source: python/trunk/Doc/library/imp.rst

:mod:`imp` --- Access the :keyword:`import` internals
=====================================================

.. module:: imp
   :synopsis: Access the implementation of the import statement.


.. index:: statement: import

This module provides an interface to the mechanisms used to implement the
:keyword:`import` statement.  It defines the following constants and functions:


.. function:: get_magic()

   .. index:: pair: file; byte-code

   Return the magic string value used to recognize byte-compiled code files
   (:file:`.pyc` files).  (This value may be different for each Python version.)


.. function:: get_suffixes()

   Return a list of 3-element tuples, each describing a particular type of
   module. Each triple has the form ``(suffix, mode, type)``, where *suffix* is
   a string to be appended to the module name to form the filename to search
   for, *mode* is the mode string to pass to the built-in :func:`open` function
   to open the file (this can be ``'r'`` for text files or ``'rb'`` for binary
   files), and *type* is the file type, which has one of the values
   :const:`PY_SOURCE`, :const:`PY_COMPILED`, or :const:`C_EXTENSION`, described
   below.


.. function:: find_module(name[, path])

   Try to find the module *name*.  If *path* is omitted or ``None``, the list of
   directory names given by ``sys.path`` is searched, but first a few special
   places are searched: the function tries to find a built-in module with the
   given name (:const:`C_BUILTIN`), then a frozen module (:const:`PY_FROZEN`),
   and on some systems some other places are looked in as well (on Windows, it
   looks in the registry which may point to a specific file).

   Otherwise, *path* must be a list of directory names; each directory is
   searched for files with any of the suffixes returned by :func:`get_suffixes`
   above.  Invalid names in the list are silently ignored (but all list items
   must be strings).

   If search is successful, the return value is a 3-element tuple ``(file,
   pathname, description)``:

   *file* is an open file object positioned at the beginning, *pathname* is the
   pathname of the file found, and *description* is a 3-element tuple as
   contained in the list returned by :func:`get_suffixes` describing the kind of
   module found.

   If the module does not live in a file, the returned *file* is ``None``,
   *pathname* is the empty string, and the *description* tuple contains empty
   strings for its suffix and mode; the module type is indicated as given in
   parentheses above.  If the search is unsuccessful, :exc:`ImportError` is
   raised.  Other exceptions indicate problems with the arguments or
   environment.

   If the module is a package, *file* is ``None``, *pathname* is the package
   path and the last item in the *description* tuple is :const:`PKG_DIRECTORY`.

   This function does not handle hierarchical module names (names containing
   dots).  In order to find *P.M*, that is, submodule *M* of package *P*, use
   :func:`find_module` and :func:`load_module` to find and load package *P*, and
   then use :func:`find_module` with the *path* argument set to ``P.__path__``.
   When *P* itself has a dotted name, apply this recipe recursively.


.. function:: load_module(name, file, pathname, description)

   .. index:: builtin: reload

   Load a module that was previously found by :func:`find_module` (or by an
   otherwise conducted search yielding compatible results).  This function does
   more than importing the module: if the module was already imported, it is
   equivalent to a :func:`reload`!  The *name* argument indicates the full
   module name (including the package name, if this is a submodule of a
   package).  The *file* argument is an open file, and *pathname* is the
   corresponding file name; these can be ``None`` and ``''``, respectively, when
   the module is a package or not being loaded from a file.  The *description*
   argument is a tuple, as would be returned by :func:`get_suffixes`, describing
   what kind of module must be loaded.

   If the load is successful, the return value is the module object; otherwise,
   an exception (usually :exc:`ImportError`) is raised.

   **Important:** the caller is responsible for closing the *file* argument, if
   it was not ``None``, even when an exception is raised.  This is best done
   using a :keyword:`try` ... :keyword:`finally` statement.


.. function:: new_module(name)

   Return a new empty module object called *name*.  This object is *not* inserted
   in ``sys.modules``.


.. function:: lock_held()

   Return ``True`` if the import lock is currently held, else ``False``. On
   platforms without threads, always return ``False``.

   On platforms with threads, a thread executing an import holds an internal lock
   until the import is complete. This lock blocks other threads from doing an
   import until the original import completes, which in turn prevents other threads
   from seeing incomplete module objects constructed by the original thread while
   in the process of completing its import (and the imports, if any, triggered by
   that).


.. function:: acquire_lock()

   Acquire the interpreter's import lock for the current thread.  This lock should
   be used by import hooks to ensure thread-safety when importing modules.

   Once a thread has acquired the import lock, the same thread may acquire it
   again without blocking; the thread must release it once for each time it has
   acquired it.

   On platforms without threads, this function does nothing.

   .. versionadded:: 2.3


.. function:: release_lock()

   Release the interpreter's import lock. On platforms without threads, this
   function does nothing.

   .. versionadded:: 2.3

The following constants with integer values, defined in this module, are used to
indicate the search result of :func:`find_module`.


.. data:: PY_SOURCE

   The module was found as a source file.


.. data:: PY_COMPILED

   The module was found as a compiled code object file.


.. data:: C_EXTENSION

   The module was found as dynamically loadable shared library.


.. data:: PKG_DIRECTORY

   The module was found as a package directory.


.. data:: C_BUILTIN

   The module was found as a built-in module.


.. data:: PY_FROZEN

   The module was found as a frozen module (see :func:`init_frozen`).

The following constant and functions are obsolete; their functionality is
available through :func:`find_module` or :func:`load_module`. They are kept
around for backward compatibility:


.. data:: SEARCH_ERROR

   Unused.


.. function:: init_builtin(name)

   Initialize the built-in module called *name* and return its module object along
   with storing it in ``sys.modules``.  If the module was already initialized, it
   will be initialized *again*.  Re-initialization involves the copying of the
   built-in module's ``__dict__`` from the cached module over the module's entry in
   ``sys.modules``.  If there is no built-in module called *name*, ``None`` is
   returned.


.. function:: init_frozen(name)

   Initialize the frozen module called *name* and return its module object.  If
   the module was already initialized, it will be initialized *again*.  If there
   is no frozen module called *name*, ``None`` is returned.  (Frozen modules are
   modules written in Python whose compiled byte-code object is incorporated
   into a custom-built Python interpreter by Python's :program:`freeze`
   utility. See :file:`Tools/freeze/` for now.)


.. function:: is_builtin(name)

   Return ``1`` if there is a built-in module called *name* which can be
   initialized again.  Return ``-1`` if there is a built-in module called *name*
   which cannot be initialized again (see :func:`init_builtin`).  Return ``0`` if
   there is no built-in module called *name*.


.. function:: is_frozen(name)

   Return ``True`` if there is a frozen module (see :func:`init_frozen`) called
   *name*, or ``False`` if there is no such module.


.. function:: load_compiled(name, pathname, [file])

   .. index:: pair: file; byte-code

   Load and initialize a module implemented as a byte-compiled code file and return
   its module object.  If the module was already initialized, it will be
   initialized *again*.  The *name* argument is used to create or access a module
   object.  The *pathname* argument points to the byte-compiled code file.  The
   *file* argument is the byte-compiled code file, open for reading in binary mode,
   from the beginning. It must currently be a real file object, not a user-defined
   class emulating a file.


.. function:: load_dynamic(name, pathname[, file])

   Load and initialize a module implemented as a dynamically loadable shared
   library and return its module object.  If the module was already initialized, it
   will be initialized *again*. Re-initialization involves copying the ``__dict__``
   attribute of the cached instance of the module over the value used in the module
   cached in ``sys.modules``.  The *pathname* argument must point to the shared
   library.  The *name* argument is used to construct the name of the
   initialization function: an external C function called ``initname()`` in the
   shared library is called.  The optional *file* argument is ignored.  (Note:
   using shared libraries is highly system dependent, and not all systems support
   it.)

   .. impl-detail::

      The import internals identify extension modules by filename, so doing
      ``foo = load_dynamic("foo", "mod.so")`` and
      ``bar = load_dynamic("bar", "mod.so")`` will result in both foo and bar
      referring to the same module, regardless of whether or not
      ``mod.so`` exports an ``initbar`` function. On systems which
      support them, symlinks can be used to import multiple modules from
      the same shared library, as each reference to the module will use
      a different file name.


.. function:: load_source(name, pathname[, file])

   Load and initialize a module implemented as a Python source file and return its
   module object.  If the module was already initialized, it will be initialized
   *again*.  The *name* argument is used to create or access a module object.  The
   *pathname* argument points to the source file.  The *file* argument is the
   source file, open for reading as text, from the beginning. It must currently be
   a real file object, not a user-defined class emulating a file.  Note that if a
   properly matching byte-compiled file (with suffix :file:`.pyc` or :file:`.pyo`)
   exists, it will be used instead of parsing the given source file.


.. class:: NullImporter(path_string)

   The :class:`NullImporter` type is a :pep:`302` import hook that handles
   non-directory path strings by failing to find any modules.  Calling this type
   with an existing directory or empty string raises :exc:`ImportError`.
   Otherwise, a :class:`NullImporter` instance is returned.

   Python adds instances of this type to ``sys.path_importer_cache`` for any path
   entries that are not directories and are not handled by any other path hooks on
   ``sys.path_hooks``.  Instances have only one method:


   .. method:: NullImporter.find_module(fullname [, path])

      This method always returns ``None``, indicating that the requested module could
      not be found.

   .. versionadded:: 2.5


.. _examples-imp:

Examples
--------

The following function emulates what was the standard import statement up to
Python 1.4 (no hierarchical module names).  (This *implementation* wouldn't work
in that version, since :func:`find_module` has been extended and
:func:`load_module` has been added in 1.4.) ::

   import imp
   import sys

   def __import__(name, globals=None, locals=None, fromlist=None):
       # Fast path: see if the module has already been imported.
       try:
           return sys.modules[name]
       except KeyError:
           pass

       # If any of the following calls raises an exception,
       # there's a problem we can't handle -- let the caller handle it.

       fp, pathname, description = imp.find_module(name)

       try:
           return imp.load_module(name, fp, pathname, description)
       finally:
           # Since we may exit via an exception, close fp explicitly.
           if fp:
               fp.close()

.. index::
   builtin: reload
   module: knee

A more complete example that implements hierarchical module names and includes a
:func:`reload` function can be found in the module :mod:`knee`.  The :mod:`knee`
module can be found in :file:`Demo/imputil/` in the Python source distribution.