path: root/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 :term:`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)

   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 will
   reload the module!  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.


.. function:: release_lock()

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


.. function:: reload(module)

   Reload a previously imported *module*.  The argument must be a module object, so
   it must have been successfully imported before.  This is useful if you have
   edited the module source file using an external editor and want to try out the
   new version without leaving the Python interpreter.  The return value is the
   module object (the same as the *module* argument).

   When ``reload(module)`` is executed:

   * Python modules' code is recompiled and the module-level code reexecuted,
     defining a new set of objects which are bound to names in the module's
     dictionary.  The ``init`` function of extension modules is not called a second
     time.

   * As with all other objects in Python the old objects are only reclaimed after
     their reference counts drop to zero.

   * The names in the module namespace are updated to point to any new or changed
     objects.

   * Other references to the old objects (such as names external to the module) are
     not rebound to refer to the new objects and must be updated in each namespace
     where they occur if that is desired.

   There are a number of other caveats:

   If a module is syntactically correct but its initialization fails, the first
   :keyword:`import` statement for it does not bind its name locally, but does
   store a (partially initialized) module object in ``sys.modules``.  To reload the
   module you must first :keyword:`import` it again (this will bind the name to the
   partially initialized module object) before you can :func:`reload` it.

   When a module is reloaded, its dictionary (containing the module's global
   variables) is retained.  Redefinitions of names will override the old
   definitions, so this is generally not a problem.  If the new version of a module
   does not define a name that was defined by the old version, the old definition
   remains.  This feature can be used to the module's advantage if it maintains a
   global table or cache of objects --- with a :keyword:`try` statement it can test
   for the table's presence and skip its initialization if desired::

      try:
          cache
      except NameError:
          cache = {}

   It is legal though generally not very useful to reload built-in or dynamically
   loaded modules, except for :mod:`sys`, :mod:`__main__` and :mod:`__builtin__`.
   In many cases, however, extension modules are not designed to be initialized
   more than once, and may fail in arbitrary ways when reloaded.

   If a module imports objects from another module using :keyword:`from` ...
   :keyword:`import` ..., calling :func:`reload` for the other module does not
   redefine the objects imported from it --- one way around this is to re-execute
   the :keyword:`from` statement, another is to use :keyword:`import` and qualified
   names (*module*.*name*) instead.

   If a module instantiates instances of a class, reloading the module that defines
   the class does not affect the method definitions of the instances --- they
   continue to use the old class definition.  The same is true for derived classes.


The following functions and data provide conveniences for handling :pep:`3147`
byte-compiled file paths.

.. versionadded:: 3.2

.. function:: cache_from_source(path, debug_override=None)

   Return the :pep:`3147` path to the byte-compiled file associated with the
   source *path*.  For example, if *path* is ``/foo/bar/baz.py`` the return
   value would be ``/foo/bar/__pycache__/baz.cpython-32.pyc`` for Python 3.2.
   The ``cpython-32`` string comes from the current magic tag (see
   :func:`get_tag`).  The returned path will end in ``.pyc`` when
   ``__debug__`` is True or ``.pyo`` for an optimized Python
   (i.e. ``__debug__`` is False).  By passing in True or False for
   *debug_override* you can override the system's value for ``__debug__`` for
   extension selection.

   *path* need not exist.


.. function:: source_from_cache(path)

   Given the *path* to a :pep:`3147` file name, return the associated source code
   file path.  For example, if *path* is
   ``/foo/bar/__pycache__/baz.cpython-32.pyc`` the returned path would be
   ``/foo/bar/baz.py``.  *path* need not exist, however if it does not conform
   to :pep:`3147` format, a ``ValueError`` is raised.


.. function:: get_tag()

   Return the :pep:`3147` magic tag string matching this version of Python's
   magic number, as returned by :func:`get_magic`.


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`).


.. 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.


.. _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:: 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.