
``importlib`` -- An implementation of ``import``
************************************************

New in version 3.1.


Introduction
============

The purpose of the ``importlib`` package is two-fold. One is to
provide an implementation of the ``import`` statement (and thus, by
extension, the ``__import__()`` function) in Python source code. This
provides an implementation of ``import`` which is portable to any
Python interpreter. This also provides a reference implementation
which is easier to comprehend than one implemented in a programming
language other than Python.

Two, the components to implement ``import`` are exposed in this
package, making it easier for users to create their own custom objects
(known generically as an *importer*) to participate in the import
process. Details on custom importers can be found in **PEP 302**.

See also:

   *The import statement*
      The language reference for the ``import`` statement.

   Packages specification
      Original specification of packages. Some semantics have changed
      since the writing of this document (e.g. redirecting based on
      ``None`` in ``sys.modules``).

   The ``__import__()`` function
      The ``import`` statement is syntactic sugar for this function.

   **PEP 235**
      Import on Case-Insensitive Platforms

   **PEP 263**
      Defining Python Source Code Encodings

   **PEP 302**
      New Import Hooks

   **PEP 328**
      Imports: Multi-Line and Absolute/Relative

   **PEP 366**
      Main module explicit relative imports

   **PEP 3120**
      Using UTF-8 as the Default Source Encoding

   **PEP 3147**
      PYC Repository Directories


Functions
=========

importlib.__import__(name, globals={}, locals={}, fromlist=list(), level=0)

   An implementation of the built-in ``__import__()`` function.

importlib.import_module(name, package=None)

   Import a module. The *name* argument specifies what module to
   import in absolute or relative terms (e.g. either ``pkg.mod`` or
   ``..mod``). If the name is specified in relative terms, then the
   *package* argument must be set to the name of the package which is
   to act as the anchor for resolving the package name (e.g.
   ``import_module('..mod', 'pkg.subpkg')`` will import ``pkg.mod``).

   The ``import_module()`` function acts as a simplifying wrapper
   around ``importlib.__import__()``. This means all semantics of the
   function are derived from ``importlib.__import__()``, including
   requiring the package from which an import is occurring to have
   been previously imported (i.e., *package* must already be
   imported). The most important difference is that
   ``import_module()`` returns the most nested package or module that
   was imported (e.g. ``pkg.mod``), while ``__import__()`` returns the
   top-level package or module (e.g. ``pkg``).


``importlib.abc`` -- Abstract base classes related to import
============================================================

The ``importlib.abc`` module contains all of the core abstract base
classes used by ``import``. Some subclasses of the core abstract base
classes are also provided to help in implementing the core ABCs.

class class importlib.abc.Finder

   An abstract base class representing a *finder*. See **PEP 302** for
   the exact definition for a finder.

   find_module(fullname, path=None)

      An abstract method for finding a *loader* for the specified
      module. If the *finder* is found on ``sys.meta_path`` and the
      module to be searched for is a subpackage or module then *path*
      will be the value of ``__path__`` from the parent package. If a
      loader cannot be found, ``None`` is returned.

class class importlib.abc.Loader

   An abstract base class for a *loader*. See **PEP 302** for the
   exact definition for a loader.

   load_module(fullname)

      An abstract method for loading a module. If the module cannot be
      loaded, ``ImportError`` is raised, otherwise the loaded module
      is returned.

      If the requested module already exists in ``sys.modules``, that
      module should be used and reloaded. Otherwise the loader should
      create a new module and insert it into ``sys.modules`` before
      any loading begins, to prevent recursion from the import. If the
      loader inserted a module and the load fails, it must be removed
      by the loader from ``sys.modules``; modules already in
      ``sys.modules`` before the loader began execution should be left
      alone. The ``importlib.util.module_for_loader()`` decorator
      handles all of these details.

      The loader should set several attributes on the module. (Note
      that some of these attributes can change when a module is
      reloaded.)

      * ``__name__``
           The name of the module.

      * ``__file__``
           The path to where the module data is stored (not set for
           built-in modules).

      * ``__path__``
           A list of strings specifying the search path within a
           package. This attribute is not set on modules.

      * ``__package__``
           The parent package for the module/package. If the module is
           top-level then it has a value of the empty string. The
           ``importlib.util.set_package()`` decorator can handle the
           details for ``__package__``.

      * ``__loader__``
           The loader used to load the module. (This is not set by the
           built-in import machinery, but it should be set whenever a
           *loader* is used.)

class class importlib.abc.ResourceLoader

   An abstract base class for a *loader* which implements the optional
   **PEP 302** protocol for loading arbitrary resources from the
   storage back-end.

   get_data(path)

      An abstract method to return the bytes for the data located at
      *path*. Loaders that have a file-like storage back-end that
      allows storing arbitrary data can implement this abstract method
      to give direct access to the data stored. ``IOError`` is to be
      raised if the *path* cannot be found. The *path* is expected to
      be constructed using a module's ``__file__`` attribute or an
      item from a package's ``__path__``.

class class importlib.abc.InspectLoader

   An abstract base class for a *loader* which implements the optional
   **PEP 302** protocol for loaders that inspect modules.

   get_code(fullname)

      An abstract method to return the ``code`` object for a module.
      ``None`` is returned if the module does not have a code object
      (e.g. built-in module).  ``ImportError`` is raised if loader
      cannot find the requested module.

   get_source(fullname)

      An abstract method to return the source of a module. It is
      returned as a text string with universal newlines. Returns
      ``None`` if no source is available (e.g. a built-in module).
      Raises ``ImportError`` if the loader cannot find the module
      specified.

   is_package(fullname)

      An abstract method to return a true value if the module is a
      package, a false value otherwise. ``ImportError`` is raised if
      the *loader* cannot find the module.

class class importlib.abc.ExecutionLoader

   An abstract base class which inherits from ``InspectLoader`` that,
   when implemented, helps a module to be executed as a script. The
   ABC represents an optional **PEP 302** protocol.

   get_filename(fullname)

      An abstract method that is to return the value of ``__file__``
      for the specified module. If no path is available,
      ``ImportError`` is raised.

      If source code is available, then the method should return the
      path to the source file, regardless of whether a bytecode was
      used to load the module.

class class importlib.abc.SourceLoader

   An abstract base class for implementing source (and optionally
   bytecode) file loading. The class inherits from both
   ``ResourceLoader`` and ``ExecutionLoader``, requiring the
   implementation of:

   * ``ResourceLoader.get_data()``

   * ``ExecutionLoader.get_filename()``
        Should only return the path to the source file; sourceless
        loading is not supported.

   The abstract methods defined by this class are to add optional
   bytecode file support. Not implementing these optional methods
   causes the loader to only work with source code. Implementing the
   methods allows the loader to work with source *and* bytecode files;
   it does not allow for *sourceless* loading where only bytecode is
   provided.  Bytecode files are an optimization to speed up loading
   by removing the parsing step of Python's compiler, and so no
   bytecode-specific API is exposed.

   path_mtime(self, path)

      Optional abstract method which returns the modification time for
      the specified path.

   set_data(self, path, data)

      Optional abstract method which writes the specified bytes to a
      file path. Any intermediate directories which do not exist are
      to be created automatically.

      When writing to the path fails because the path is read-only
      (``errno.EACCES``), do not propagate the exception.

   get_code(self, fullname)

      Concrete implementation of ``InspectLoader.get_code()``.

   load_module(self, fullname)

      Concrete implementation of ``Loader.load_module()``.

   get_source(self, fullname)

      Concrete implementation of ``InspectLoader.get_source()``.

   is_package(self, fullname)

      Concrete implementation of ``InspectLoader.is_package()``. A
      module is determined to be a package if its file path is a file
      named ``__init__`` when the file extension is removed.

class class importlib.abc.PyLoader

   An abstract base class inheriting from ``ExecutionLoader`` and
   ``ResourceLoader`` designed to ease the loading of Python source
   modules (bytecode is not handled; see ``SourceLoader`` for a
   source/bytecode ABC). A subclass implementing this ABC will only
   need to worry about exposing how the source code is stored; all
   other details for loading Python source code will be handled by the
   concrete implementations of key methods.

   Deprecated since version 3.2: This class has been deprecated in
   favor of ``SourceLoader`` and is slated for removal in Python 3.4.
   See below for how to create a subclass that is compatible with
   Python 3.1 onwards.

   If compatibility with Python 3.1 is required, then use the
   following idiom to implement a subclass that will work with Python
   3.1 onwards (make sure to implement
   ``ExecutionLoader.get_filename()``):

      try:
          from importlib.abc import SourceLoader
      except ImportError:
          from importlib.abc import PyLoader as SourceLoader


      class CustomLoader(SourceLoader):
          def get_filename(self, fullname):
              """Return the path to the source file."""
              # Implement ...

          def source_path(self, fullname):
              """Implement source_path in terms of get_filename."""
              try:
                  return self.get_filename(fullname)
              except ImportError:
                  return None

          def is_package(self, fullname):
              """Implement is_package by looking for an __init__ file
              name as returned by get_filename."""
              filename = os.path.basename(self.get_filename(fullname))
              return os.path.splitext(filename)[0] == '__init__'

   source_path(fullname)

      An abstract method that returns the path to the source code for
      a module. Should return ``None`` if there is no source code.
      Raises ``ImportError`` if the loader knows it cannot handle the
      module.

   get_filename(fullname)

      A concrete implementation of
      ``importlib.abc.ExecutionLoader.get_filename()`` that relies on
      ``source_path()``. If ``source_path()`` returns ``None``, then
      ``ImportError`` is raised.

   load_module(fullname)

      A concrete implementation of
      ``importlib.abc.Loader.load_module()`` that loads Python source
      code. All needed information comes from the abstract methods
      required by this ABC. The only pertinent assumption made by this
      method is that when loading a package ``__path__`` is set to
      ``[os.path.dirname(__file__)]``.

   get_code(fullname)

      A concrete implementation of
      ``importlib.abc.InspectLoader.get_code()`` that creates code
      objects from Python source code, by requesting the source code
      (using ``source_path()`` and ``get_data()``) and compiling it
      with the built-in ``compile()`` function.

   get_source(fullname)

      A concrete implementation of
      ``importlib.abc.InspectLoader.get_source()``. Uses
      ``importlib.abc.ResourceLoader.get_data()`` and
      ``source_path()`` to get the source code.  It tries to guess the
      source encoding using ``tokenize.detect_encoding()``.

class class importlib.abc.PyPycLoader

   An abstract base class inheriting from ``PyLoader``. This ABC is
   meant to help in creating loaders that support both Python source
   and bytecode.

   Deprecated since version 3.2: This class has been deprecated in
   favor of ``SourceLoader`` and to properly support **PEP 3147**. If
   compatibility is required with Python 3.1, implement both
   ``SourceLoader`` and ``PyLoader``; instructions on how to do so are
   included in the documentation for ``PyLoader``. Do note that this
   solution will not support sourceless/bytecode-only loading; only
   source *and* bytecode loading.

   source_mtime(fullname)

      An abstract method which returns the modification time for the
      source code of the specified module. The modification time
      should be an integer. If there is no source code, return
      ``None``. If the module cannot be found then ``ImportError`` is
      raised.

   bytecode_path(fullname)

      An abstract method which returns the path to the bytecode for
      the specified module, if it exists. It returns ``None`` if no
      bytecode exists (yet). Raises ``ImportError`` if the loader
      knows it cannot handle the module.

   get_filename(fullname)

      A concrete implementation of ``ExecutionLoader.get_filename()``
      that relies on ``PyLoader.source_path()`` and
      ``bytecode_path()``. If ``source_path()`` returns a path, then
      that value is returned. Else if ``bytecode_path()`` returns a
      path, that path will be returned. If a path is not available
      from both methods, ``ImportError`` is raised.

   write_bytecode(fullname, bytecode)

      An abstract method which has the loader write *bytecode* for
      future use. If the bytecode is written, return ``True``. Return
      ``False`` if the bytecode could not be written. This method
      should not be called if ``sys.dont_write_bytecode`` is true. The
      *bytecode* argument should be a bytes string or bytes array.


``importlib.machinery`` -- Importers and path hooks
===================================================

This module contains the various objects that help ``import`` find and
load modules.

class class importlib.machinery.BuiltinImporter

   An *importer* for built-in modules. All known built-in modules are
   listed in ``sys.builtin_module_names``. This class implements the
   ``importlib.abc.Finder`` and ``importlib.abc.InspectLoader`` ABCs.

   Only class methods are defined by this class to alleviate the need
   for instantiation.

class class importlib.machinery.FrozenImporter

   An *importer* for frozen modules. This class implements the
   ``importlib.abc.Finder`` and ``importlib.abc.InspectLoader`` ABCs.

   Only class methods are defined by this class to alleviate the need
   for instantiation.

class class importlib.machinery.PathFinder

   *Finder* for ``sys.path``. This class implements the
   ``importlib.abc.Finder`` ABC.

   This class does not perfectly mirror the semantics of ``import`` in
   terms of ``sys.path``. No implicit path hooks are assumed for
   simplification of the class and its semantics.

   Only class methods are defined by this class to alleviate the need
   for instantiation.

   classmethod find_module(fullname, path=None)

      Class method that attempts to find a *loader* for the module
      specified by *fullname* on ``sys.path`` or, if defined, on
      *path*. For each path entry that is searched,
      ``sys.path_importer_cache`` is checked. If an non-false object
      is found then it is used as the *finder* to look for the module
      being searched for. If no entry is found in
      ``sys.path_importer_cache``, then ``sys.path_hooks`` is searched
      for a finder for the path entry and, if found, is stored in
      ``sys.path_importer_cache`` along with being queried about the
      module. If no finder is ever found then ``None`` is returned.


``importlib.util`` -- Utility code for importers
================================================

This module contains the various objects that help in the construction
of an *importer*.

@importlib.util.module_for_loader

   A *decorator* for a *loader* method, to handle selecting the proper
   module object to load with. The decorated method is expected to
   have a call signature taking two positional arguments (e.g.
   ``load_module(self, module)``) for which the second argument will
   be the module **object** to be used by the loader. Note that the
   decorator will not work on static methods because of the assumption
   of two arguments.

   The decorated method will take in the **name** of the module to be
   loaded as expected for a *loader*. If the module is not found in
   ``sys.modules`` then a new one is constructed with its ``__name__``
   attribute set. Otherwise the module found in ``sys.modules`` will
   be passed into the method. If an exception is raised by the
   decorated method and a module was added to ``sys.modules`` it will
   be removed to prevent a partially initialized module from being in
   left in ``sys.modules``. If the module was already in
   ``sys.modules`` then it is left alone.

   Use of this decorator handles all the details of which module
   object a loader should initialize as specified by **PEP 302**.

@importlib.util.set_loader

   A *decorator* for a *loader* method, to set the ``__loader__``
   attribute on loaded modules. If the attribute is already set the
   decorator does nothing. It is assumed that the first positional
   argument to the wrapped method is what ``__loader__`` should be set
   to.

@importlib.util.set_package

   A *decorator* for a *loader* to set the ``__package__`` attribute
   on the module returned by the loader. If ``__package__`` is set and
   has a value other than ``None`` it will not be changed. Note that
   the module returned by the loader is what has the attribute set on
   and not the module found in ``sys.modules``.

   Reliance on this decorator is discouraged when it is possible to
   set ``__package__`` before the execution of the code is possible.
   By setting it before the code for the module is executed it allows
   the attribute to be used at the global level of the module during
   initialization.
