cpython/Doc/c-api/import.rst

.. highlight:: c

.. _importing:

Importing Modules
=================


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

   .. index::
      single: package variable; __all__
      single: __all__ (package variable)
      single: modules (in module sys)

   This is a simplified interface to :c:func:`PyImport_ImportModuleEx` below,
   leaving the *globals* and *locals* arguments set to *NULL* and *level* set
   to 0.  When the *name*
   argument contains a dot (when it specifies a submodule of a package), the
   *fromlist* argument is set to the list ``['*']`` so that the return value is the
   named module rather than the top-level package containing it as would otherwise
   be the case.  (Unfortunately, this has an additional side effect when *name* in
   fact specifies a subpackage instead of a submodule: the submodules specified in
   the package's ``__all__`` variable are  loaded.)  Return a new reference to the
   imported module, or *NULL* with an exception set on failure.  A failing
   import of a module doesn't leave the module in :data:`sys.modules`.

   This function always uses absolute imports.


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

   This function is a deprecated alias of :c:func:`PyImport_ImportModule`.

   .. versionchanged:: 3.3
      This function used to fail immediately when the import lock was held
      by another thread.  In Python 3.3 though, the locking scheme switched
      to per-module locks for most purposes, so this function's special
      behaviour isn't needed anymore.


.. c:function:: PyObject* PyImport_ImportModuleEx(const char *name, PyObject *globals, PyObject *locals, PyObject *fromlist)

   .. index:: builtin: __import__

   Import a module.  This is best described by referring to the built-in Python
   function :func:`__import__`.

   The return value is a new reference to the imported module or top-level
   package, or *NULL* with an exception set on failure.  Like for
   :func:`__import__`, the return value when a submodule of a package was
   requested is normally the top-level package, unless a non-empty *fromlist*
   was given.

   Failing imports remove incomplete module objects, like with
   :c:func:`PyImport_ImportModule`.


.. c:function:: PyObject* PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals, PyObject *locals, PyObject *fromlist, int level)

   Import a module.  This is best described by referring to the built-in Python
   function :func:`__import__`, as the standard :func:`__import__` function calls
   this function directly.

   The return value is a new reference to the imported module or top-level package,
   or *NULL* with an exception set on failure.  Like for :func:`__import__`,
   the return value when a submodule of a package was requested is normally the
   top-level package, unless a non-empty *fromlist* was given.

   .. versionadded:: 3.3


.. c:function:: PyObject* PyImport_ImportModuleLevel(const char *name, PyObject *globals, PyObject *locals, PyObject *fromlist, int level)

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

   .. versionchanged:: 3.3
         Negative values for *level* are no longer accepted.

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

   This is a higher-level interface that calls the current "import hook
   function" (with an explicit *level* of 0, meaning absolute import).  It
   invokes the :func:`__import__` function from the ``__builtins__`` of the
   current globals.  This means that the import is done using whatever import
   hooks are installed in the current environment.

   This function always uses absolute imports.


.. c:function:: PyObject* PyImport_ReloadModule(PyObject *m)

   Reload a module.  Return a new reference to the reloaded module, or *NULL* with
   an exception set on failure (the module still exists in this case).


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

   Return the module object corresponding to a module name.  The *name* argument
   may be of the form ``package.module``. First check the modules dictionary if
   there's one there, and if not, create a new one and insert it in the modules
   dictionary. Return *NULL* with an exception set on failure.

   .. note::

      This function does not load or import the module; if the module wasn't already
      loaded, you will get an empty module object. Use :c:func:`PyImport_ImportModule`
      or one of its variants to import a module.  Package structures implied by a
      dotted name for *name* are not created if not already present.

   .. versionadded:: 3.3


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

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


.. c:function:: PyObject* PyImport_ExecCodeModule(const char *name, PyObject *co)

   .. index:: builtin: compile

   Given a module name (possibly of the form ``package.module``) and a code object
   read from a Python bytecode file or obtained from the built-in function
   :func:`compile`, load the module.  Return a new reference to the module object,
   or *NULL* with an exception set if an error occurred.  *name*
   is removed from :attr:`sys.modules` in error cases, even if *name* was already
   in :attr:`sys.modules` on entry to :c:func:`PyImport_ExecCodeModule`.  Leaving
   incompletely initialized modules in :attr:`sys.modules` is dangerous, as imports of
   such modules have no way to know that the module object is an unknown (and
   probably damaged with respect to the module author's intents) state.

   The module's :attr:`__spec__` and :attr:`__loader__` will be set, if
   not set already, with the appropriate values.  The spec's loader will
   be set to the module's ``__loader__`` (if set) and to an instance of
   :class:`SourceFileLoader` otherwise.

   The module's :attr:`__file__` attribute will be set to the code object's
   :c:member:`co_filename`.  If applicable, :attr:`__cached__` will also
   be set.

   This function will reload the module if it was already imported.  See
   :c:func:`PyImport_ReloadModule` for the intended way to reload a module.

   If *name* points to a dotted name of the form ``package.module``, any package
   structures not already created will still not be created.

   See also :c:func:`PyImport_ExecCodeModuleEx` and
   :c:func:`PyImport_ExecCodeModuleWithPathnames`.


.. c:function:: PyObject* PyImport_ExecCodeModuleEx(const char *name, PyObject *co, const char *pathname)

   Like :c:func:`PyImport_ExecCodeModule`, but the :attr:`__file__` attribute of
   the module object is set to *pathname* if it is non-``NULL``.

   See also :c:func:`PyImport_ExecCodeModuleWithPathnames`.


.. c:function:: PyObject* PyImport_ExecCodeModuleObject(PyObject *name, PyObject *co, PyObject *pathname, PyObject *cpathname)

   Like :c:func:`PyImport_ExecCodeModuleEx`, but the :attr:`__cached__`
   attribute of the module object is set to *cpathname* if it is
   non-``NULL``.  Of the three functions, this is the preferred one to use.

   .. versionadded:: 3.3


.. c:function:: PyObject* PyImport_ExecCodeModuleWithPathnames(const char *name, PyObject *co, const char *pathname, const char *cpathname)

   Like :c:func:`PyImport_ExecCodeModuleObject`, but *name*, *pathname* and
   *cpathname* are UTF-8 encoded strings. Attempts are also made to figure out
   what the value for *pathname* should be from *cpathname* if the former is
   set to ``NULL``.

   .. versionadded:: 3.2
   .. versionchanged:: 3.3
      Uses :func:`imp.source_from_cache()` in calculating the source path if
      only the bytecode path is provided.


.. c:function:: long PyImport_GetMagicNumber()

   Return the magic number for Python bytecode files (a.k.a. :file:`.pyc` file).
   The magic number should be present in the first four bytes of the bytecode
   file, in little-endian byte order. Returns ``-1`` on error.

   .. versionchanged:: 3.3
      Return value of ``-1`` upon failure.


.. c:function:: const char * PyImport_GetMagicTag()

   Return the magic tag string for :pep:`3147` format Python bytecode file
   names.  Keep in mind that the value at ``sys.implementation.cache_tag`` is
   authoritative and should be used instead of this function.

   .. versionadded:: 3.2

.. c:function:: PyObject* PyImport_GetModuleDict()

   Return the dictionary used for the module administration (a.k.a.
   ``sys.modules``).  Note that this is a per-interpreter variable.

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

   Return the already imported module with the given name.  If the
   module has not been imported yet then returns NULL but does not set
   an error.  Returns NULL and sets an error if the lookup failed.

   .. versionadded:: 3.7

.. c:function:: PyObject* PyImport_GetImporter(PyObject *path)

   Return a finder object for a :data:`sys.path`/:attr:`pkg.__path__` item
   *path*, possibly by fetching it from the :data:`sys.path_importer_cache`
   dict.  If it wasn't yet cached, traverse :data:`sys.path_hooks` until a hook
   is found that can handle the path item.  Return ``None`` if no hook could;
   this tells our caller that the :term:`path based finder` could not find a
   finder for this path item. Cache the result in :data:`sys.path_importer_cache`.
   Return a new reference to the finder object.


.. c:function:: void _PyImport_Init()

   Initialize the import mechanism.  For internal use only.


.. c:function:: void PyImport_Cleanup()

   Empty the module table.  For internal use only.


.. c:function:: void _PyImport_Fini()

   Finalize the import mechanism.  For internal use only.


.. c:function:: int PyImport_ImportFrozenModuleObject(PyObject *name)

   Load a frozen module named *name*.  Return ``1`` for success, ``0`` if the
   module is not found, and ``-1`` with an exception set if the initialization
   failed.  To access the imported module on a successful load, use
   :c:func:`PyImport_ImportModule`.  (Note the misnomer --- this function would
   reload the module if it was already imported.)

   .. versionadded:: 3.3

   .. versionchanged:: 3.4
      The ``__file__`` attribute is no longer set on the module.


.. c:function:: int PyImport_ImportFrozenModule(const char *name)

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


.. c:type:: struct _frozen

   .. index:: single: freeze utility

   This is the structure type definition for frozen module descriptors, as
   generated by the :program:`freeze` utility (see :file:`Tools/freeze/` in the
   Python source distribution).  Its definition, found in :file:`Include/import.h`,
   is::

      struct _frozen {
          const char *name;
          const unsigned char *code;
          int size;
      };


.. c:var:: const struct _frozen* PyImport_FrozenModules

   This pointer is initialized to point to an array of :c:type:`struct _frozen`
   records, terminated by one whose members are all *NULL* or zero.  When a frozen
   module is imported, it is searched in this table.  Third-party code could play
   tricks with this to provide a dynamically created collection of frozen modules.


.. c:function:: int PyImport_AppendInittab(const char *name, PyObject* (*initfunc)(void))

   Add a single module to the existing table of built-in modules.  This is a
   convenience wrapper around :c:func:`PyImport_ExtendInittab`, returning ``-1`` if
   the table could not be extended.  The new module can be imported by the name
   *name*, and uses the function *initfunc* as the initialization function called
   on the first attempted import.  This should be called before
   :c:func:`Py_Initialize`.


.. c:type:: struct _inittab

   Structure describing a single entry in the list of built-in modules.  Each of
   these structures gives the name and initialization function for a module built
   into the interpreter.  The name is an ASCII encoded string.  Programs which
   embed Python may use an array of these structures in conjunction with
   :c:func:`PyImport_ExtendInittab` to provide additional built-in modules.
   The structure is defined in :file:`Include/import.h` as::

      struct _inittab {
          const char *name;           /* ASCII encoded string */
          PyObject* (*initfunc)(void);
      };


.. c:function:: int PyImport_ExtendInittab(struct _inittab *newtab)

   Add a collection of modules to the table of built-in modules.  The *newtab*
   array must end with a sentinel entry which contains *NULL* for the :attr:`name`
   field; failure to provide the sentinel value can result in a memory fault.
   Returns ``0`` on success or ``-1`` if insufficient memory could be allocated to
   extend the internal table.  In the event of failure, no modules are added to the
   internal table.  This should be called before :c:func:`Py_Initialize`.