cpython/Doc/c-api/import.rst

.. highlightlang:: c

.. _importing:

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


.. cfunction:: 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 :cfunc:`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`.


.. cfunction:: PyObject* PyImport_ImportModuleNoBlock(const char *name)

   This version of :cfunc:`PyImport_ImportModule` does not block. It's intended
   to be used in C functions that import other modules to execute a function.
   The import may block if another thread holds the import lock. The function
   :cfunc:`PyImport_ImportModuleNoBlock` never blocks. It first tries to fetch
   the module from sys.modules and falls back to :cfunc:`PyImport_ImportModule`
   unless the lock is held, in which case the function will raise an
   :exc:`ImportError`.


.. cfunction:: PyObject* PyImport_ImportModuleEx(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__`, 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.

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


.. cfunction:: PyObject* PyImport_ImportModuleLevel(char *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.


.. cfunction:: 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.


.. cfunction:: 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).


.. cfunction:: PyObject* PyImport_AddModule(const char *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 :cfunc:`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.


.. cfunction:: PyObject* PyImport_ExecCodeModule(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 :cfunc:`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.

   This function will reload the module if it was already imported.  See
   :cfunc:`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.


.. cfunction:: long PyImport_GetMagicNumber()

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


.. cfunction:: PyObject* PyImport_GetModuleDict()

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


.. cfunction:: PyObject* PyImport_GetImporter(PyObject *path)

   Return an importer 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 it should fall back to the built-in import mechanism.
   Cache the result in :data:`sys.path_importer_cache`.  Return a new reference
   to the importer object.


.. cfunction:: void _PyImport_Init()

   Initialize the import mechanism.  For internal use only.


.. cfunction:: void PyImport_Cleanup()

   Empty the module table.  For internal use only.


.. cfunction:: void _PyImport_Fini()

   Finalize the import mechanism.  For internal use only.


.. cfunction:: PyObject* _PyImport_FindExtension(char *, char *)

   For internal use only.


.. cfunction:: PyObject* _PyImport_FixupExtension(char *, char *)

   For internal use only.


.. cfunction:: int PyImport_ImportFrozenModule(char *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
   :cfunc:`PyImport_ImportModule`.  (Note the misnomer --- this function would
   reload the module if it was already imported.)


.. ctype:: 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 {
          char *name;
          unsigned char *code;
          int size;
      };


.. cvar:: struct _frozen* PyImport_FrozenModules

   This pointer is initialized to point to an array of :ctype:`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.


.. cfunction:: 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 :cfunc:`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
   :cfunc:`Py_Initialize`.


.. ctype:: 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.  Programs which embed Python may use an array of these
   structures in conjunction with :cfunc:`PyImport_ExtendInittab` to provide
   additional built-in modules.  The structure is defined in
   :file:`Include/import.h` as::

      struct _inittab {
          char *name;
          PyObject* (*initfunc)(void);
      };


.. cfunction:: 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 :cfunc:`Py_Initialize`.