: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* on the search path *path*.  If *path* is 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 *path* is

   omitted or ``None``, the list of directory names given by ``sys.path`` is

   searched, but first it searches a few special places: it 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).

   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. On

   platforms without threads, this function does nothing.

   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.)

.. 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.