Integration of importdocs from the features/pep-420 repo.

Doc/glossary.rst

@@ -209,9 +209,9 @@ Glossary
 
    finder
       An object that tries to find the :term:`loader` for a module. It must
-      implement a method named :meth:`find_module`. See :pep:`302` for
+      implement either a method named :meth:`find_loader` or a method named
-      details and :class:`importlib.abc.Finder` for an
+      :meth:`find_module`. See :pep:`302` and :pep:`420` for details and
-      :term:`abstract base class`.
+      :class:`importlib.abc.Finder` for an :term:`abstract base class`.
 
    floor division
       Mathematical division that rounds down to nearest integer.  The floor
@@ -315,6 +315,10 @@ Glossary
       role in places where a constant hash value is needed, for example as a key
       in a dictionary.
 
+   importing
+      The process by which Python code in one module is made available to
+      Python code in another module.
+
    importer
       An object that both finds and loads a module; both a
       :term:`finder` and :term:`loader` object.
@@ -440,6 +444,11 @@ Glossary
       include :class:`dict`, :class:`collections.defaultdict`,
       :class:`collections.OrderedDict` and :class:`collections.Counter`.
 
+   meta path finder
+      A finder returned by a search of :data:`sys.meta_path`.  Meta path
+      finders are related to, but different from :term:`sys path finders <sys
+      path finder>`.
+
    metaclass
       The class of a class.  Class definitions create a class name, a class
       dictionary, and a list of base classes.  The metaclass is responsible for
@@ -464,6 +473,11 @@ Glossary
       for a member during lookup. See `The Python 2.3 Method Resolution Order
       <http://www.python.org/download/releases/2.3/mro/>`_.
 
+   module
+      An object that serves as an organizational unit of Python code.  Modules
+      have a namespace contain arbitrary Python objects.  Modules are loaded
+      into Python by the process of :term:`importing`.
+
    MRO
       See :term:`method resolution order`.
 
@@ -496,6 +510,12 @@ Glossary
       functions are implemented by the :mod:`random` and :mod:`itertools`
       modules, respectively.
 
+   namespace package
+      A :pep:`420` :term:`package` which serves only as a container for
+      subpackages.  Namespace packages may have no physical representation,
+      and specifically are not like a :term:`regular package` because they
+      have no ``__init__.py`` file.
+
    nested scope
       The ability to refer to a variable in an enclosing definition.  For
       instance, a function defined inside another function can refer to
@@ -516,6 +536,19 @@ Glossary
       (methods).  Also the ultimate base class of any :term:`new-style
       class`.
 
+   package
+      A Python module which can contain submodules or recursively,
+      subpackages.  Technically, a package is a Python module with an
+      ``__path__`` attribute.
+
+   path importer
+      A built-in :term:`finder` / :term:`loader` that knows how to find and
+      load modules from the file system.
+
+   portion
+      A set of files in a single directory (possibly stored in a zip file)
+      that contribute to a namespace package, as defined in :pep:`420`.
+
    positional argument
       The arguments assigned to local names inside a function or method,
       determined by the order in which they were given in the call.  ``*`` is
@@ -524,8 +557,8 @@ Glossary
       :term:`argument`.
 
    provisional package
-      A provisional package is one which has been deliberately excluded from the
+      A provisional package is one which has been deliberately excluded from
-      standard library's backwards compatibility guarantees.  While major
+      the standard library's backwards compatibility guarantees.  While major
       changes to such packages are not expected, as long as they are marked
       provisional, backwards incompatible changes (up to and including removal
       of the package) may occur if deemed necessary by core developers.  Such
@@ -533,13 +566,13 @@ Glossary
       flaws are uncovered that were missed prior to the inclusion of the
       package.
 
-      This process allows the standard library to continue to evolve over time,
+      This process allows the standard library to continue to evolve over
-      without locking in problematic design errors for extended periods of time.
+      time, without locking in problematic design errors for extended periods
-      See :pep:`411` for more details.
+      of time.  See :pep:`411` for more details.
 
    Python 3000
-      Nickname for the Python 3.x release line (coined long ago when the release
+      Nickname for the Python 3.x release line (coined long ago when the
-      of version 3 was something in the distant future.)  This is also
+      release of version 3 was something in the distant future.)  This is also
       abbreviated "Py3k".
 
    Pythonic
@@ -576,6 +609,14 @@ Glossary
          >>> C.D.meth.__qualname__
          'C.D.meth'
 
+      When used to refer to modules, the *fully qualified name* means the
+      entire dotted path to the module, including any parent packages,
+      e.g. ``email.mime.text``::
+
+         >>> import email.mime.text
+         >>> email.mime.text.__name__
+         'email.mime.text'
+
    reference count
       The number of references to an object.  When the reference count of an
       object drops to zero, it is deallocated.  Reference counting is
@@ -584,6 +625,10 @@ Glossary
       :func:`~sys.getrefcount` function that programmers can call to return the
       reference count for a particular object.
 
+   regular package
+      A traditional :term:`package`, such as a directory containing an
+      ``__init__.py`` file.
+
    __slots__
       A declaration inside a class that saves memory by pre-declaring space for
       instance attributes and eliminating instance dictionaries.  Though
@@ -626,6 +671,11 @@ Glossary
       :meth:`~collections.somenamedtuple._asdict`. Examples of struct sequences
       include :data:`sys.float_info` and the return value of :func:`os.stat`.
 
+   sys path finder
+      A finder returned by a search of :data:`sys.path` by the :term:`path
+      importer`.  Sys path finders are related to, but different from
+      :term:`meta path finders <meta path finder>`.
+
    triple-quoted string
       A string which is bound by three instances of either a quotation mark
       (") or an apostrophe (').  While they don't provide any functionality

Doc/library/importlib.rst

@@ -128,6 +128,16 @@ are also provided to help in implementing the core ABCs.
     An abstract base class representing a :term:`finder`.
     See :pep:`302` for the exact definition for a finder.
 
+    .. method:: find_loader(self, fullname):
+
+        An abstract method for finding a :term:`loader` for the specified
+        module.  Returns a 2-tuple of ``(loader, portion)`` where portion is a
+        sequence of file system locations contributing to part of a namespace
+        package.  The sequence may be empty.  When present, `find_loader()` is
+        preferred over `find_module()`.
+
+        .. versionadded: 3.3
+
     .. method:: find_module(fullname, path=None)
 
         An abstract method for finding a :term:`loader` for the specified
@@ -190,6 +200,13 @@ are also provided to help in implementing the core ABCs.
             (This is not set by the built-in import machinery,
             but it should be set whenever a :term:`loader` is used.)
 
+    .. method:: module_repr(module)
+
+        An abstract method which when implemented calculates and returns the
+        given module's repr, as a string.
+
+        .. versionadded: 3.3
+
 
 .. class:: ResourceLoader
 
@@ -270,14 +287,13 @@ are also provided to help in implementing the core ABCs.
 
       Path to the file of the module.
 
-   .. method:: load_module(fullname=None)
+   .. method:: load_module(fullname)
 
-      Calls
+      Calls super's ``load_module()``.
-      ``super().load_module(fullname if fullname is not None else self.name)``.
 
    .. method:: get_filename(fullname)
 
-      Returns :attr:`path` when ``fullname`` equals :attr:`name` or ``None``.
+      Returns :attr:`path`.
 
    .. method:: get_data(path)
 
@@ -538,7 +554,6 @@ find and load modules.
 
    .. versionadded:: 3.3
 
-
 .. function:: all_suffixes()
 
    Returns a combined list of strings representing all file suffixes for
@@ -727,7 +742,7 @@ find and load modules.
 
       Path to the extension module.
 
-   .. method:: load_module(fullname=None)
+   .. method:: load_module(fullname)
 
       Loads the extension module if and only if *fullname* is the same as
       :attr:`name` or is ``None``.

Doc/reference/datamodel.rst

@@ -651,17 +651,19 @@ Modules
       statement: import
       object: module
 
-   Modules are imported by the :keyword:`import` statement (see section
+   Modules are a basic organizational unit of Python code, and are created by
-   :ref:`import`). A module object has a
+   the :ref:`importmachinery` as invoked either by the :keyword:`import`
-   namespace implemented by a dictionary object (this is the dictionary referenced
+   statement (see section :ref:`import`) or by calling the built in
-   by the __globals__ attribute of functions defined in the module).  Attribute
+   :func:`__import__` function.  A module object has a namespace implemented
+   by a dictionary object (this is the dictionary referenced by the
+   ``__globals__`` attribute of functions defined in the module).  Attribute
    references are translated to lookups in this dictionary, e.g., ``m.x`` is
-   equivalent to ``m.__dict__["x"]``. A module object does not contain the code
+   equivalent to ``m.__dict__["x"]``. A module object does not contain the
-   object used to initialize the module (since it isn't needed once the
+   code object used to initialize the module (since it isn't needed once the
    initialization is done).
 
-   Attribute assignment updates the module's namespace dictionary, e.g., ``m.x =
+   Attribute assignment updates the module's namespace dictionary, e.g.,
-   1`` is equivalent to ``m.__dict__["x"] = 1``.
+   ``m.x = 1`` is equivalent to ``m.__dict__["x"] = 1``.
 
    .. index:: single: __dict__ (module attribute)
 
@@ -683,11 +685,12 @@ Modules
 
    Predefined (writable) attributes: :attr:`__name__` is the module's name;
    :attr:`__doc__` is the module's documentation string, or ``None`` if
-   unavailable; :attr:`__file__` is the pathname of the file from which the module
+   unavailable; :attr:`__file__` is the pathname of the file from which the
-   was loaded, if it was loaded from a file. The :attr:`__file__` attribute is not
+   module was loaded, if it was loaded from a file. The :attr:`__file__`
-   present for C modules that are statically linked into the interpreter; for
+   attribute may be missing for certain types of modules, such as C modules
-   extension modules loaded dynamically from a shared library, it is the pathname
+   that are statically linked into the interpreter; for extension modules
-   of the shared library file.
+   loaded dynamically from a shared library, it is the pathname of the shared
+   library file.
 
 Custom classes
    Custom class types are typically created by class definitions (see section

Doc/reference/import_machinery.rst

@@ -0,0 +1,547 @@
+
+.. _importmachinery:
+
+****************
+Import machinery
+****************
+
+.. index:: single: import machinery
+
+Python code in one :term:`module` gains access to the code in another module
+by the process of :term:`importing` it.  Most commonly, the :keyword:`import`
+statement is used to invoke the import machinery, but it can also be invoked
+by calling the built-in :func:`__import__` function.
+
+The :keyword:`import` statement combines two operations; it searches for the
+named module, then it binds the results of that search to a name in the local
+scope.  The search operation of the :keyword:`import` statement is defined as
+a call to the :func:`__import__` function, with the appropriate arguments.
+The return value of :func:`__import__` is used to perform the name binding
+operation of the :keyword:`import` statement.  See the :keyword:`import`
+statement for the exact details of that name binding operation.
+
+A direct call to :func:`__import__` performs only the search for the module.
+The function's return value is used like any other function call in Python;
+there is no special side-effects (e.g. name binding) associated with
+:func:`__import__`.
+
+When a module is first imported, Python searches for the module and if found,
+it creates a module object, initializing it.  If the named module cannot be
+found, an :exc:`ImportError` is raised.  Python implements various strategies
+to search for the named module when the import machinery is invoked.  These
+strategies can be modified and extended by using various hooks described in
+the sections below.  The entire import machinery itself can be overridden by
+replacing built-in :func:`__import__`.
+
+
+Packages
+========
+
+.. index::
+    single: package
+
+Python has only one type of module object, and all modules are of this type,
+regardless of whether the module is implemented in Python, C, or something
+else.  To help organize modules and provide a naming hierarchy, Python has a
+concept of :term:`packages <package>`.  It's important to keep in mind that
+all packages are modules, but not all modules are packages.  Or put another
+way, packages are just a special kind of module.  Although usually
+unnecessary, introspection of various module object attributes can determine
+whether a module is a package or not.
+
+Packages can contain other packages and modules, while modules generally do
+not contain other modules or packages.  You can think of packages as the
+directories on a file system and modules as files within directories, but
+don't take this analogy too literally since packages and modules need not
+originate from the file system.  For the purposes of this documentation, we'll
+use this convenient analogy of directories and files.
+
+All modules have a name.  Packages also have names, and subpackages can be
+nested arbitrarily deeply.  Subpackage names are separated from their parent
+package by dots, akin to Python's standard attribute access syntax.  Thus you
+might have a module called :mod:`sys` and a package called :mod:`email`, which
+in turn has a subpackage called :mod:`email.mime` and a module within that
+subpackage called :mod:`email.mime.text`.
+
+
+Regular packages
+----------------
+
+.. index::
+    pair: package; regular
+
+Python defines two types of packages, :term:`regular packages <regular
+package>` and :term:`namespace packages <namespace package>`.  Regular
+packages are traditional packages as they existed in Python 3.2 and earlier.
+A regular package is typically implemented as a directory containing an
+``__init__.py`` file.  When a regular package is imported, this
+``__init__.py`` file is implicitly imported, and the objects it defines are
+bound to names in the package's namespace.  The ``__init__.py`` file can
+contain the same Python code that any other module can contain, and Python
+will add some additional attributes to the module when it is imported.
+
+
+Namespace packages
+------------------
+
+.. index::
+    pair:: package; namespace
+    pair:: package; portion
+
+A namespace package is a composite of various :term:`portions <portion>`,
+where each portion contributes a subpackage to the parent package.  Portions
+may reside in different locations on the file system.  Portions may also be
+found in zip files, on the network, or anywhere else that Python searches
+during import.  Namespace packages may or may not correspond directly to
+objects on the file system; they may be virtual modules that have no concrete
+representation.
+
+For example, the following file system layout defines a top level ``parent``
+package with three subpackages::
+
+    parent/
+        __init__.py
+        one/
+            __init__.py
+        two/
+            __init__.py
+        three/
+            __init__.py
+
+Importing ``parent.one`` will implicitly import ``parent/__init__.py`` and
+``parent/one/__init__.py``.  Subsequent imports of ``parent.two`` or
+``parent.three`` will import ``parent/two/__init__.py`` and
+``parent/three/__init__.py`` respectively.
+
+With namespace packages, there is no ``parent/__init__.py`` file.  In fact,
+there may be multiple ``parent`` directories found during import search, where
+each one is provided by a separate vendor installed container, and none of
+them contain an ``__init__.py`` file.  Thus ``parent/one`` may not be
+physically located next to ``parent/two``.  In this case, Python will create a
+namespace package for the top-level ``parent`` package whenever it or one of
+its subpackages is imported.
+
+
+Searching
+=========
+
+To begin the search, Python needs the :term:`fully qualified <qualified name>`
+name of the module (or package, but for the purposes of this discussion, the
+difference is immaterial) being imported.  This name may come from various
+arguments to the :keyword:`import` statement, or from the parameters to the
+:func:`__import__` function.
+
+This name will be used in various phases of the import search, and it may be
+the dotted path to a submodule, e.g. ``foo.bar.baz``.  In this case, Python
+first tries to import ``foo``, then ``foo.bar``, and finally ``foo.bar.baz``.
+If any of the intermediate imports fail, an :exc:`ImportError` is raised.
+
+
+The module cache
+----------------
+
+.. index::
+    single: sys.modules
+
+The first place checked during import search is :data:`sys.modules`.  This
+mapping serves as a cache of all modules that have been previously imported,
+including the intermediate paths.  So if ``foo.bar.baz`` was previously
+imported, :data:`sys.modules` will contain entries for ``foo``, ``foo.bar``,
+and ``foo.bar.baz``.  Each key will have as its value the corresponding module
+object.
+
+During import, the module name is looked up in :data:`sys.modules` and if
+present, the associated value is the module satisfying the import, and the
+process completes.  However, if the value is ``None``, then an
+:exc:`ImportError` is raised.  If the module name is missing, Python will
+continue searching for the module.
+
+:data:`sys.modules` is writable.  Deleting a key will generally not destroy
+the associated module, but it will invalidate the cache entry for the named
+module, causing Python to search anew for the named module upon its next
+import.  Beware though, because if you keep a reference to the module object,
+invalidate its cache entry in :data:`sys.modules`, and then re-import the
+named module, the two module objects will *not* be the same.  The key can also
+be assigned to ``None``, forcing the next import of the module to result in an
+:exc:`ImportError`.
+
+
+Finders and loaders
+-------------------
+
+.. index::
+    single: finder
+    single: loader
+
+If the named module is not found in :data:`sys.modules` then Python's import
+protocol is invoked to find and load the module.  As this implies, the import
+protocol consists of two conceptual objects, :term:`finders <finder>` and
+:term:`loaders <loader>`.  A finder's job is to determine whether it can find
+the named module using whatever strategy it knows about.  For example, there
+is a file system finder which know how to search the file system for the named
+module.  Other finders may know how to search a zip file, a web page, or a
+database to find the named module.  The import machinery is extensible, so new
+finders can be added to extend the range and scope of module searching.
+
+Finders do not actually load modules.  If they can find the named module, they
+return a loader, which the import machinery later invokes to load the module
+and create the corresponding module object.
+
+There are actually two types of finders, and two different but related APIs
+for finders, depending on whether it is a :term:`meta path finder` or a
+:term:`sys path finder`.  Meta path processing occurs at the beginning of
+import processing, while sys path processing happens later, by the :term:`path
+importer`.
+
+The following sections describe the protocol for finders and loaders in more
+detail, including how you can create and register new ones to extend the
+import machinery.
+
+
+Import hooks
+------------
+
+.. index::
+   single: import hooks
+   single: meta hooks
+   single: path hooks
+   pair: hooks; import
+   pair: hooks; meta
+   pair: hooks; path
+
+The import machinery is designed to be extensible; the primary mechanism for
+this are the *import hooks*.  There are two types of import hooks: *meta
+hooks* and *path hooks*.
+
+Meta hooks are called at the start of import processing, before any other
+import processing has occurred.  This allows meta hooks to override
+:data:`sys.path` processing, frozen modules, or even built-in modules.  Meta
+hooks are registered by adding new finder objects to :data:`sys.meta_path`, as
+described below.
+
+Path hooks are called as part of :data:`sys.path` (or ``package.__path__``)
+processing, at the point where their associated path item is encountered.
+Path hooks are registered by adding new callables to :data:`sys.path_hooks` as
+described below.
+
+
+The meta path
+-------------
+
+.. index::
+    single: sys.meta_path
+    pair: finder; find_module
+    pair: finder; find_loader
+
+When the named module is not found in :data:`sys.modules`, Python next
+searches :data:`sys.meta_path`, which contains a list of meta path finder
+objects.  These finders are queried in order to see if they know how to handle
+the named module.  Meta path finders must implement a method called
+:meth:`find_module()` which takes two arguments, a name and a path.  The meta
+path finder can use any strategy it wants to determine whether it can handle
+the named module or not.
+
+If the meta path finder knows how to handle the named module, it returns a
+loader object.  If it cannot handle the named module, it returns ``None``.  If
+:data:`sys.meta_path` processing reaches the end of its list without returning
+a loader, then an :exc:`ImportError` is raised.  Any other exceptions raised
+are simply propagated up, aborting the import process.
+
+The :meth:`find_module()` method of meta path finders is called with two
+arguments.  The first is the fully qualified name of the module being
+imported, for example ``foo.bar.baz``.  The second argument is the relative
+path for the module search.  For top-level modules, the second argument is
+``None``, but for submodules or subpackages, the second argument is the value
+of the parent package's ``__path__`` attribute, which must exist or an
+:exc:`ImportError` is raised.
+
+Python's default :data:`sys.meta_path` has three meta path finders, one that
+knows how to import built-in modules, one that knows how to import frozen
+modules, and one that knows how to import modules from the file system
+(i.e. the :term:`path importer`).
+
+
+Meta path loaders
+-----------------
+
+Once a loader is found via a meta path finder, the loader's
+:meth:`load_module()` method is called, with a single argument, the fully
+qualified name of the module being imported.  This method has several
+responsibilities, and should return the module object it has loaded [#fn1]_.
+If it cannot load the module, it should raise an :exc:`ImportError`, although
+any other exception raised during :meth:`load_module()` will be propagated.
+
+In many cases, the meta path finder and loader can be the same object,
+e.g. :meth:`finder.find_module()` would just return ``self``.
+
+Loaders must satisfy the following requirements:
+
+ * If there is an existing module object with the given name in
+   :data:`sys.modules`, the loader must use that existing module.  (Otherwise,
+   the :func:`reload()` builtin will not work correctly.)  If the named module
+   does not exist in :data:`sys.modules`, the loader must create a new module
+   object and add it to :data:`sys.modules`.
+
+   Note that the module *must* exist in :data:`sys.modules` before the loader
+   executes the module code.  This is crucial because the module code may
+   (directly or indirectly) import itself; adding it to :data:`sys.modules`
+   beforehand prevents unbounded recursion in the worst case and multiple
+   loading in the best.
+
+   If the load fails, the loader needs to remove any modules it may have
+   inserted into ``sys.modules``.  If the module was already in
+   ``sys.modules`` then the loader should leave it alone.
+
+ * The loader may set the ``__file__`` attribute of the module.  If set, this
+   attribute's value must be a string.  The loader may opt to leave
+   ``__file__`` unset if it has no semantic meaning (e.g. a module loaded from
+   a database).
+
+ * The loader may set the ``__name__`` attribute of the module.  While not
+   required, setting this attribute is highly recommended so that the
+   :meth:`repr()` of the module is more informative.
+
+ * If module is a package (either regular or namespace), the loader must set
+   the module object's ``__path__`` attribute.  The value must be a list, but
+   may be empty if ``__path__`` has no further significance to the importer.
+   More details on the semantics of ``__path__`` are given below.
+
+ * The ``__loader__`` attribute must be set to the loader object that loaded
+   the module.  This is mostly for introspection and reloading, but can be
+   used for additional importer-specific functionality, for example getting
+   data associated with an importer.
+
+ * The module's ``__package__`` attribute should be set.  Its value must be a
+   string, but it can be the same value as its ``__name__``.  This is the
+   recommendation when the module is a package.  When the module is not a
+   package, ``__package__`` should be set to the parent package's name.
+
+   This attribute is used instead of ``__name__`` to calculate explicit
+   relative imports for main modules, as defined in :pep:`366`.
+
+ * If the module is a Python module (as opposed to a built-in module or a
+   dynamically loaded extension), it should execute the module's code in the
+   module's global name space (``module.__dict__``).
+
+
+Module reprs
+------------
+
+By default, all modules have a usable repr, however depending on the
+attributes set above, and hooks in the loader, you can more tightly control
+the repr of module objects.
+
+Loaders may implement a :meth:`module_repr()` method which takes a single
+argument, the module object.  When ``repr(module)`` is called for a module
+with a loader supporting this protocol, whatever is returned from
+``loader.module_repr(module)`` is returned as the module's repr without
+further processing.  This return value must be a string.
+
+If the module has no ``__loader__`` attribute, or the loader has no
+:meth:`module_repr()` method, then the module object implementation itself
+will craft a default repr using whatever information is available.  It will
+try to use the ``module.__name__``, ``module.__file__``, and
+``module.__loader__`` as input into the repr, with defaults for whatever
+information is missing.
+
+Here are the exact rules used:
+
+ * If the module has an ``__loader__`` and that loader has a
+   :meth:`module_repr()` method, call it with a single argument, which is the
+   module object.  The value returned is used as the module's repr.
+
+ * If an exception occurs in :meth:`module_repr()`, the exception is caught
+   and discarded, and the calculation of the module's repr continues as if
+   :meth:`module_repr()` did not exist.
+
+ * If the module has an ``__file__`` attribute, this is used as part of the
+   module's repr.
+
+ * If the module has no ``__file__`` but does have an ``__loader__``, then the
+   loader's repr is used as part of the module's repr.
+
+ * Otherwise, just use the module's ``__name__`` in the repr.
+
+This example, from :pep:`420` shows how a loader can craft its own module
+repr::
+
+    class NamespaceLoader:
+        @classmethod
+        def module_repr(cls, module):
+            return "<module '{}' (namespace)>".format(module.__name__)
+
+
+module.__path__
+---------------
+
+By definition, if a module has an ``__path__`` attribute, it is a package,
+regardless of its value.
+
+A package's ``__path__`` attribute is used during imports of its subpackages.
+Within the import machinery, it functions much the same as :data:`sys.path`,
+i.e. providing a list of locations to search for modules during import.
+However, ``__path__`` is typically much more constrained than
+:data:`sys.path`.
+
+``__path__`` must be a list, but it may be empty.  The same rules used for
+:data:`sys.path` also apply to a package's ``__path__``, and
+:data:`sys.path_hooks` (described below) are consulted when traversing a
+package's ``__path__``.
+
+A package's ``__init__.py`` file may set or alter the package's ``__path__``
+attribute, and this was typically the way namespace packages were implemented
+prior to :pep:`420`.  With the adoption of :pep:`420`, namespace packages no
+longer need to supply ``__init__.py`` files containing only ``__path__``
+manipulation code; the namespace loader automatically sets ``__path__``
+correctly for the namespace package.
+
+
+The Path Importer
+=================
+
+.. index::
+    single: path importer
+
+As mentioned previously, Python comes with several default meta path finders.
+One of these, called the :term:`path importer`, knows how to provide
+traditional file system imports.  It implements all the semantics for finding
+modules on the file system, handling special file types such as Python source
+code (``.py`` files), Python byte code (``.pyc`` and ``.pyo`` files) and
+shared libraries (e.g. ``.so`` files).
+
+In addition to being able to find such modules, there is built-in support for
+loading these modules.  To accomplish these two related tasks, additional
+hooks and protocols are provided so that you can extend and customize the path
+importer semantics.
+
+A word of warning: this section and the previous both use the term *finder*,
+distinguishing between them by using the terms :term:`meta path finder` and
+:term:`sys path finder`.  Meta path finders and sys path finders are very
+similar, support similar protocols, and function in similar ways during the
+import process, but it's important to keep in mind that they are subtly
+different.  In particular, meta path finders operate at the beginning of the
+import process, as keyed off the :data:`sys.meta_path` traversal.
+
+On the other hand, sys path finders are in a sense an implementation detail of
+the path importer, and in fact, if the path importer were to be removed from
+:data:`sys.meta_path`, none of the sys path finder semantics would be invoked.
+
+
+sys path finders
+----------------
+
+.. index::
+    single: sys.path
+    single: sys.path_hooks
+    single: sys.path_importer_cache
+    single: PYTHONPATH
+
+The path importer is responsible for finding and loading Python modules and
+packages from the file system.  As a meta path finder, it implements the
+:meth:`find_module()` protocol previously described, however it exposes
+additional hooks that can be used to customize how modules are found and
+loaded from the file system.
+
+Three variables are used during file system import, :data:`sys.path`,
+:data:`sys.path_hooks` and :data:`sys.path_importer_cache`.  These provide
+additional ways that the import machinery can be customized, in this case
+specifically during file system path import.
+
+:data:`sys.path` contains a list of strings providing search locations for
+modules and packages.  It is initialized from the :data:`PYTHONPATH`
+environment variable and various other installation- and
+implementation-specific defaults.  Entries in :data:`sys.path` can name
+directories on the file system, zip files, and potentially other "locations"
+that should be searched for modules.
+
+The path importer is a meta path finder, so the import machinery begins file
+system search by calling the path importer's :meth:`find_module()` method as
+described previously.  When the ``path`` argument to :meth:`find_module()` is
+given, it will be a list of string paths to traverse.  If not,
+:data:`sys.path` is used.
+
+The path importer iterates over every entry in the search path, and for each
+of these, searches for an appropriate sys path finder for the path entry.
+Because this can be an expensive operation (e.g. there are `stat()` call
+overheads for this search), the path importer maintains a cache mapping path
+entries to sys path finders.  This cache is maintained in
+:data:`sys.path_importer_cache`.  In this way, the expensive search for a
+particular path location's sys path finder need only be done once.  User code
+is free to remove cache entries from :data:`sys.path_importer_cache` forcing
+the path importer to perform the path search again.
+
+If the path entry is not present in the cache, the path importer iterates over
+every callable in :data:`sys.path_hooks`.  Each entry in this list is called
+with a single argument, the path entry being searched.  This callable may
+either return a sys path finder that can handle the path entry, or it may
+raise :exc:`ImportError`.  An :exc:`ImportError` is used by the path importer
+to signal that the hook cannot find a sys path finder for that path entry.
+The exception is ignored and :data:`sys.path_hooks` iteration continues.
+
+If :data:`sys.path_hooks` iteration ends with no sys path finder being
+returned then the path importer's :meth:`find_module()` method will return
+``None`` and an :exc:`ImportError` will be raised.
+
+If a sys path finder *is* returned by one of the callables on
+:data:`sys.path_hooks`, then the following protocol is used to ask the sys
+path finder for a module loader.  If a loader results from this step, it is
+used to load the module as previously described (i.e. its
+:meth:`load_module()` method is called).
+
+
+sys path finder protocol
+------------------------
+
+sys path finders support the same, traditional :meth:`find_module()` method
+that meta path finders support, however sys path finder :meth:`find_module()`
+methods are never called with a ``path`` argument.
+
+The :meth:`find_module()` method on sys path finders is deprecated though, and
+instead sys path finders should implement the :meth:`find_loader()` method.
+If it exists on the sys path finder, :meth:`find_loader()` will always be
+called instead of :meth:`find_module()`.
+
+:meth:`find_loader()` takes one argument, the fully qualified name of the
+module being imported.  :meth:`find_loader()` returns a 2-tuple where the
+first item is the loader and the second item is a namespace :term:`portion`.
+When the first item (i.e. the loader) is ``None``, this means that while the
+sys path finder does not have a loader for the named module, it knows that the
+path entry contributes to a namespace portion for the named module.  This will
+almost always be the case where Python is asked to import a namespace package
+that has no physical presence on the file system.  When a sys path finder
+returns ``None`` for the loader, the second item of the 2-tuple return value
+must be a sequence, although it can be empty.
+
+If :meth:`find_loader()` returns a non-``None`` loader value, the portion is
+ignored and the loader is returned from the path importer, terminating the
+:data:`sys.path` search.
+
+
+Open issues
+===========
+
+XXX What to say about `imp.NullImporter` when it's found in
+:data:`sys.path_importer_cache`?
+
+XXX It would be really nice to have a diagram.
+
+.. [#fn1] The importlib implementation appears not to use the return value
+   directly. Instead, it gets the module object by looking the module name up
+   in ``sys.modules``.)
+
+
+References
+==========
+
+The import machinery has evolved considerably since Python's early days.  The
+original `specification for packages
+<http://www.python.org/doc/essays/packages.html>`_ is still available to read,
+although some details have changed since the writing of that document.
+
+The original specification for :data:`sys.meta_path` was :pep:`302`, with
+subsequent extension in :pep:`420`, which also introduced namespace packages
+without ``__init__.py`` files in Python 3.3.  :pep:`420` also introduced the
+:meth:`find_loader` protocol as an alternative to :meth:`find_module`.
+
+:pep:`366` describes the addition of the ``__package__`` attribute for
+explicit relative imports in main modules.

Doc/reference/index.rst

@@ -24,6 +24,7 @@ interfaces available to C/C++ programmers in detail.
    lexical_analysis.rst
    datamodel.rst
    executionmodel.rst
+   import_machinery.rst
    expressions.rst
    simple_stmts.rst
    compound_stmts.rst

Doc/reference/simple_stmts.rst

@@ -660,162 +660,50 @@ The :keyword:`import` statement
    relative_module: "."* `module` | "."+
    name: `identifier`
 
-Import statements are executed in two steps: (1) find a module, and initialize
+Import statements are executed in two steps: (1) find a module, loading and
-it if necessary; (2) define a name or names in the local namespace (of the scope
+initializing it if necessary; (2) define a name or names in the local
-where the :keyword:`import` statement occurs). The statement comes in two
+namespace (of the scope where the :keyword:`import` statement occurs). The
-forms differing on whether it uses the :keyword:`from` keyword. The first form
+statement comes in two forms differing on whether it uses the :keyword:`from`
-(without :keyword:`from`) repeats these steps for each identifier in the list.
+keyword. The first form (without :keyword:`from`) repeats these steps for each
-The form with :keyword:`from` performs step (1) once, and then performs step
+identifier in the list.  The form with :keyword:`from` performs step (1) once,
-(2) repeatedly. For a reference implementation of step (1), see the
+and then performs step (2) repeatedly.
-:mod:`importlib` module.
 
-.. index::
+The details of step (1), finding and loading modules is described in greater
-    single: package
+detail in the section on the :ref:`import machinery <importmachinery>`, which
-
+also describes the various types of packages and modules that can be imported,
-To understand how step (1) occurs, one must first understand how Python handles
+as well as all the hooks that can be used to customize Python's import.
-hierarchical naming of modules. To help organize modules and provide a
-hierarchy in naming, Python has a concept of packages. A package can contain
-other packages and modules while modules cannot contain other modules or
-packages. From a file system perspective, packages are directories and modules
-are files. The original `specification for packages
-<http://www.python.org/doc/essays/packages.html>`_ is still available to read,
-although minor details have changed since the writing of that document.
-
-.. index::
-    single: sys.modules
-
-Once the name of the module is known (unless otherwise specified, the term
-"module" will refer to both packages and modules), searching
-for the module or package can begin. The first place checked is
-:data:`sys.modules`, the cache of all modules that have been imported
-previously. If the module is found there then it is used in step (2) of import
-unless ``None`` is found in :data:`sys.modules`, in which case
-:exc:`ImportError` is raised.
-
-.. index::
-    single: sys.meta_path
-    single: finder
-    pair: finder; find_module
-    single: __path__
-
-If the module is not found in the cache, then :data:`sys.meta_path` is searched
-(the specification for :data:`sys.meta_path` can be found in :pep:`302`).
-The object is a list of :term:`finder` objects which are queried in order as to
-whether they know how to load the module by calling their :meth:`find_module`
-method with the name of the module. If the module happens to be contained
-within a package (as denoted by the existence of a dot in the name), then a
-second argument to :meth:`find_module` is given as the value of the
-:attr:`__path__` attribute from the parent package (everything up to the last
-dot in the name of the module being imported). If a finder can find the module
-it returns a :term:`loader` (discussed later) or returns ``None``.
-
-.. index::
-    single: sys.path_hooks
-    single: sys.path_importer_cache
-    single: sys.path
-
-If none of the finders on :data:`sys.meta_path` are able to find the module
-then some implicitly defined finders are queried. Implementations of Python
-vary in what implicit meta path finders are defined. The one they all do
-define, though, is one that handles :data:`sys.path_hooks`,
-:data:`sys.path_importer_cache`, and :data:`sys.path`.
-
-The implicit finder searches for the requested module in the "paths" specified
-in one of two places ("paths" do not have to be file system paths). If the
-module being imported is supposed to be contained within a package then the
-second argument passed to :meth:`find_module`, :attr:`__path__` on the parent
-package, is used as the source of paths. If the module is not contained in a
-package then :data:`sys.path` is used as the source of paths.
-
-Once the source of paths is chosen it is iterated over to find a finder that
-can handle that path. The dict at :data:`sys.path_importer_cache` caches
-finders for paths and is checked for a finder. If the path does not have a
-finder cached then :data:`sys.path_hooks` is searched by calling each object in
-the list with a single argument of the path, returning a finder or raises
-:exc:`ImportError`. If a finder is returned then it is cached in
-:data:`sys.path_importer_cache` and then used for that path entry. If no finder
-can be found but the path exists then a value of ``None`` is
-stored in :data:`sys.path_importer_cache` to signify that an implicit,
-file-based finder that handles modules stored as individual files should be
-used for that path. If the path does not exist then a finder which always
-returns ``None`` is placed in the cache for the path.
-
-.. index::
-    single: loader
-    pair: loader; load_module
-    exception: ImportError
-
-If no finder can find the module then :exc:`ImportError` is raised. Otherwise
-some finder returned a loader whose :meth:`load_module` method is called with
-the name of the module to load (see :pep:`302` for the original definition of
-loaders). A loader has several responsibilities to perform on a module it
-loads. First, if the module already exists in :data:`sys.modules` (a
-possibility if the loader is called outside of the import machinery) then it
-is to use that module for initialization and not a new module. But if the
-module does not exist in :data:`sys.modules` then it is to be added to that
-dict before initialization begins. If an error occurs during loading of the
-module and it was added to :data:`sys.modules` it is to be removed from the
-dict. If an error occurs but the module was already in :data:`sys.modules` it
-is left in the dict.
-
-.. index::
-    single: __name__
-    single: __file__
-    single: __path__
-    single: __package__
-    single: __loader__
-
-The loader must set several attributes on the module. :data:`__name__` is to be
-set to the name of the module. :data:`__file__` is to be the "path" to the file
-unless the module is built-in (and thus listed in
-:data:`sys.builtin_module_names`) in which case the attribute is not set.
-If what is being imported is a package then :data:`__path__` is to be set to a
-list of paths to be searched when looking for modules and packages contained
-within the package being imported. :data:`__package__` is optional but should
-be set to the name of package that contains the module or package (the empty
-string is used for module not contained in a package). :data:`__loader__` is
-also optional but should be set to the loader object that is loading the
-module. While loaders are required to return the module they loaded, import
-itself always retrieves any modules it returns from :data:`sys.modules`.
-
-.. index::
-    exception: ImportError
-
-If an error occurs during loading then the loader raises :exc:`ImportError` if
-some other exception is not already being propagated. Otherwise the loader
-returns the module that was loaded and initialized.
 
 When step (1) finishes without raising an exception, step (2) can begin.
 
-The first form of :keyword:`import` statement binds the module name in the local
+The first form of :keyword:`import` statement binds the module name in the
-namespace to the module object, and then goes on to import the next identifier,
+local namespace to the module object, and then goes on to import the next
-if any.  If the module name is followed by :keyword:`as`, the name following
+identifier, if any.  If the module name is followed by :keyword:`as`, the name
-:keyword:`as` is used as the local name for the module.
+following :keyword:`as` is used as the local name for the module.
 
 .. index::
    pair: name; binding
    exception: ImportError
 
-The :keyword:`from` form does not bind the module name: it goes through the list
+The :keyword:`from` form does not bind the module name: it goes through the
-of identifiers, looks each one of them up in the module found in step (1), and
+list of identifiers, looks each one of them up in the module found in step
-binds the name in the local namespace to the object thus found.  As with the
+(1), and binds the name in the local namespace to the object thus found.  As
-first form of :keyword:`import`, an alternate local name can be supplied by
+with the first form of :keyword:`import`, an alternate local name can be
-specifying ":keyword:`as` localname".  If a name is not found,
+supplied by specifying ":keyword:`as` localname".  If a name is not found,
-:exc:`ImportError` is raised.  If the list of identifiers is replaced by a star
+:exc:`ImportError` is raised.  If the list of identifiers is replaced by a
-(``'*'``), all public names defined in the module are bound in the local
+star (``'*'``), all public names defined in the module are bound in the local
 namespace of the :keyword:`import` statement.
 
 .. index:: single: __all__ (optional module attribute)
 
 The *public names* defined by a module are determined by checking the module's
-namespace for a variable named ``__all__``; if defined, it must be a sequence of
+namespace for a variable named ``__all__``; if defined, it must be a sequence
-strings which are names defined or imported by that module.  The names given in
+of strings which are names defined or imported by that module.  The names
-``__all__`` are all considered public and are required to exist.  If ``__all__``
+given in ``__all__`` are all considered public and are required to exist.  If
-is not defined, the set of public names includes all names found in the module's
+``__all__`` is not defined, the set of public names includes all names found
-namespace which do not begin with an underscore character (``'_'``).
+in the module's namespace which do not begin with an underscore character
-``__all__`` should contain the entire public API. It is intended to avoid
+(``'_'``).  ``__all__`` should contain the entire public API. It is intended
-accidentally exporting items that are not part of the API (such as library
+to avoid accidentally exporting items that are not part of the API (such as
-modules which were imported and used within the module).
+library modules which were imported and used within the module).
 
 The :keyword:`from` form with ``*`` may only occur in a module scope.  The wild
 card form of import --- ``import *`` --- is only allowed at the module level.

Lib/importlib/abc.py

@@ -33,16 +33,33 @@ class Loader(metaclass=abc.ABCMeta):
         The fullname is a str."""
         raise NotImplementedError
 
+    @abs.abstractmethod
+    def module_repr(self, module):
+        """Abstract method which when implemented calculates and returns the
+        given module's repr."""
+        raise NotImplementedError
+
 
 class Finder(metaclass=abc.ABCMeta):
 
     """Abstract base class for import finders."""
 
+    @abs.abstractmethod
+    def find_loader(self, fullname):
+        """Abstract method which when implemented returns a module loader.
+        The fullname is a str.  Returns a 2-tuple of (Loader, portion) where
+        portion is a sequence of file system locations contributing to part of
+        a namespace package.  The sequence may be empty.  When present,
+        `find_loader()` is preferred over `find_module()`.
+        """
+        raise NotImplementedError
+
     @abc.abstractmethod
     def find_module(self, fullname, path=None):
         """Abstract method which when implemented should find a module.
         The fullname is a str and the optional path is a str or None.
-        Returns a Loader object.
+        Returns a Loader object.  This method is only called if
+        `find_loader()` is not present.
         """
         raise NotImplementedError