36.2. "imp" — Access the import internals
*****************************************

**Source code:** Lib/imp.py

Deprecated since version 3.4: The "imp" package is pending deprecation
in favor of "importlib".

======================================================================

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.)

   Deprecated since version 3.4: Use "importlib.util.MAGIC_NUMBER"
   instead.

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.

   Deprecated since version 3.3: Use the constants defined on
   "importlib.machinery" instead.

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.

   Deprecated since version 3.3: Use "importlib.util.find_spec()"
   instead unless Python 3.3 compatibility is required, in which case
   use "importlib.find_loader()".

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.

   Deprecated since version 3.3: If previously used in conjunction
   with "imp.find_module()" then consider using
   "importlib.import_module()", otherwise use the loader returned by
   the replacement you chose for "imp.find_module()". If you called
   "imp.load_module()" and related functions directly then use the
   classes in "importlib.machinery", e.g.
   "importlib.machinery.SourceFileLoader(name, path).load_module()".

imp.new_module(name)

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

   Deprecated since version 3.4: Use "types.ModuleType" instead.

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:

   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
   "builtins". 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.

   Changed in version 3.3: Relies on both "__name__" and "__loader__"
   being defined on the module being reloaded instead of just
   "__name__".

   Deprecated since version 3.4: Use "importlib.reload()" instead.

The following functions are conveniences for handling **PEP 3147**
byte-compiled file paths.

New in version 3.2.

imp.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
   "get_tag()"; if "sys.implementation.cache_tag" is not defined then
   "NotImplementedError" will be raised). By passing in "True" or
   "False" for *debug_override* you can override the system’s value
   for "__debug__", leading to optimized bytecode.

   *path* need not exist.

   Changed in version 3.3: If "sys.implementation.cache_tag" is
   "None", then "NotImplementedError" is raised.

   Deprecated since version 3.4: Use
   "importlib.util.cache_from_source()" instead.

   Changed in version 3.5: The *debug_override* parameter no longer
   creates a ".pyo" file.

imp.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. If
   "sys.implementation.cache_tag" is not defined,
   "NotImplementedError" is raised.

   Changed in version 3.3: Raise "NotImplementedError" when
   "sys.implementation.cache_tag" is not defined.

   Deprecated since version 3.4: Use
   "importlib.util.source_from_cache()" instead.

imp.get_tag()

   Return the **PEP 3147** magic tag string matching this version of
   Python’s magic number, as returned by "get_magic()".

   Deprecated since version 3.4: Use "sys.implementation.cache_tag"
   directly starting in Python 3.3.

The following functions help interact with the import system’s
internal locking mechanism.  Locking semantics of imports are an
implementation detail which may vary from release to release.
However, Python ensures that circular imports work without any
deadlocks.

imp.lock_held()

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

   On platforms with threads, a thread executing an import first holds
   a global import lock, then sets up a per-module lock for the rest
   of the import.  This blocks other threads from importing the same
   module until the original import completes, preventing other
   threads from seeing incomplete module objects constructed by the
   original thread.  An exception is made for circular imports, which
   by construction have to expose an incomplete module object at some
   point.

   Changed in version 3.3: The locking scheme has changed to per-
   module locks for the most part.  A global import lock is kept for
   some critical tasks, such as initializing the per-module locks.

   Deprecated since version 3.4.

imp.acquire_lock()

   Acquire the interpreter’s global 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.

   Changed in version 3.3: The locking scheme has changed to per-
   module locks for the most part.  A global import lock is kept for
   some critical tasks, such as initializing the per-module locks.

   Deprecated since version 3.4.

imp.release_lock()

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

   Changed in version 3.3: The locking scheme has changed to per-
   module locks for the most part.  A global import lock is kept for
   some critical tasks, such as initializing the per-module locks.

   Deprecated since version 3.4.

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.

   Deprecated since version 3.3.

imp.PY_COMPILED

   The module was found as a compiled code object file.

   Deprecated since version 3.3.

imp.C_EXTENSION

   The module was found as dynamically loadable shared library.

   Deprecated since version 3.3.

imp.PKG_DIRECTORY

   The module was found as a package directory.

   Deprecated since version 3.3.

imp.C_BUILTIN

   The module was found as a built-in module.

   Deprecated since version 3.3.

imp.PY_FROZEN

   The module was found as a frozen module.

   Deprecated since version 3.3.

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.

   Instances have only one method:

   find_module(fullname[, path])

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

   Changed in version 3.3: "None" is inserted into
   "sys.path_importer_cache" instead of an instance of "NullImporter".

   Deprecated since version 3.4: Insert "None" into
   "sys.path_importer_cache" instead.


36.2.1. 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()
