
``imp`` --- Access the ``import`` internals
*******************************************

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

imp.get_magic()

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

imp.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 ``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 ``PY_SOURCE``,
   ``PY_COMPILED``, or ``C_EXTENSION``, described below.

imp.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 (``C_BUILTIN``), then a
   frozen module (``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
   ``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
   ``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, ``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
   ``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 ``find_module()`` and ``load_module()`` to find
   and load package *P*, and then use ``find_module()`` with the
   *path* argument set to ``P.__path__``. When *P* itself has a dotted
   name, apply this recipe recursively.

imp.load_module(name, file, pathname, description)

   Load a module that was previously found by ``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 ``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 ``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 ``try`` ... ``finally`` statement.

imp.new_module(name)

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

imp.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).

imp.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.

imp.release_lock()

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

imp.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 ``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 ``import`` it
   again (this will bind the name to the partially initialized module
   object) before you can ``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 ``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 ``sys``, ``__main__`` and
   ``__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 ``from`` ...
   ``import`` ..., calling ``reload()`` for the other module does not
   redefine the objects imported from it --- one way around this is to
   re-execute the ``from`` statement, another is to use ``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 constants with integer values, defined in this module,
are used to indicate the search result of ``find_module()``.

imp.PY_SOURCE

   The module was found as a source file.

imp.PY_COMPILED

   The module was found as a compiled code object file.

imp.C_EXTENSION

   The module was found as dynamically loadable shared library.

imp.PKG_DIRECTORY

   The module was found as a package directory.

imp.C_BUILTIN

   The module was found as a built-in module.

imp.PY_FROZEN

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

class class imp.NullImporter(path_string)

   The ``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
   ``ImportError``. Otherwise, a ``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:

   find_module(fullname[, path])

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


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
``find_module()`` has been extended and ``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()

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