects",           /* tp_doc */
-    0,                         /* tp_traverse */
-    0,                         /* tp_clear */
-    0,                         /* tp_richcompare */
-    0,                         /* tp_weaklistoffset */
-    0,                         /* tp_iter */
-    0,                         /* tp_iternext */
-    0,                         /* tp_methods */
-    0,                         /* tp_members */
-    0,                         /* tp_getset */
-    0,                         /* tp_base */
-    0,                         /* tp_dict */
-    0,                         /* tp_descr_get */
-    0,                         /* tp_descr_set */
-    0,                         /* tp_dictoffset */
-    0,                         /* tp_init */
-    0,                         /* tp_alloc */
-    PyType_GenericNew,         /* tp_new */
-};
-
-static PyModuleDef noddymodule = {
-    PyModuleDef_HEAD_INIT,
-    "noddy",
-    "Example module that creates an extension type.",
-    -1,
-    NULL, NULL, NULL, NULL, NULL
-};
-
-PyMODINIT_FUNC
-PyInit_noddy(void)
-{
-    PyObject* m;
-
-    if (PyType_Ready(&noddy_NoddyType) < 0)
-        return NULL;
-
-    m = PyModule_Create(&noddymodule);
-    if (m == NULL)
-        return NULL;
-
-    Py_INCREF(&noddy_NoddyType);
-    PyModule_AddObject(m, "Noddy", (PyObject *)&noddy_NoddyType);
-    return m;
-}
diff --git a/Doc/includes/noddy2.c b/Doc/includes/noddy2.c
deleted file mode 100644
index 9641558..0000000
--- a/Doc/includes/noddy2.c
+++ /dev/null
@@ -1,172 +0,0 @@
-#include <Python.h>
-#include "structmember.h"
-
-typedef struct {
-    PyObject_HEAD
-    PyObject *first; /* first name */
-    PyObject *last;  /* last name */
-    int number;
-} Noddy;
-
-static void
-Noddy_dealloc(Noddy* self)
-{
-    Py_XDECREF(self->first);
-    Py_XDECREF(self->last);
-    Py_TYPE(self)->tp_free((PyObject*)self);
-}
-
-static PyObject *
-Noddy_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
-{
-    Noddy *self;
-
-    self = (Noddy *)type->tp_alloc(type, 0);
-    if (self != NULL) {
-        self->first = PyUnicode_FromString("");
-        if (self->first == NULL) {
-            Py_DECREF(self);
-            return NULL;
-        }
-
-        self->last = PyUnicode_FromString("");
-        if (self->last == NULL) {
-            Py_DECREF(self);
-            return NULL;
-        }
-
-        self->number = 0;
-    }
-
-    return (PyObject *)self;
-}
-
-static int
-Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
-{
-    PyObject *first=NULL, *last=NULL, *tmp;
-
-    static char *kwlist[] = {"first", "last", "number", NULL};
-
-    if (! PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
-                                      &first, &last,
-                                      &self->number))
-        return -1;
-
-    if (first) {
-        tmp = self->first;
-        Py_INCREF(first);
-        self->first = first;
-        Py_XDECREF(tmp);
-    }
-
-    if (last) {
-        tmp = self->last;
-        Py_INCREF(last);
-        self->last = last;
-        Py_XDECREF(tmp);
-    }
-
-    return 0;
-}
-
-
-static PyMemberDef Noddy_members[] = {
-    {"first", T_OBJECT_EX, offsetof(Noddy, first), 0,
-     "first name"},
-    {"last", T_OBJECT_EX, offsetof(Noddy, last), 0,
-     "last name"},
-    {"number", T_INT, offsetof(Noddy, number), 0,
-     "noddy number"},
-    {NULL}  /* Sentinel */
-};
-
-static PyObject *
-Noddy_name(Noddy* self)
-{
-    if (self->first == NULL) {
-        PyErr_SetString(PyExc_AttributeError, "first");
-        return NULL;
-    }
-
-    if (self->last == NULL) {
-        PyErr_SetString(PyExc_AttributeError, "last");
-        return NULL;
-    }
-
-    return PyUnicode_FromFormat("%S %S", self->first, self->last);
-}
-
-static PyMethodDef Noddy_methods[] = {
-    {"name", (PyCFunction)Noddy_name, METH_NOARGS,
-     "Return the name, combining the first and last name"
-    },
-    {NULL}  /* Sentinel */
-};
-
-static PyTypeObject NoddyType = {
-    PyVarObject_HEAD_INIT(NULL, 0)
-    "noddy.Noddy",             /* tp_name */
-    sizeof(Noddy),             /* tp_basicsize */
-    0,                         /* tp_itemsize */
-    (destructor)Noddy_dealloc, /* tp_dealloc */
-    0,                         /* tp_print */
-    0,                         /* tp_getattr */
-    0,                         /* tp_setattr */
-    0,                         /* tp_reserved */
-    0,                         /* tp_repr */
-    0,                         /* tp_as_number */
-    0,                         /* tp_as_sequence */
-    0,                         /* tp_as_mapping */
-    0,                         /* tp_hash  */
-    0,                         /* tp_call */
-    0,                         /* tp_str */
-    0,                         /* tp_getattro */
-    0,                         /* tp_setattro */
-    0,                         /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT |
-        Py_TPFLAGS_BASETYPE,   /* tp_flags */
-    "Noddy objects",           /* tp_doc */
-    0,                         /* tp_traverse */
-    0,                         /* tp_clear */
-    0,                         /* tp_richcompare */
-    0,                         /* tp_weaklistoffset */
-    0,                         /* tp_iter */
-    0,                         /* tp_iternext */
-    Noddy_methods,             /* tp_methods */
-    Noddy_members,             /* tp_members */
-    0,                         /* tp_getset */
-    0,                         /* tp_base */
-    0,                         /* tp_dict */
-    0,                         /* tp_descr_get */
-    0,                         /* tp_descr_set */
-    0,                         /* tp_dictoffset */
-    (initproc)Noddy_init,      /* tp_init */
-    0,                         /* tp_alloc */
-    Noddy_new,                 /* tp_new */
-};
-
-static PyModuleDef noddy2module = {
-    PyModuleDef_HEAD_INIT,
-    "noddy2",
-    "Example module that creates an extension type.",
-    -1,
-    NULL, NULL, NULL, NULL, NULL
-};
-
-PyMODINIT_FUNC
-PyInit_noddy2(void)
-{
-    PyObject* m;
-
-    if (PyType_Ready(&NoddyType) < 0)
-        return NULL;
-
-    m = PyModule_Create(&noddy2module);
-    if (m == NULL)
-        return NULL;
-
-    Py_INCREF(&NoddyType);
-    PyModule_AddObject(m, "Noddy", (PyObject *)&NoddyType);
-    return m;
-}
diff --git a/Doc/includes/noddy3.c b/Doc/includes/noddy3.c
deleted file mode 100644
index 8a5a753..0000000
--- a/Doc/includes/noddy3.c
+++ /dev/null
@@ -1,225 +0,0 @@
-#include <Python.h>
-#include "structmember.h"
-
-typedef struct {
-    PyObject_HEAD
-    PyObject *first;
-    PyObject *last;
-    int number;
-} Noddy;
-
-static void
-Noddy_dealloc(Noddy* self)
-{
-    Py_XDECREF(self->first);
-    Py_XDECREF(self->last);
-    Py_TYPE(self)->tp_free((PyObject*)self);
-}
-
-static PyObject *
-Noddy_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
-{
-    Noddy *self;
-
-    self = (Noddy *)type->tp_alloc(type, 0);
-    if (self != NULL) {
-        self->first = PyUnicode_FromString("");
-        if (self->first == NULL) {
-            Py_DECREF(self);
-            return NULL;
-        }
-
-        self->last = PyUnicode_FromString("");
-        if (self->last == NULL) {
-            Py_DECREF(self);
-            return NULL;
-        }
-
-        self->number = 0;
-    }
-
-    return (PyObject *)self;
-}
-
-static int
-Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
-{
-    PyObject *first=NULL, *last=NULL, *tmp;
-
-    static char *kwlist[] = {"first", "last", "number", NULL};
-
-    if (! PyArg_ParseTupleAndKeywords(args, kwds, "|SSi", kwlist,
-                                      &first, &last,
-                                      &self->number))
-        return -1;
-
-    if (first) {
-        tmp = self->first;
-        Py_INCREF(first);
-        self->first = first;
-        Py_DECREF(tmp);
-    }
-
-    if (last) {
-        tmp = self->last;
-        Py_INCREF(last);
-        self->last = last;
-        Py_DECREF(tmp);
-    }
-
-    return 0;
-}
-
-static PyMemberDef Noddy_members[] = {
-    {"number", T_INT, offsetof(Noddy, number), 0,
-     "noddy number"},
-    {NULL}  /* Sentinel */
-};
-
-static PyObject *
-Noddy_getfirst(Noddy *self, void *closure)
-{
-    Py_INCREF(self->first);
-    return self->first;
-}
-
-static int
-Noddy_setfirst(Noddy *self, PyObject *value, void *closure)
-{
-    if (value == NULL) {
-        PyErr_SetString(PyExc_TypeError, "Cannot delete the first attribute");
-        return -1;
-    }
-
-    if (! PyUnicode_Check(value)) {
-        PyErr_SetString(PyExc_TypeError,
-                        "The first attribute value must be a string");
-        return -1;
-    }
-
-    Py_DECREF(self->first);
-    Py_INCREF(value);
-    self->first = value;
-
-    return 0;
-}
-
-static PyObject *
-Noddy_getlast(Noddy *self, void *closure)
-{
-    Py_INCREF(self->last);
-    return self->last;
-}
-
-static int
-Noddy_setlast(Noddy *self, PyObject *value, void *closure)
-{
-    if (value == NULL) {
-        PyErr_SetString(PyExc_TypeError, "Cannot delete the last attribute");
-        return -1;
-    }
-
-    if (! PyUnicode_Check(value)) {
-        PyErr_SetString(PyExc_TypeError,
-                        "The last attribute value must be a string");
-        return -1;
-    }
-
-    Py_DECREF(self->last);
-    Py_INCREF(value);
-    self->last = value;
-
-    return 0;
-}
-
-static PyGetSetDef Noddy_getseters[] = {
-    {"first",
-     (getter)Noddy_getfirst, (setter)Noddy_setfirst,
-     "first name",
-     NULL},
-    {"last",
-     (getter)Noddy_getlast, (setter)Noddy_setlast,
-     "last name",
-     NULL},
-    {NULL}  /* Sentinel */
-};
-
-static PyObject *
-Noddy_name(Noddy* self)
-{
-    return PyUnicode_FromFormat("%S %S", self->first, self->last);
-}
-
-static PyMethodDef Noddy_methods[] = {
-    {"name", (PyCFunction)Noddy_name, METH_NOARGS,
-     "Return the name, combining the first and last name"
-    },
-    {NULL}  /* Sentinel */
-};
-
-static PyTypeObject NoddyType = {
-    PyVarObject_HEAD_INIT(NULL, 0)
-    "noddy.Noddy",             /* tp_name */
-    sizeof(Noddy),             /* tp_basicsize */
-    0,                         /* tp_itemsize */
-    (destructor)Noddy_dealloc, /* tp_dealloc */
-    0,                         /* tp_print */
-    0,                         /* tp_getattr */
-    0,                         /* tp_setattr */
-    0,                         /* tp_reserved */
-    0,                         /* tp_repr */
-    0,                         /* tp_as_number */
-    0,                         /* tp_as_sequence */
-    0,                         /* tp_as_mapping */
-    0,                         /* tp_hash  */
-    0,                         /* tp_call */
-    0,                         /* tp_str */
-    0,                         /* tp_getattro */
-    0,                         /* tp_setattro */
-    0,                         /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT |
-        Py_TPFLAGS_BASETYPE,   /* tp_flags */
-    "Noddy objects",           /* tp_doc */
-    0,                         /* tp_traverse */
-    0,                         /* tp_clear */
-    0,                         /* tp_richcompare */
-    0,                         /* tp_weaklistoffset */
-    0,                         /* tp_iter */
-    0,                         /* tp_iternext */
-    Noddy_methods,             /* tp_methods */
-    Noddy_members,             /* tp_members */
-    Noddy_getseters,           /* tp_getset */
-    0,                         /* tp_base */
-    0,                         /* tp_dict */
-    0,                         /* tp_descr_get */
-    0,                         /* tp_descr_set */
-    0,                         /* tp_dictoffset */
-    (initproc)Noddy_init,      /* tp_init */
-    0,                         /* tp_alloc */
-    Noddy_new,                 /* tp_new */
-};
-
-static PyModuleDef noddy3module = {
-    PyModuleDef_HEAD_INIT,
-    "noddy3",
-    "Example module that creates an extension type.",
-    -1,
-    NULL, NULL, NULL, NULL, NULL
-};
-
-PyMODINIT_FUNC
-PyInit_noddy3(void)
-{
-    PyObject* m;
-
-    if (PyType_Ready(&NoddyType) < 0)
-        return NULL;
-
-    m = PyModule_Create(&noddy3module);
-    if (m == NULL)
-        return NULL;
-
-    Py_INCREF(&NoddyType);
-    PyModule_AddObject(m, "Noddy", (PyObject *)&NoddyType);
-    return m;
-}
diff --git a/Doc/includes/noddy4.c b/Doc/includes/noddy4.c
deleted file mode 100644
index 08ba4c3..0000000
--- a/Doc/includes/noddy4.c
+++ /dev/null
@@ -1,208 +0,0 @@
-#include <Python.h>
-#include "structmember.h"
-
-typedef struct {
-    PyObject_HEAD
-    PyObject *first;
-    PyObject *last;
-    int number;
-} Noddy;
-
-static int
-Noddy_traverse(Noddy *self, visitproc visit, void *arg)
-{
-    int vret;
-
-    if (self->first) {
-        vret = visit(self->first, arg);
-        if (vret != 0)
-            return vret;
-    }
-    if (self->last) {
-        vret = visit(self->last, arg);
-        if (vret != 0)
-            return vret;
-    }
-
-    return 0;
-}
-
-static int
-Noddy_clear(Noddy *self)
-{
-    PyObject *tmp;
-
-    tmp = self->first;
-    self->first = NULL;
-    Py_XDECREF(tmp);
-
-    tmp = self->last;
-    self->last = NULL;
-    Py_XDECREF(tmp);
-
-    return 0;
-}
-
-static void
-Noddy_dealloc(Noddy* self)
-{
-    PyObject_GC_UnTrack(self);
-    Noddy_clear(self);
-    Py_TYPE(self)->tp_free((PyObject*)self);
-}
-
-static PyObject *
-Noddy_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
-{
-    Noddy *self;
-
-    self = (Noddy *)type->tp_alloc(type, 0);
-    if (self != NULL) {
-        self->first = PyUnicode_FromString("");
-        if (self->first == NULL) {
-            Py_DECREF(self);
-            return NULL;
-        }
-
-        self->last = PyUnicode_FromString("");
-        if (self->last == NULL) {
-            Py_DECREF(self);
-            return NULL;
-        }
-
-        self->number = 0;
-    }
-
-    return (PyObject *)self;
-}
-
-static int
-Noddy_init(Noddy *self, PyObject *args, PyObject *kwds)
-{
-    PyObject *first=NULL, *last=NULL, *tmp;
-
-    static char *kwlist[] = {"first", "last", "number", NULL};
-
-    if (! PyArg_ParseTupleAndKeywords(args, kwds, "|OOi", kwlist,
-                                      &first, &last,
-                                      &self->number))
-        return -1;
-
-    if (first) {
-        tmp = self->first;
-        Py_INCREF(first);
-        self->first = first;
-        Py_XDECREF(tmp);
-    }
-
-    if (last) {
-        tmp = self->last;
-        Py_INCREF(last);
-        self->last = last;
-        Py_XDECREF(tmp);
-    }
-
-    return 0;
-}
-
-
-static PyMemberDef Noddy_members[] = {
-    {"first", T_OBJECT_EX, offsetof(Noddy, first), 0,
-     "first name"},
-    {"last", T_OBJECT_EX, offsetof(Noddy, last), 0,
-     "last name"},
-    {"number", T_INT, offsetof(Noddy, number), 0,
-     "noddy number"},
-    {NULL}  /* Sentinel */
-};
-
-static PyObject *
-Noddy_name(Noddy* self)
-{
-    if (self->first == NULL) {
-        PyErr_SetString(PyExc_AttributeError, "first");
-        return NULL;
-    }
-
-    if (self->last == NULL) {
-        PyErr_SetString(PyExc_AttributeError, "last");
-        return NULL;
-    }
-
-    return PyUnicode_FromFormat("%S %S", self->first, self->last);
-}
-
-static PyMethodDef Noddy_methods[] = {
-    {"name", (PyCFunction)Noddy_name, METH_NOARGS,
-     "Return the name, combining the first and last name"
-    },
-    {NULL}  /* Sentinel */
-};
-
-static PyTypeObject NoddyType = {
-    PyVarObject_HEAD_INIT(NULL, 0)
-    "noddy.Noddy",             /* tp_name */
-    sizeof(Noddy),             /* tp_basicsize */
-    0,                         /* tp_itemsize */
-    (destructor)Noddy_dealloc, /* tp_dealloc */
-    0,                         /* tp_print */
-    0,                         /* tp_getattr */
-    0,                         /* tp_setattr */
-    0,                         /* tp_reserved */
-    0,                         /* tp_repr */
-    0,                         /* tp_as_number */
-    0,                         /* tp_as_sequence */
-    0,                         /* tp_as_mapping */
-    0,                         /* tp_hash  */
-    0,                         /* tp_call */
-    0,                         /* tp_str */
-    0,                         /* tp_getattro */
-    0,                         /* tp_setattro */
-    0,                         /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT |
-        Py_TPFLAGS_BASETYPE |
-        Py_TPFLAGS_HAVE_GC,    /* tp_flags */
-    "Noddy objects",           /* tp_doc */
-    (traverseproc)Noddy_traverse,   /* tp_traverse */
-    (inquiry)Noddy_clear,           /* tp_clear */
-    0,                         /* tp_richcompare */
-    0,                         /* tp_weaklistoffset */
-    0,                         /* tp_iter */
-    0,                         /* tp_iternext */
-    Noddy_methods,             /* tp_methods */
-    Noddy_members,             /* tp_members */
-    0,                         /* tp_getset */
-    0,                         /* tp_base */
-    0,                         /* tp_dict */
-    0,                         /* tp_descr_get */
-    0,                         /* tp_descr_set */
-    0,                         /* tp_dictoffset */
-    (initproc)Noddy_init,      /* tp_init */
-    0,                         /* tp_alloc */
-    Noddy_new,                 /* tp_new */
-};
-
-static PyModuleDef noddy4module = {
-    PyModuleDef_HEAD_INIT,
-    "noddy4",
-    "Example module that creates an extension type.",
-    -1,
-    NULL, NULL, NULL, NULL, NULL
-};
-
-PyMODINIT_FUNC
-PyInit_noddy4(void)
-{
-    PyObject* m;
-
-    if (PyType_Ready(&NoddyType) < 0)
-        return NULL;
-
-    m = PyModule_Create(&noddy4module);
-    if (m == NULL)
-        return NULL;
-
-    Py_INCREF(&NoddyType);
-    PyModule_AddObject(m, "Noddy", (PyObject *)&NoddyType);
-    return m;
-}
diff --git a/Doc/includes/shoddy.c b/Doc/includes/shoddy.c
deleted file mode 100644
index 0ef4765..0000000
--- a/Doc/includes/shoddy.c
+++ /dev/null
@@ -1,99 +0,0 @@
-#include <Python.h>
-
-typedef struct {
-    PyListObject list;
-    int state;
-} Shoddy;
-
-
-static PyObject *
-Shoddy_increment(Shoddy *self, PyObject *unused)
-{
-    self->state++;
-    return PyLong_FromLong(self->state);
-}
-
-
-static PyMethodDef Shoddy_methods[] = {
-    {"increment", (PyCFunction)Shoddy_increment, METH_NOARGS,
-     PyDoc_STR("increment state counter")},
-    {NULL},
-};
-
-static int
-Shoddy_init(Shoddy *self, PyObject *args, PyObject *kwds)
-{
-    if (PyList_Type.tp_init((PyObject *)self, args, kwds) < 0)
-        return -1;
-    self->state = 0;
-    return 0;
-}
-
-
-static PyTypeObject ShoddyType = {
-    PyVarObject_HEAD_INIT(NULL, 0)
-    "shoddy.Shoddy",         /* tp_name */
-    sizeof(Shoddy),          /* tp_basicsize */
-    0,                       /* tp_itemsize */
-    0,                       /* tp_dealloc */
-    0,                       /* tp_print */
-    0,                       /* tp_getattr */
-    0,                       /* tp_setattr */
-    0,                       /* tp_reserved */
-    0,                       /* tp_repr */
-    0,                       /* tp_as_number */
-    0,                       /* tp_as_sequence */
-    0,                       /* tp_as_mapping */
-    0,                       /* tp_hash */
-    0,                       /* tp_call */
-    0,                       /* tp_str */
-    0,                       /* tp_getattro */
-    0,                       /* tp_setattro */
-    0,                       /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT |
-        Py_TPFLAGS_BASETYPE, /* tp_flags */
-    0,                       /* tp_doc */
-    0,                       /* tp_traverse */
-    0,                       /* tp_clear */
-    0,                       /* tp_richcompare */
-    0,                       /* tp_weaklistoffset */
-    0,                       /* tp_iter */
-    0,                       /* tp_iternext */
-    Shoddy_methods,          /* tp_methods */
-    0,                       /* tp_members */
-    0,                       /* tp_getset */
-    0,                       /* tp_base */
-    0,                       /* tp_dict */
-    0,                       /* tp_descr_get */
-    0,                       /* tp_descr_set */
-    0,                       /* tp_dictoffset */
-    (initproc)Shoddy_init,   /* tp_init */
-    0,                       /* tp_alloc */
-    0,                       /* tp_new */
-};
-
-static PyModuleDef shoddymodule = {
-    PyModuleDef_HEAD_INIT,
-    "shoddy",
-    "Shoddy module",
-    -1,
-    NULL, NULL, NULL, NULL, NULL
-};
-
-PyMODINIT_FUNC
-PyInit_shoddy(void)
-{
-    PyObject *m;
-
-    ShoddyType.tp_base = &PyList_Type;
-    if (PyType_Ready(&ShoddyType) < 0)
-        return NULL;
-
-    m = PyModule_Create(&shoddymodule);
-    if (m == NULL)
-        return NULL;
-
-    Py_INCREF(&ShoddyType);
-    PyModule_AddObject(m, "Shoddy", (PyObject *) &ShoddyType);
-    return m;
-}
diff --git a/Doc/includes/sublist.c b/Doc/includes/sublist.c
new file mode 100644
index 0000000..376dddf
--- /dev/null
+++ b/Doc/includes/sublist.c
@@ -0,0 +1,63 @@
+#include <Python.h>
+
+typedef struct {
+    PyListObject list;
+    int state;
+} SubListObject;
+
+static PyObject *
+SubList_increment(SubListObject *self, PyObject *unused)
+{
+    self->state++;
+    return PyLong_FromLong(self->state);
+}
+
+static PyMethodDef SubList_methods[] = {
+    {"increment", (PyCFunction) SubList_increment, METH_NOARGS,
+     PyDoc_STR("increment state counter")},
+    {NULL},
+};
+
+static int
+SubList_init(SubListObject *self, PyObject *args, PyObject *kwds)
+{
+    if (PyList_Type.tp_init((PyObject *) self, args, kwds) < 0)
+        return -1;
+    self->state = 0;
+    return 0;
+}
+
+static PyTypeObject SubListType = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "sublist.SubList",
+    .tp_doc = "SubList objects",
+    .tp_basicsize = sizeof(SubListObject),
+    .tp_itemsize = 0,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .tp_init = (initproc) SubList_init,
+    .tp_methods = SubList_methods,
+};
+
+static PyModuleDef sublistmodule = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "sublist",
+    .m_doc = "Example module that creates an extension type.",
+    .m_size = -1,
+};
+
+PyMODINIT_FUNC
+PyInit_sublist(void)
+{
+    PyObject *m;
+    SubListType.tp_base = &PyList_Type;
+    if (PyType_Ready(&SubListType) < 0)
+        return NULL;
+
+    m = PyModule_Create(&sublistmodule);
+    if (m == NULL)
+        return NULL;
+
+    Py_INCREF(&SubListType);
+    PyModule_AddObject(m, "SubList", (PyObject *) &SubListType);
+    return m;
+}
diff --git a/Doc/includes/test.py b/Doc/includes/test.py
index 9e9d4a6..09ebe3f 100644
--- a/Doc/includes/test.py
+++ b/Doc/includes/test.py
@@ -1,181 +1,168 @@
-"""Test module for the noddy examples
+"""Test module for the custom examples
 
-Noddy 1:
+Custom 1:
 
->>> import noddy
->>> n1 = noddy.Noddy()
->>> n2 = noddy.Noddy()
->>> del n1
->>> del n2
+>>> import custom
+>>> c1 = custom.Custom()
+>>> c2 = custom.Custom()
+>>> del c1
+>>> del c2
 
 
-Noddy 2
+Custom 2
 
->>> import noddy2
->>> n1 = noddy2.Noddy('jim', 'fulton', 42)
->>> n1.first
+>>> import custom2
+>>> c1 = custom2.Custom('jim', 'fulton', 42)
+>>> c1.first
 'jim'
->>> n1.last
+>>> c1.last
 'fulton'
->>> n1.number
+>>> c1.number
 42
->>> n1.name()
+>>> c1.name()
 'jim fulton'
->>> n1.first = 'will'
->>> n1.name()
+>>> c1.first = 'will'
+>>> c1.name()
 'will fulton'
->>> n1.last = 'tell'
->>> n1.name()
+>>> c1.last = 'tell'
+>>> c1.name()
 'will tell'
->>> del n1.first
->>> n1.name()
+>>> del c1.first
+>>> c1.name()
 Traceback (most recent call last):
 ...
 AttributeError: first
->>> n1.first
+>>> c1.first
 Traceback (most recent call last):
 ...
 AttributeError: first
->>> n1.first = 'drew'
->>> n1.first
+>>> c1.first = 'drew'
+>>> c1.first
 'drew'
->>> del n1.number
+>>> del c1.number
 Traceback (most recent call last):
 ...
 TypeError: can't delete numeric/char attribute
->>> n1.number=2
->>> n1.number
+>>> c1.number=2
+>>> c1.number
 2
->>> n1.first = 42
->>> n1.name()
+>>> c1.first = 42
+>>> c1.name()
 '42 tell'
->>> n2 = noddy2.Noddy()
->>> n2.name()
+>>> c2 = custom2.Custom()
+>>> c2.name()
 ' '
->>> n2.first
+>>> c2.first
 ''
->>> n2.last
+>>> c2.last
 ''
->>> del n2.first
->>> n2.first
+>>> del c2.first
+>>> c2.first
 Traceback (most recent call last):
 ...
 AttributeError: first
->>> n2.first
+>>> c2.first
 Traceback (most recent call last):
 ...
 AttributeError: first
->>> n2.name()
+>>> c2.name()
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
 AttributeError: first
->>> n2.number
+>>> c2.number
 0
->>> n3 = noddy2.Noddy('jim', 'fulton', 'waaa')
+>>> n3 = custom2.Custom('jim', 'fulton', 'waaa')
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
-TypeError: an integer is required
->>> del n1
->>> del n2
+TypeError: an integer is required (got type str)
+>>> del c1
+>>> del c2
 
 
-Noddy 3
+Custom 3
 
->>> import noddy3
->>> n1 = noddy3.Noddy('jim', 'fulton', 42)
->>> n1 = noddy3.Noddy('jim', 'fulton', 42)
->>> n1.name()
+>>> import custom3
+>>> c1 = custom3.Custom('jim', 'fulton', 42)
+>>> c1 = custom3.Custom('jim', 'fulton', 42)
+>>> c1.name()
 'jim fulton'
->>> del n1.first
+>>> del c1.first
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
 TypeError: Cannot delete the first attribute
->>> n1.first = 42
+>>> c1.first = 42
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
 TypeError: The first attribute value must be a string
->>> n1.first = 'will'
->>> n1.name()
+>>> c1.first = 'will'
+>>> c1.name()
 'will fulton'
->>> n2 = noddy3.Noddy()
->>> n2 = noddy3.Noddy()
->>> n2 = noddy3.Noddy()
->>> n3 = noddy3.Noddy('jim', 'fulton', 'waaa')
+>>> c2 = custom3.Custom()
+>>> c2 = custom3.Custom()
+>>> c2 = custom3.Custom()
+>>> n3 = custom3.Custom('jim', 'fulton', 'waaa')
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
-TypeError: an integer is required
->>> del n1
->>> del n2
+TypeError: an integer is required (got type str)
+>>> del c1
+>>> del c2
 
-Noddy 4
+Custom 4
 
->>> import noddy4
->>> n1 = noddy4.Noddy('jim', 'fulton', 42)
->>> n1.first
+>>> import custom4
+>>> c1 = custom4.Custom('jim', 'fulton', 42)
+>>> c1.first
 'jim'
->>> n1.last
+>>> c1.last
 'fulton'
->>> n1.number
+>>> c1.number
 42
->>> n1.name()
+>>> c1.name()
 'jim fulton'
->>> n1.first = 'will'
->>> n1.name()
+>>> c1.first = 'will'
+>>> c1.name()
 'will fulton'
->>> n1.last = 'tell'
->>> n1.name()
+>>> c1.last = 'tell'
+>>> c1.name()
 'will tell'
->>> del n1.first
->>> n1.name()
+>>> del c1.first
 Traceback (most recent call last):
 ...
-AttributeError: first
->>> n1.first
-Traceback (most recent call last):
-...
-AttributeError: first
->>> n1.first = 'drew'
->>> n1.first
+TypeError: Cannot delete the first attribute
+>>> c1.name()
+'will tell'
+>>> c1.first = 'drew'
+>>> c1.first
 'drew'
->>> del n1.number
+>>> del c1.number
 Traceback (most recent call last):
 ...
 TypeError: can't delete numeric/char attribute
->>> n1.number=2
->>> n1.number
+>>> c1.number=2
+>>> c1.number
 2
->>> n1.first = 42
->>> n1.name()
-'42 tell'
->>> n2 = noddy4.Noddy()
->>> n2 = noddy4.Noddy()
->>> n2 = noddy4.Noddy()
->>> n2 = noddy4.Noddy()
->>> n2.name()
+>>> c1.first = 42
+Traceback (most recent call last):
+...
+TypeError: The first attribute value must be a string
+>>> c1.name()
+'drew tell'
+>>> c2 = custom4.Custom()
+>>> c2 = custom4.Custom()
+>>> c2 = custom4.Custom()
+>>> c2 = custom4.Custom()
+>>> c2.name()
 ' '
->>> n2.first
+>>> c2.first
 ''
->>> n2.last
+>>> c2.last
 ''
->>> del n2.first
->>> n2.first
-Traceback (most recent call last):
-...
-AttributeError: first
->>> n2.first
-Traceback (most recent call last):
-...
-AttributeError: first
->>> n2.name()
-Traceback (most recent call last):
-  File "<stdin>", line 1, in ?
-AttributeError: first
->>> n2.number
+>>> c2.number
 0
->>> n3 = noddy4.Noddy('jim', 'fulton', 'waaa')
+>>> n3 = custom4.Custom('jim', 'fulton', 'waaa')
 Traceback (most recent call last):
-  File "<stdin>", line 1, in ?
-TypeError: an integer is required
+...
+TypeError: an integer is required (got type str)
 
 
 Test cyclic gc(?)
@@ -183,15 +170,14 @@ Test cyclic gc(?)
 >>> import gc
 >>> gc.disable()
 
->>> x = []
->>> l = [x]
->>> n2.first = l
->>> n2.first
-[[]]
->>> l.append(n2)
->>> del l
->>> del n1
->>> del n2
+>>> class Subclass(custom4.Custom): pass
+...
+>>> s = Subclass()
+>>> s.cycle = [s]
+>>> s.cycle.append(s.cycle)
+>>> x = object()
+>>> s.x = x
+>>> del s
 >>> sys.getrefcount(x)
 3
 >>> ignore = gc.collect()
diff --git a/Misc/NEWS.d/next/Documentation/2018-04-01-21-03-41.bpo-33201.aa8Lkl.rst b/Misc/NEWS.d/next/Documentation/2018-04-01-21-03-41.bpo-33201.aa8Lkl.rst
new file mode 100644
index 0000000..bdee48b
--- /dev/null
+++ b/Misc/NEWS.d/next/Documentation/2018-04-01-21-03-41.bpo-33201.aa8Lkl.rst
@@ -0,0 +1 @@
+Modernize documentation for writing C extension types.