Issue #15502: Bring the importlib.PathFinder docs and docstring more in line with the new import system documentation, and fix various parts of the new docs that weren't quite right given PEP 420 or were otherwise a bit misleading. Also note the key terminology problem still being discussed in the issue

2 files changed, 153 insertions, 83 deletions

diff --git a/Doc/library/importlib.rst b/Doc/library/importlib.rst
index 06d997f..0081e04 100644
--- a/Doc/library/importlib.rst
+++ b/Doc/library/importlib.rst
@@ -606,7 +606,7 @@ find and load modules.
 .. class:: WindowsRegistryFinder
 
    :term:`Finder` for modules declared in the Windows registry.  This class
-   implements the :class:`importlib.abc.MetaPathFinder` ABC.
+   implements the :class:`importlib.abc.Finder` ABC.
 
    Only class methods are defined by this class to alleviate the need for
    instantiation.
@@ -616,14 +616,8 @@ find and load modules.
 
 .. class:: PathFinder
 
-    :term:`Finder` for :data:`sys.path`. This class implements the
-    :class:`importlib.abc.MetaPathFinder` ABC.
-
-    This class does not perfectly mirror the semantics of :keyword:`import` in
-    terms of :data:`sys.path`. No implicit path hooks are assumed for
-    simplification of the class and its semantics. This implies that when
-    ``None`` is found in :data:`sys.path_importer_cache` that it is simply
-    ignored instead of implying a default finder.
+    :term:`Finder` for :data:`sys.path` and package ``__path__`` attributes.
+    This class implements the :class:`importlib.abc.MetaPathFinder` ABC.
 
     Only class methods are defined by this class to alleviate the need for
     instantiation.
@@ -639,7 +633,8 @@ find and load modules.
         :data:`sys.path_importer_cache`, then :data:`sys.path_hooks` is
         searched for a finder for the path entry and, if found, is stored in
         :data:`sys.path_importer_cache` along with being queried about the
-        module. If no finder is ever found then ``None`` is returned.
+        module. If no finder is ever found then ``None`` is both stored in
+        the cache and returned.
 
 
 .. class:: FileFinder(path, \*loader_details)
diff --git a/Doc/reference/import.rst b/Doc/reference/import.rst
index 50c041c..9eb093c 100644
--- a/Doc/reference/import.rst
+++ b/Doc/reference/import.rst
@@ -16,8 +16,8 @@ way.  Functions such as :func:`importlib.import_module` and built-in
 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 built-in :func:`__import__` function, with the appropriate
-arguments.  The return value of :func:`__import__` is used to perform the name
+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.
@@ -28,13 +28,19 @@ such as the importing of parent packages, and the updating of various caches
 (including :data:`sys.modules`), only the :keyword:`import` statement performs
 a name binding operation.
 
+When calling :func:`__import__` as part of an import statement, the
+import system first checks the module global namespace for a function by
+that name. If it is not found, then the standard builtin :func:`__import__`
+is called. Other mechanisms for invoking the import system (such as
+:func:`importlib.import_module`) do not perform this check and will always
+use the standard import system.
+
 When a module is first imported, Python searches for the module and if found,
 it creates a module object [#fnmo]_, 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.  More coarse-grained overriding of the import
-system can be accomplished by replacing built-in :func:`__import__`.
+described in the sections below.
 
 
 :mod:`importlib`
@@ -69,7 +75,7 @@ regular modules.
 
 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.  Specifically, any module that contains an ``__path__`` attribute is
+module.  Specifically, any module that contains a ``__path__`` attribute is
 considered a package.
 
 All modules have a name.  Subpackage names are separated from their parent
@@ -90,7 +96,7 @@ 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
+``__init__.py`` file is implicitly executed, 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.
@@ -107,9 +113,9 @@ package with three subpackages::
         three/
             __init__.py
 
-Importing ``parent.one`` will implicitly import ``parent/__init__.py`` and
+Importing ``parent.one`` will implicitly execute ``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`` will execute ``parent/two/__init__.py`` and
 ``parent/three/__init__.py`` respectively.
 
 
@@ -128,6 +134,12 @@ 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.
 
+Namespace packages do not use an ordinary list for their ``__path__``
+attribute. They instead use a custom iterable type which will automatically
+perform a new search for package portions on the next import attempt within
+that package if the path of their parent package (or :data:`sys.path` for a
+top level package) changes.
+
 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 different portion.  Thus ``parent/one`` may not be
@@ -172,14 +184,18 @@ 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 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,
+:data:`sys.modules` is writable.  Deleting a key may not destroy the
+associated module (as other modules may hold references to it),
+but it will invalidate the cache entry for the named module, causing
+Python to search anew for the named module upon its next
+import. The key can also be assigned to ``None``, forcing the next import
+of the module to result in an :exc:`ImportError`.
+
+Beware though, as 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`.
+named module, the two module objects will *not* be the same. By contrast,
+:func:`imp.reload` will reuse the *same* module object, and simply
+reinitialise the module contents by rerunning the module's code.
 
 
 Finders and loaders
@@ -193,14 +209,16 @@ If the named module is not found in :data:`sys.modules`, then Python's import
 protocol is invoked to find and load the module.  This 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.
+whatever strategy it knows about. Objects that implement both of these
+interfaces are referred to as :term:`importers <importer>` - they return
+themselves when they find that they can load the requested module.
 
-By default, Python comes with several default finders.  One knows how to
-locate frozen modules, and another knows how to locate built-in modules.  A
-third default finder searches an :term:`import path` for modules.  The
-:term:`import path` is a list of locations that may name file system paths or
-zip files.  It can also be extended to search for any locatable resource, such
-as those identified by URLs.
+By default, Python comes with several default finders and importers.  One
+knows how to locate frozen modules, and another knows how to locate
+built-in modules.  A third default finder searches an :term:`import path`
+for modules.  The :term:`import path` is a list of locations that may
+name file system paths or zip files.  It can also be extended to search
+for any locatable resource, such as those identified by URLs.
 
 The import machinery is extensible, so new finders can be added to extend the
 range and scope of module searching.
@@ -265,11 +283,26 @@ 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
-import path for the module search.  For top-level modules, this second
-argument will always be ``None``, but for submodules or subpackages, the
-second argument is the value of the parent package's ``__path__`` attribute,
-which must exist on the parent module or an :exc:`ImportError` is raised.
+imported, for example ``foo.bar.baz``.  The second argument is the path
+entries to use 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. If
+the appropriate ``__path__`` attribute cannot be accessed, an
+:exc:`ImportError` is raised.
+
+The meta path may be traversed multiple times for a single import request.
+For example, assuming none of the modules involved has already been cached,
+importing ``foo.bar.baz`` will first perform a top level import, calling
+``mpf.find_module("foo", None)`` on each meta path finder (``mpf``). After
+``foo`` has been imported, ``foo.bar`` will be imported by traversing the
+meta path a second time, calling
+``mpf.find_module("foo.bar", foo.__path__)``. Once ``foo.bar`` has been
+imported, the final traversal will call
+``mpf.find_module("foo.bar.baz", foo.bar.__path__)``.
+
+Some meta path finders only support top level imports. These importers will
+always return ``None`` when anything other than ``None`` is passed as the
+second argument.
 
 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
@@ -295,7 +328,7 @@ 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:`imp.reload` will not work correctly.)  If the named module does
+   :func:`imp.reload` 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`.
 
@@ -320,10 +353,12 @@ Loaders must satisfy the following requirements:
    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.
+ * If the module is a package (either regular or namespace), the loader must
+   set the module object's ``__path__`` attribute.  The value must be
+   iterable, but may be empty if ``__path__`` has no further significance
+   to the importer. If ``__path__`` is not empty, it must produce strings
+   when iterated over. More details on the semantics of ``__path__`` are
+   given :ref`below <package-path-rules>`.
 
  * The ``__loader__`` attribute must be set to the loader object that loaded
    the module.  This is mostly for introspection and reloading, but can be
@@ -369,7 +404,7 @@ information is missing.
 
 Here are the exact rules used:
 
- * If the module has an ``__loader__`` and that loader has a
+ * If the module has a ``__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.
 
@@ -377,10 +412,10 @@ Here are the exact rules used:
    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
+ * If the module has a ``__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
+ * If the module has no ``__file__`` but does have a ``__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.
@@ -394,6 +429,8 @@ repr::
             return "<module '{}' (namespace)>".format(module.__name__)
 
 
+.. _package-path-rules:
+
 module.__path__
 ---------------
 
@@ -406,10 +443,10 @@ 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) is consulted when traversing a
-package's ``__path__``.
+``__path__`` must be an iterable of strings, 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
@@ -430,15 +467,20 @@ One of these, called the :term:`path importer`, searches an :term:`import
 path`, which contains a list of :term:`path entries <path entry>`.  Each path
 entry names a location to search for modules.
 
-Path entries may name file system locations, and by default 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).
+The path importer itself doesn't know how to import anything. Instead, it
+traverses the individual path entries, associating each of them with a
+path entry finder that knows how to handle that particular kind of path.
+
+The default set of path entry finders implement 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). When supported by the :mod:`zipimport`
+module in the standard library, the default path entry finders also handle
+loading all of these file types (other than shared libraries) from zipfiles.
 
 Path entries need not be limited to file system locations.  They can refer to
-the contents of zip files, URLs, database queries, or any other location that
-can be specified as a string.
+the URLs, database queries, or any other location that can be specified as a
+string.
 
 The :term:`path importer` provides additional hooks and protocols so that you
 can extend and customize the types of searchable path entries.  For example,
@@ -483,8 +525,8 @@ loaded from the :term:`import path`.
 
 Three variables are used by the :term:`path importer`, :data:`sys.path`,
 :data:`sys.path_hooks` and :data:`sys.path_importer_cache`.  The ``__path__``
-attribute on package objects is also used.  These provide additional ways that
-the import machinery can be customized.
+attributes on package objects are also used.  These provide additional ways
+that the import machinery can be customized.
 
 :data:`sys.path` contains a list of strings providing search locations for
 modules and packages.  It is initialized from the :data:`PYTHONPATH`
@@ -495,11 +537,12 @@ directories on the file system, zip files, and potentially other "locations"
 URLs, or database queries.
 
 The :term:`path importer` is a :term:`meta path finder`, so the import
-machinery begins :term:`import path` 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 the ``path`` argument is not given or is ``None``,
-:data:`sys.path` is used.
+machinery begins the :term:`import path` 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 - typically a package's ``__path__``
+attribute for an import within that package.  If the ``path`` argument
+is ``None``, this indicates a top level import and :data:`sys.path` is used.
 
 The :term:`path importer` iterates over every entry in the search path, and
 for each of these, looks for an appropriate :term:`path entry finder` for the
@@ -523,8 +566,9 @@ exception is ignored and :term:`import path` iteration continues.
 
 If :data:`sys.path_hooks` iteration ends with no :term:`path entry finder`
 being returned, then the path importer's :meth:`find_module()` method will
-return ``None``, indicating that this :term:`meta path finder` could not find
-the module.
+store ``None`` in :data:`sys.path_importer_cache` (to indicate that there
+is no finder for this path entry) and return ``None``, indicating that
+this :term:`meta path finder` could not find the module.
 
 If a :term:`path entry finder` *is* returned by one of the :term:`path entry
 hook` callables on :data:`sys.path_hooks`, then the following protocol is used
@@ -534,29 +578,59 @@ to ask the finder for a module loader, which is then used to load the module.
 Path entry finder protocol
 --------------------------
 
-Path entry finders support the same :meth:`find_module()` method that meta
-path finders support, however path entry finder's :meth:`find_module()`
-methods are never called with a ``path`` argument.
-
-The :meth:`find_module()` method on path entry finders is deprecated though,
-and instead path entry finders should implement the :meth:`find_loader()`
-method.  If it exists on the path entry finder, :meth:`find_loader()` will
-always be called instead of :meth:`find_module()`.
+In order to support imports of modules and initialized packages and also to
+contribute portions to namespace packages, path entry finders must implement
+the :meth:`find_loader()` method.
 
 :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
-path entry finder does not have a loader for the named module, it knows that
-the :term:`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
-:term:`namespace package` that has no physical presence on the file system.
-When a path entry finder returns ``None`` for the loader, the second item of
-the 2-tuple return value must be a sequence, although it can be empty.
+path entry 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 path entry 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 :term:`path importer`, terminating
-the :term:`import path` search.
+ignored and the loader is returned from the path importer, terminating the
+search through the path entries.
+
+For backwards compatibility with other implementations of the import
+protocol, many path entry finders also support the same,
+traditional :meth:`find_module()` method that meta path finders support.
+However path entry finder :meth:`find_module()` methods are never called
+with a ``path`` argument (they are expected to record the appropriate
+path information from the initial call to the path hook).
+
+The :meth:`find_module()` method on path entry finders is deprecated,
+as it does not allow the path entry finder to contribute portions to
+namespace packages. Instead path entry finders should implement the
+:meth:`find_loader()` method as described above. If it exists on the path
+entry finder, the import system will always call :meth:`find_loader()`
+in preference to :meth:`find_module()`.
+
+
+Replacing the standard import system
+====================================
+
+The most reliable mechanism for replacing the entire import system is to
+delete the default contents of :data:`sys.meta_path`, replacing them
+entirely with a custom meta path hook.
+
+If it is acceptable to only alter the behaviour of import statements
+without affecting other APIs that access the import system, then replacing
+the builtin :func:`__import__` function may be sufficient. This technique
+may also be employed at the module level to only alter the behaviour of
+import statements within that module.
+
+To selectively prevent import of some modules from a hook early on the
+meta path (rather than disabling the standard import system entirely),
+it is sufficient to raise :exc:`ImportError` directly from
+:meth:`find_module` instead of returning ``None``. The latter indicates
+that the meta path search should continue. while raising an exception
+terminates it immediately.
 
 
 Open issues
@@ -568,11 +642,12 @@ XXX * (import_machinery.rst) how about a section devoted just to the
 attributes of modules and packages, perhaps expanding upon or supplanting the
 related entries in the data model reference page?
 
-XXX Module reprs: how does module.__qualname__ fit in?
-
 XXX runpy, pkgutil, et al in the library manual should all get "See Also"
 links at the top pointing to the new import system section.
 
+XXX The :term:`path importer` is not, in fact, an :term:`importer`. That's
+why the corresponding implementation class is :class:`importlib.PathFinder`.
+
 
 References
 ==========