.. highlight:: c .. _common-structs: Common Object Structures ======================== There are a large number of structures which are used in the definition of object types for Python. This section describes these structures and how they are used. Base object types and macros ---------------------------- All Python objects ultimately share a small number of fields at the beginning of the object's representation in memory. These are represented by the :c:type:`PyObject` and :c:type:`PyVarObject` types, which are defined, in turn, by the expansions of some macros also used, whether directly or indirectly, in the definition of all other Python objects. .. c:type:: PyObject All object types are extensions of this type. This is a type which contains the information Python needs to treat a pointer to an object as an object. In a normal "release" build, it contains only the object's reference count and a pointer to the corresponding type object. Nothing is actually declared to be a :c:type:`PyObject`, but every pointer to a Python object can be cast to a :c:type:`PyObject*`. Access to the members must be done by using the macros :c:macro:`Py_REFCNT` and :c:macro:`Py_TYPE`. .. c:type:: PyVarObject This is an extension of :c:type:`PyObject` that adds the :attr:`ob_size` field. This is only used for objects that have some notion of *length*. This type does not often appear in the Python/C API. Access to the members must be done by using the macros :c:macro:`Py_REFCNT`, :c:macro:`Py_TYPE`, and :c:macro:`Py_SIZE`. .. c:macro:: PyObject_HEAD This is a macro used when declaring new types which represent objects without a varying length. The PyObject_HEAD macro expands to:: PyObject ob_base; See documentation of :c:type:`PyObject` above. .. c:macro:: PyObject_VAR_HEAD This is a macro used when declaring new types which represent objects with a length that varies from instance to instance. The PyObject_VAR_HEAD macro expands to:: PyVarObject ob_base; See documentation of :c:type:`PyVarObject` above. .. c:macro:: Py_TYPE(o) This macro is used to access the :attr:`ob_type` member of a Python object. It expands to:: (((PyObject*)(o))->ob_type) .. c:function:: int Py_IS_TYPE(PyObject *o, PyTypeObject *type) Return non-zero if the object *o* type is *type*. Return zero otherwise. Equivalent to: ``Py_TYPE(o) == type``. .. versionadded:: 3.9 .. c:function:: void Py_SET_TYPE(PyObject *o, PyTypeObject *type) Set the object *o* type to *type*. .. versionadded:: 3.9 .. c:macro:: Py_REFCNT(o) This macro is used to access the :attr:`ob_refcnt` member of a Python object. It expands to:: (((PyObject*)(o))->ob_refcnt) .. c:function:: void Py_SET_REFCNT(PyObject *o, Py_ssize_t refcnt) Set the object *o* reference counter to *refcnt*. .. versionadded:: 3.9 .. c:macro:: Py_SIZE(o) This macro is used to access the :attr:`ob_size` member of a Python object. It expands to:: (((PyVarObject*)(o))->ob_size) .. c:function:: void Py_SET_SIZE(PyVarObject *o, Py_ssize_t size) Set the object *o* size to *size*. .. versionadded:: 3.9 .. c:macro:: PyObject_HEAD_INIT(type) This is a macro which expands to initialization values for a new :c:type:`PyObject` type. This macro expands to:: _PyObject_EXTRA_INIT 1, type, .. c:macro:: PyVarObject_HEAD_INIT(type, size) This is a macro which expands to initialization values for a new :c:type:`PyVarObject` type, including the :attr:`ob_size` field. This macro expands to:: _PyObject_EXTRA_INIT 1, type, size, Implementing functions and methods ---------------------------------- .. c:type:: PyCFunction Type of the functions used to implement most Python callables in C. Functions of this type take two :c:type:`PyObject*` parameters and return one such value. If the return value is ``NULL``, an exception shall have been set. If not ``NULL``, the return value is interpreted as the return value of the function as exposed in Python. The function must return a new reference. The function signature is:: PyObject *PyCFunction(PyObject *self, PyObject *args); .. c:type:: PyCFunctionWithKeywords Type of the functions used to implement Python callables in C with signature :const:`METH_VARARGS | METH_KEYWORDS`. The function signature is:: PyObject *PyCFunctionWithKeywords(PyObject *self, PyObject *args, PyObject *kwargs); .. c:type:: _PyCFunctionFast Type of the functions used to implement Python callables in C with signature :const:`METH_FASTCALL`. The function signature is:: PyObject *_PyCFunctionFast(PyObject *self, PyObject *const *args, Py_ssize_t nargs); .. c:type:: _PyCFunctionFastWithKeywords Type of the functions used to implement Python callables in C with signature :const:`METH_FASTCALL | METH_KEYWORDS`. The function signature is:: PyObject *_PyCFunctionFastWithKeywords(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames); .. c:type:: PyCMethod Type of the functions used to implement Python callables in C with signature :const:`METH_METHOD | METH_FASTCALL | METH_KEYWORDS`. The function signature is:: PyObject *PyCMethod(PyObject *self, PyTypeObject *defining_class, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames) .. versionadded:: 3.9 .. c:type:: PyMethodDef Structure used to describe a method of an extension type. This structure has four fields: +------------------+---------------+-------------------------------+ | Field | C Type | Meaning | +==================+===============+===============================+ | :attr:`ml_name` | const char \* | name of the method | +------------------+---------------+-------------------------------+ | :attr:`ml_meth` | PyCFunction | pointer to the C | | | | implementation | +------------------+---------------+-------------------------------+ | :attr:`ml_flags` | int | flag bits indicating how the | | | | call should be constructed | +------------------+---------------+-------------------------------+ | :attr:`ml_doc` | const char \* | points to the contents of the | | | | docstring | +------------------+---------------+-------------------------------+ The :attr:`ml_meth` is a C function pointer. The functions may be of different types, but they always return :c:type:`PyObject*`. If the function is not of the :c:type:`PyCFunction`, the compiler will require a cast in the method table. Even though :c:type:`PyCFunction` defines the first parameter as :c:type:`PyObject*`, it is common that the method implementation uses the specific C type of the *self* object. The :attr:`ml_flags` field is a bitfield which can include the following flags. The individual flags indicate either a calling convention or a binding convention. There are these calling conventions: .. data:: METH_VARARGS This is the typical calling convention, where the methods have the type :c:type:`PyCFunction`. The function expects two :c:type:`PyObject*` values. The first one is the *self* object for methods; for module functions, it is the module object. The second parameter (often called *args*) is a tuple object representing all arguments. This parameter is typically processed using :c:func:`PyArg_ParseTuple` or :c:func:`PyArg_UnpackTuple`. .. data:: METH_VARARGS | METH_KEYWORDS Methods with these flags must be of type :c:type:`PyCFunctionWithKeywords`. The function expects three parameters: *self*, *args*, *kwargs* where *kwargs* is a dictionary of all the keyword arguments or possibly ``NULL`` if there are no keyword arguments. The parameters are typically processed using :c:func:`PyArg_ParseTupleAndKeywords`. .. data:: METH_FASTCALL Fast calling convention supporting only positional arguments. The methods have the type :c:type:`_PyCFunctionFast`. The first parameter is *self*, the second parameter is a C array of :c:type:`PyObject*` values indicating the arguments and the third parameter is the number of arguments (the length of the array). This is not part of the :ref:`limited API `. .. versionadded:: 3.7 .. data:: METH_FASTCALL | METH_KEYWORDS Extension of :const:`METH_FASTCALL` supporting also keyword arguments, with methods of type :c:type:`_PyCFunctionFastWithKeywords`. Keyword arguments are passed the same way as in the :ref:`vectorcall protocol `: there is an additional fourth :c:type:`PyObject*` parameter which is a tuple representing the names of the keyword arguments (which are guaranteed to be strings) or possibly ``NULL`` if there are no keywords. The values of the keyword arguments are stored in the *args* array, after the positional arguments. This is not part of the :ref:`limited API `. .. versionadded:: 3.7 .. data:: METH_METHOD | METH_FASTCALL | METH_KEYWORDS Extension of :const:`METH_FASTCALL | METH_KEYWORDS` supporting the *defining class*, that is, the class that contains the method in question. The defining class might be a superclass of ``Py_TYPE(self)``. The method needs to be of type :c:type:`PyCMethod`, the same as for ``METH_FASTCALL | METH_KEYWORDS`` with ``defining_class`` argument added after ``self``. .. versionadded:: 3.9 .. data:: METH_NOARGS Methods without parameters don't need to check whether arguments are given if they are listed with the :const:`METH_NOARGS` flag. They need to be of type :c:type:`PyCFunction`. The first parameter is typically named *self* and will hold a reference to the module or object instance. In all cases the second parameter will be ``NULL``. .. data:: METH_O Methods with a single object argument can be listed with the :const:`METH_O` flag, instead of invoking :c:func:`PyArg_ParseTuple` with a ``"O"`` argument. They have the type :c:type:`PyCFunction`, with the *self* parameter, and a :c:type:`PyObject*` parameter representing the single argument. These two constants are not used to indicate the calling convention but the binding when use with methods of classes. These may not be used for functions defined for modules. At most one of these flags may be set for any given method. .. data:: METH_CLASS .. index:: builtin: classmethod The method will be passed the type object as the first parameter rather than an instance of the type. This is used to create *class methods*, similar to what is created when using the :func:`classmethod` built-in function. .. data:: METH_STATIC .. index:: builtin: staticmethod The method will be passed ``NULL`` as the first parameter rather than an instance of the type. This is used to create *static methods*, similar to what is created when using the :func:`staticmethod` built-in function. One other constant controls whether a method is loaded in place of another definition with the same method name. .. data:: METH_COEXIST The method will be loaded in place of existing definitions. Without *METH_COEXIST*, the default is to skip repeated definitions. Since slot wrappers are loaded before the method table, the existence of a *sq_contains* slot, for example, would generate a wrapped method named :meth:`__contains__` and preclude the loading of a corresponding PyCFunction with the same name. With the flag defined, the PyCFunction will be loaded in place of the wrapper object and will co-exist with the slot. This is helpful because calls to PyCFunctions are optimized more than wrapper object calls. Accessing attributes of extension types --------------------------------------- .. c:type:: PyMemberDef Structure which describes an attribute of a type which corresponds to a C struct member. Its fields are: +------------------+---------------+-------------------------------+ | Field | C Type | Meaning | +==================+===============+===============================+ | :attr:`name` | const char \* | name of the member | +------------------+---------------+-------------------------------+ | :attr:`!type` | int | the type of the member in the | | | | C struct | +------------------+---------------+-------------------------------+ | :attr:`offset` | Py_ssize_t | the offset in bytes that the | | | | member is located on the | | | | type's object struct | +------------------+---------------+-------------------------------+ | :attr:`flags` | int | flag bits indicating if the | | | | field should be read-only or | | | | writable | +------------------+---------------+-------------------------------+ | :attr:`doc` | const char \* | points to the contents of the | | | | docstring | +------------------+---------------+-------------------------------+ :attr:`!type` can be one of many ``T_`` macros corresponding to various C types. When the member is accessed in Python, it will be converted to the equivalent Python type. =============== ================== Macro name C type =============== ================== T_SHORT short T_INT int T_LONG long T_FLOAT float T_DOUBLE double T_STRING const char \* T_OBJECT PyObject \* T_OBJECT_EX PyObject \* T_CHAR char T_BYTE char T_UBYTE unsigned char T_UINT unsigned int T_USHORT unsigned short T_ULONG unsigned long T_BOOL char T_LONGLONG long long T_ULONGLONG unsigned long long T_PYSSIZET Py_ssize_t =============== ================== :c:macro:`T_OBJECT` and :c:macro:`T_OBJECT_EX` differ in that :c:macro:`T_OBJECT` returns ``None`` if the member is ``NULL`` and :c:macro:`T_OBJECT_EX` raises an :exc:`AttributeError`. Try to use :c:macro:`T_OBJECT_EX` over :c:macro:`T_OBJECT` because :c:macro:`T_OBJECT_EX` handles use of the :keyword:`del` statement on that attribute more correctly than :c:macro:`T_OBJECT`. :attr:`flags` can be ``0`` for write and read access or :c:macro:`READONLY` for read-only access. Using :c:macro:`T_STRING` for :attr:`type` implies :c:macro:`READONLY`. :c:macro:`T_STRING` data is interpreted as UTF-8. Only :c:macro:`T_OBJECT` and :c:macro:`T_OBJECT_EX` members can be deleted. (They are set to ``NULL``). .. _pymemberdef-offsets: Heap allocated types (created using :c:func:`PyType_FromSpec` or similar), ``PyMemberDef`` may contain definitions for the special members ``__dictoffset__``, ``__weaklistoffset__`` and ``__vectorcalloffset__``, corresponding to :c:member:`~PyTypeObject.tp_dictoffset`, :c:member:`~PyTypeObject.tp_weaklistoffset` and :c:member:`~PyTypeObject.tp_vectorcall_offset` in type objects. These must be defined with ``T_PYSSIZET`` and ``READONLY``, for example:: static PyMemberDef spam_type_members[] = { {"__dictoffset__", T_PYSSIZET, offsetof(Spam_object, dict), READONLY}, {NULL} /* Sentinel */ }; .. c:function:: PyObject* PyMember_GetOne(const char *obj_addr, struct PyMemberDef *m) Get an attribute belonging to the object at address *obj_addr*. The attribute is described by ``PyMemberDef`` *m*. Returns ``NULL`` on error. .. c:function:: int PyMember_SetOne(char *obj_addr, struct PyMemberDef *m, PyObject *o) Set an attribute belonging to the object at address *obj_addr* to object *o*. The attribute to set is described by ``PyMemberDef`` *m*. Returns ``0`` if successful and a negative value on failure. .. c:type:: PyGetSetDef Structure to define property-like access for a type. See also description of the :c:member:`PyTypeObject.tp_getset` slot. +-------------+------------------+-----------------------------------+ | Field | C Type | Meaning | +=============+==================+===================================+ | name | const char \* | attribute name | +-------------+------------------+-----------------------------------+ | get | getter | C Function to get the attribute | +-------------+------------------+-----------------------------------+ | set | setter | optional C function to set or | | | | delete the attribute, if omitted | | | | the attribute is readonly | +-------------+------------------+-----------------------------------+ | doc | const char \* | optional docstring | +-------------+------------------+-----------------------------------+ | closure | void \* | optional function pointer, | | | | providing additional data for | | | | getter and setter | +-------------+------------------+-----------------------------------+ The ``get`` function takes one :c:type:`PyObject*` parameter (the instance) and a function pointer (the associated ``closure``):: typedef PyObject *(*getter)(PyObject *, void *); It should return a new reference on success or ``NULL`` with a set exception on failure. ``set`` functions take two :c:type:`PyObject*` parameters (the instance and the value to be set) and a function pointer (the associated ``closure``):: typedef int (*setter)(PyObject *, PyObject *, void *); In case the attribute should be deleted the second parameter is ``NULL``. Should return ``0`` on success or ``-1`` with a set exception on failure.