path: root/Doc/c-api/module.rst

.. highlight:: c

.. _moduleobjects:

Module Objects
--------------

.. index:: pair: object; module


.. c:var:: PyTypeObject PyModule_Type

   .. index:: single: ModuleType (in module types)

   This instance of :c:type:`PyTypeObject` represents the Python module type.  This
   is exposed to Python programs as ``types.ModuleType``.


.. c:function:: int PyModule_Check(PyObject *p)

   Return true if *p* is a module object, or a subtype of a module object.
   This function always succeeds.


.. c:function:: int PyModule_CheckExact(PyObject *p)

   Return true if *p* is a module object, but not a subtype of
   :c:data:`PyModule_Type`.  This function always succeeds.


.. c:function:: PyObject* PyModule_NewObject(PyObject *name)

   .. index::
      single: __name__ (module attribute)
      single: __doc__ (module attribute)
      single: __file__ (module attribute)
      single: __package__ (module attribute)
      single: __loader__ (module attribute)

   Return a new module object with the :attr:`__name__` attribute set to *name*.
   The module's :attr:`__name__`, :attr:`__doc__`, :attr:`__package__`, and
   :attr:`__loader__` attributes are filled in (all but :attr:`__name__` are set
   to ``None``); the caller is responsible for providing a :attr:`__file__`
   attribute.

   .. versionadded:: 3.3

   .. versionchanged:: 3.4
      :attr:`__package__` and :attr:`__loader__` are set to ``None``.


.. c:function:: PyObject* PyModule_New(const char *name)

   Similar to :c:func:`PyModule_NewObject`, but the name is a UTF-8 encoded
   string instead of a Unicode object.


.. c:function:: PyObject* PyModule_GetDict(PyObject *module)

   .. index:: single: __dict__ (module attribute)

   Return the dictionary object that implements *module*'s namespace; this object
   is the same as the :attr:`~object.__dict__` attribute of the module object.
   If *module* is not a module object (or a subtype of a module object),
   :exc:`SystemError` is raised and ``NULL`` is returned.

   It is recommended extensions use other ``PyModule_*`` and
   ``PyObject_*`` functions rather than directly manipulate a module's
   :attr:`~object.__dict__`.


.. c:function:: PyObject* PyModule_GetNameObject(PyObject *module)

   .. index::
      single: __name__ (module attribute)
      single: SystemError (built-in exception)

   Return *module*'s :attr:`__name__` value.  If the module does not provide one,
   or if it is not a string, :exc:`SystemError` is raised and ``NULL`` is returned.

   .. versionadded:: 3.3


.. c:function:: const char* PyModule_GetName(PyObject *module)

   Similar to :c:func:`PyModule_GetNameObject` but return the name encoded to
   ``'utf-8'``.

.. c:function:: void* PyModule_GetState(PyObject *module)

   Return the "state" of the module, that is, a pointer to the block of memory
   allocated at module creation time, or ``NULL``.  See
   :c:member:`PyModuleDef.m_size`.


.. c:function:: PyModuleDef* PyModule_GetDef(PyObject *module)

   Return a pointer to the :c:type:`PyModuleDef` struct from which the module was
   created, or ``NULL`` if the module wasn't created from a definition.


.. c:function:: PyObject* PyModule_GetFilenameObject(PyObject *module)

   .. index::
      single: __file__ (module attribute)
      single: SystemError (built-in exception)

   Return the name of the file from which *module* was loaded using *module*'s
   :attr:`__file__` attribute.  If this is not defined, or if it is not a
   unicode string, raise :exc:`SystemError` and return ``NULL``; otherwise return
   a reference to a Unicode object.

   .. versionadded:: 3.2


.. c:function:: const char* PyModule_GetFilename(PyObject *module)

   Similar to :c:func:`PyModule_GetFilenameObject` but return the filename
   encoded to 'utf-8'.

   .. deprecated:: 3.2
      :c:func:`PyModule_GetFilename` raises :c:type:`UnicodeEncodeError` on
      unencodable filenames, use :c:func:`PyModule_GetFilenameObject` instead.


.. _initializing-modules:

Initializing C modules
^^^^^^^^^^^^^^^^^^^^^^

Modules objects are usually created from extension modules (shared libraries
which export an initialization function), or compiled-in modules
(where the initialization function is added using :c:func:`PyImport_AppendInittab`).
See :ref:`building` or :ref:`extending-with-embedding` for details.

The initialization function can either pass a module definition instance
to :c:func:`PyModule_Create`, and return the resulting module object,
or request "multi-phase initialization" by returning the definition struct itself.

.. c:type:: PyModuleDef

   The module definition struct, which holds all information needed to create
   a module object. There is usually only one statically initialized variable
   of this type for each module.

   .. c:member:: PyModuleDef_Base m_base

      Always initialize this member to :c:macro:`PyModuleDef_HEAD_INIT`.

   .. c:member:: const char *m_name

      Name for the new module.

   .. c:member:: const char *m_doc

      Docstring for the module; usually a docstring variable created with
      :c:macro:`PyDoc_STRVAR` is used.

   .. c:member:: Py_ssize_t m_size

      Module state may be kept in a per-module memory area that can be
      retrieved with :c:func:`PyModule_GetState`, rather than in static globals.
      This makes modules safe for use in multiple sub-interpreters.

      This memory area is allocated based on *m_size* on module creation,
      and freed when the module object is deallocated, after the
      :c:member:`m_free` function has been called, if present.

      Setting ``m_size`` to ``-1`` means that the module does not support
      sub-interpreters, because it has global state.

      Setting it to a non-negative value means that the module can be
      re-initialized and specifies the additional amount of memory it requires
      for its state. Non-negative ``m_size`` is required for multi-phase
      initialization.

      See :PEP:`3121` for more details.

   .. c:member:: PyMethodDef* m_methods

      A pointer to a table of module-level functions, described by
      :c:type:`PyMethodDef` values.  Can be ``NULL`` if no functions are present.

   .. c:member:: PyModuleDef_Slot* m_slots

      An array of slot definitions for multi-phase initialization, terminated by
      a ``{0, NULL}`` entry.
      When using single-phase initialization, *m_slots* must be ``NULL``.

      .. versionchanged:: 3.5

         Prior to version 3.5, this member was always set to ``NULL``,
         and was defined as:

           .. c:member:: inquiry m_reload

   .. c:member:: traverseproc m_traverse

      A traversal function to call during GC traversal of the module object, or
      ``NULL`` if not needed.

      This function is not called if the module state was requested but is not
      allocated yet. This is the case immediately after the module is created
      and before the module is executed (:c:data:`Py_mod_exec` function). More
      precisely, this function is not called if :c:member:`m_size` is greater
      than 0 and the module state (as returned by :c:func:`PyModule_GetState`)
      is ``NULL``.

      .. versionchanged:: 3.9
         No longer called before the module state is allocated.

   .. c:member:: inquiry m_clear

      A clear function to call during GC clearing of the module object, or
      ``NULL`` if not needed.

      This function is not called if the module state was requested but is not
      allocated yet. This is the case immediately after the module is created
      and before the module is executed (:c:data:`Py_mod_exec` function). More
      precisely, this function is not called if :c:member:`m_size` is greater
      than 0 and the module state (as returned by :c:func:`PyModule_GetState`)
      is ``NULL``.

      Like :c:member:`PyTypeObject.tp_clear`, this function is not *always*
      called before a module is deallocated. For example, when reference
      counting is enough to determine that an object is no longer used,
      the cyclic garbage collector is not involved and
      :c:member:`~PyModuleDef.m_free` is called directly.

      .. versionchanged:: 3.9
         No longer called before the module state is allocated.

   .. c:member:: freefunc m_free

      A function to call during deallocation of the module object, or ``NULL``
      if not needed.

      This function is not called if the module state was requested but is not
      allocated yet. This is the case immediately after the module is created
      and before the module is executed (:c:data:`Py_mod_exec` function). More
      precisely, this function is not called if :c:member:`m_size` is greater
      than 0 and the module state (as returned by :c:func:`PyModule_GetState`)
      is ``NULL``.

      .. versionchanged:: 3.9
         No longer called before the module state is allocated.

Single-phase initialization
...........................

The module initialization function may create and return the module object
directly. This is referred to as "single-phase initialization", and uses one
of the following two module creation functions:

.. c:function:: PyObject* PyModule_Create(PyModuleDef *def)

   Create a new module object, given the definition in *def*.  This behaves
   like :c:func:`PyModule_Create2` with *module_api_version* set to
   :c:macro:`PYTHON_API_VERSION`.


.. c:function:: PyObject* PyModule_Create2(PyModuleDef *def, int module_api_version)

   Create a new module object, given the definition in *def*, assuming the
   API version *module_api_version*.  If that version does not match the version
   of the running interpreter, a :exc:`RuntimeWarning` is emitted.

   .. note::

      Most uses of this function should be using :c:func:`PyModule_Create`
      instead; only use this if you are sure you need it.

Before it is returned from in the initialization function, the resulting module
object is typically populated using functions like :c:func:`PyModule_AddObjectRef`.

.. _multi-phase-initialization:

Multi-phase initialization
..........................

An alternate way to specify extensions is to request "multi-phase initialization".
Extension modules created this way behave more like Python modules: the
initialization is split between the *creation phase*, when the module object
is created, and the *execution phase*, when it is populated.
The distinction is similar to the :py:meth:`__new__` and :py:meth:`__init__` methods
of classes.

Unlike modules created using single-phase initialization, these modules are not
singletons: if the *sys.modules* entry is removed and the module is re-imported,
a new module object is created, and the old module is subject to normal garbage
collection -- as with Python modules.
By default, multiple modules created from the same definition should be
independent: changes to one should not affect the others.
This means that all state should be specific to the module object (using e.g.
using :c:func:`PyModule_GetState`), or its contents (such as the module's
:attr:`__dict__` or individual classes created with :c:func:`PyType_FromSpec`).

All modules created using multi-phase initialization are expected to support
:ref:`sub-interpreters <sub-interpreter-support>`. Making sure multiple modules
are independent is typically enough to achieve this.

To request multi-phase initialization, the initialization function
(PyInit_modulename) returns a :c:type:`PyModuleDef` instance with non-empty
:c:member:`~PyModuleDef.m_slots`. Before it is returned, the ``PyModuleDef``
instance must be initialized with the following function:

.. c:function:: PyObject* PyModuleDef_Init(PyModuleDef *def)

   Ensures a module definition is a properly initialized Python object that
   correctly reports its type and reference count.

   Returns *def* cast to ``PyObject*``, or ``NULL`` if an error occurred.

   .. versionadded:: 3.5

The *m_slots* member of the module definition must point to an array of
``PyModuleDef_Slot`` structures:

.. c:type:: PyModuleDef_Slot

   .. c:member:: int slot

      A slot ID, chosen from the available values explained below.

   .. c:member:: void* value

      Value of the slot, whose meaning depends on the slot ID.

   .. versionadded:: 3.5

The *m_slots* array must be terminated by a slot with id 0.

The available slot types are:

.. c:macro:: Py_mod_create

   Specifies a function that is called to create the module object itself.
   The *value* pointer of this slot must point to a function of the signature:

   .. c:function:: PyObject* create_module(PyObject *spec, PyModuleDef *def)

   The function receives a :py:class:`~importlib.machinery.ModuleSpec`
   instance, as defined in :PEP:`451`, and the module definition.
   It should return a new module object, or set an error
   and return ``NULL``.

   This function should be kept minimal. In particular, it should not
   call arbitrary Python code, as trying to import the same module again may
   result in an infinite loop.

   Multiple ``Py_mod_create`` slots may not be specified in one module
   definition.

   If ``Py_mod_create`` is not specified, the import machinery will create
   a normal module object using :c:func:`PyModule_New`. The name is taken from
   *spec*, not the definition, to allow extension modules to dynamically adjust
   to their place in the module hierarchy and be imported under different
   names through symlinks, all while sharing a single module definition.

   There is no requirement for the returned object to be an instance of
   :c:type:`PyModule_Type`. Any type can be used, as long as it supports
   setting and getting import-related attributes.
   However, only ``PyModule_Type`` instances may be returned if the
   ``PyModuleDef`` has non-``NULL`` ``m_traverse``, ``m_clear``,
   ``m_free``; non-zero ``m_size``; or slots other than ``Py_mod_create``.

.. c:macro:: Py_mod_exec

   Specifies a function that is called to *execute* the module.
   This is equivalent to executing the code of a Python module: typically,
   this function adds classes and constants to the module.
   The signature of the function is:

   .. c:function:: int exec_module(PyObject* module)

   If multiple ``Py_mod_exec`` slots are specified, they are processed in the
   order they appear in the *m_slots* array.

See :PEP:`489` for more details on multi-phase initialization.

Low-level module creation functions
...................................

The following functions are called under the hood when using multi-phase
initialization. They can be used directly, for example when creating module
objects dynamically. Note that both ``PyModule_FromDefAndSpec`` and
``PyModule_ExecDef`` must be called to fully initialize a module.

.. c:function:: PyObject * PyModule_FromDefAndSpec(PyModuleDef *def, PyObject *spec)

   Create a new module object, given the definition in *def* and the
   ModuleSpec *spec*.  This behaves like :c:func:`PyModule_FromDefAndSpec2`
   with *module_api_version* set to :c:macro:`PYTHON_API_VERSION`.

   .. versionadded:: 3.5

.. c:function:: PyObject * PyModule_FromDefAndSpec2(PyModuleDef *def, PyObject *spec, int module_api_version)

   Create a new module object, given the definition in *def* and the
   ModuleSpec *spec*, assuming the API version *module_api_version*.
   If that version does not match the version of the running interpreter,
   a :exc:`RuntimeWarning` is emitted.

   .. note::

      Most uses of this function should be using :c:func:`PyModule_FromDefAndSpec`
      instead; only use this if you are sure you need it.

   .. versionadded:: 3.5

.. c:function:: int PyModule_ExecDef(PyObject *module, PyModuleDef *def)

   Process any execution slots (:c:data:`Py_mod_exec`) given in *def*.

   .. versionadded:: 3.5

.. c:function:: int PyModule_SetDocString(PyObject *module, const char *docstring)

   Set the docstring for *module* to *docstring*.
   This function is called automatically when creating a module from
   ``PyModuleDef``, using either ``PyModule_Create`` or
   ``PyModule_FromDefAndSpec``.

   .. versionadded:: 3.5

.. c:function:: int PyModule_AddFunctions(PyObject *module, PyMethodDef *functions)

   Add the functions from the ``NULL`` terminated *functions* array to *module*.
   Refer to the :c:type:`PyMethodDef` documentation for details on individual
   entries (due to the lack of a shared module namespace, module level
   "functions" implemented in C typically receive the module as their first
   parameter, making them similar to instance methods on Python classes).
   This function is called automatically when creating a module from
   ``PyModuleDef``, using either ``PyModule_Create`` or
   ``PyModule_FromDefAndSpec``.

   .. versionadded:: 3.5

Support functions
.................

The module initialization function (if using single phase initialization) or
a function called from a module execution slot (if using multi-phase
initialization), can use the following functions to help initialize the module
state:

.. c:function:: int PyModule_AddObjectRef(PyObject *module, const char *name, PyObject *value)

   Add an object to *module* as *name*.  This is a convenience function which
   can be used from the module's initialization function.

   On success, return ``0``. On error, raise an exception and return ``-1``.

   Return ``NULL`` if *value* is ``NULL``. It must be called with an exception
   raised in this case.

   Example usage::

       static int
       add_spam(PyObject *module, int value)
       {
           PyObject *obj = PyLong_FromLong(value);
           if (obj == NULL) {
               return -1;
           }
           int res = PyModule_AddObjectRef(module, "spam", obj);
           Py_DECREF(obj);
           return res;
        }

   The example can also be written without checking explicitly if *obj* is
   ``NULL``::

       static int
       add_spam(PyObject *module, int value)
       {
           PyObject *obj = PyLong_FromLong(value);
           int res = PyModule_AddObjectRef(module, "spam", obj);
           Py_XDECREF(obj);
           return res;
        }

   Note that ``Py_XDECREF()`` should be used instead of ``Py_DECREF()`` in
   this case, since *obj* can be ``NULL``.

   .. versionadded:: 3.10


.. c:function:: int PyModule_AddObject(PyObject *module, const char *name, PyObject *value)

   Similar to :c:func:`PyModule_AddObjectRef`, but steals a reference to
   *value* on success (if it returns ``0``).

   The new :c:func:`PyModule_AddObjectRef` function is recommended, since it is
   easy to introduce reference leaks by misusing the
   :c:func:`PyModule_AddObject` function.

   .. note::

      Unlike other functions that steal references, ``PyModule_AddObject()``
      only decrements the reference count of *value* **on success**.

      This means that its return value must be checked, and calling code must
      :c:func:`Py_DECREF` *value* manually on error.

   Example usage::

      static int
      add_spam(PyObject *module, int value)
      {
          PyObject *obj = PyLong_FromLong(value);
          if (obj == NULL) {
              return -1;
          }
          if (PyModule_AddObject(module, "spam", obj) < 0) {
              Py_DECREF(obj);
              return -1;
          }
          // PyModule_AddObject() stole a reference to obj:
          // Py_DECREF(obj) is not needed here
          return 0;
      }

   The example can also be written without checking explicitly if *obj* is
   ``NULL``::

      static int
      add_spam(PyObject *module, int value)
      {
          PyObject *obj = PyLong_FromLong(value);
          if (PyModule_AddObject(module, "spam", obj) < 0) {
              Py_XDECREF(obj);
              return -1;
          }
          // PyModule_AddObject() stole a reference to obj:
          // Py_DECREF(obj) is not needed here
          return 0;
      }

   Note that ``Py_XDECREF()`` should be used instead of ``Py_DECREF()`` in
   this case, since *obj* can be ``NULL``.


.. c:function:: int PyModule_AddIntConstant(PyObject *module, const char *name, long value)

   Add an integer constant to *module* as *name*.  This convenience function can be
   used from the module's initialization function. Return ``-1`` on error, ``0`` on
   success.


.. c:function:: int PyModule_AddStringConstant(PyObject *module, const char *name, const char *value)

   Add a string constant to *module* as *name*.  This convenience function can be
   used from the module's initialization function.  The string *value* must be
   ``NULL``-terminated.  Return ``-1`` on error, ``0`` on success.


.. c:function:: int PyModule_AddIntMacro(PyObject *module, macro)

   Add an int constant to *module*. The name and the value are taken from
   *macro*. For example ``PyModule_AddIntMacro(module, AF_INET)`` adds the int
   constant *AF_INET* with the value of *AF_INET* to *module*.
   Return ``-1`` on error, ``0`` on success.


.. c:function:: int PyModule_AddStringMacro(PyObject *module, macro)

   Add a string constant to *module*.

.. c:function:: int PyModule_AddType(PyObject *module, PyTypeObject *type)

   Add a type object to *module*.
   The type object is finalized by calling internally :c:func:`PyType_Ready`.
   The name of the type object is taken from the last component of
   :c:member:`~PyTypeObject.tp_name` after dot.
   Return ``-1`` on error, ``0`` on success.

   .. versionadded:: 3.9


Module lookup
^^^^^^^^^^^^^

Single-phase initialization creates singleton modules that can be looked up
in the context of the current interpreter. This allows the module object to be
retrieved later with only a reference to the module definition.

These functions will not work on modules created using multi-phase initialization,
since multiple such modules can be created from a single definition.

.. c:function:: PyObject* PyState_FindModule(PyModuleDef *def)

   Returns the module object that was created from *def* for the current interpreter.
   This method requires that the module object has been attached to the interpreter state with
   :c:func:`PyState_AddModule` beforehand. In case the corresponding module object is not
   found or has not been attached to the interpreter state yet, it returns ``NULL``.

.. c:function:: int PyState_AddModule(PyObject *module, PyModuleDef *def)

   Attaches the module object passed to the function to the interpreter state. This allows
   the module object to be accessible via :c:func:`PyState_FindModule`.

   Only effective on modules created using single-phase initialization.

   Python calls ``PyState_AddModule`` automatically after importing a module,
   so it is unnecessary (but harmless) to call it from module initialization
   code. An explicit call is needed only if the module's own init code
   subsequently calls ``PyState_FindModule``.
   The function is mainly intended for implementing alternative import
   mechanisms (either by calling it directly, or by referring to its
   implementation for details of the required state updates).

   The caller must hold the GIL.

   Return 0 on success or -1 on failure.

   .. versionadded:: 3.3

.. c:function:: int PyState_RemoveModule(PyModuleDef *def)

   Removes the module object created from *def* from the interpreter state.
   Return 0 on success or -1 on failure.

   The caller must hold the GIL.

   .. versionadded:: 3.3