Issue #17093,17566,17567: Methods from classes in importlib.abc now raise/return

the default exception/value when called instead of raising/returning
NotimplementedError/NotImplemented (except where appropriate).
This should allow for the ABCs to act as the bottom/end of the MRO with expected
default results.

As part of this work, also make importlib.abc.Loader.module_repr()
optional instead of an abstractmethod.

Doc/library/importlib.rst

@@ -153,6 +153,10 @@ ABC hierarchy::
       module.  Originally specified in :pep:`302`, this method was meant
       for use in :data:`sys.meta_path` and in the path-based import subsystem.
 
+      .. versionchanged:: 3.4
+         Returns ``None`` when called instead of raising
+         :exc:`NotImplementedError`.
+
 
 .. class:: MetaPathFinder
 
@@ -169,12 +173,19 @@ ABC hierarchy::
       will be the value of :attr:`__path__` from the parent
       package. If a loader cannot be found, ``None`` is returned.
 
+      .. versionchanged:: 3.4
+         Returns ``None`` when called instead of raising
+         :exc:`NotImplementedError`.
+
    .. method:: invalidate_caches()
 
       An optional method which, when called, should invalidate any internal
       cache used by the finder. Used by :func:`importlib.invalidate_caches`
       when invalidating the caches of all finders on :data:`sys.meta_path`.
 
+      .. versionchanged:: 3.4
+         Returns ``None`` when called instead of ``NotImplemented``.
+
 
 .. class:: PathEntryFinder
 
@@ -182,7 +193,7 @@ ABC hierarchy::
    it bears some similarities to :class:`MetaPathFinder`, ``PathEntryFinder``
    is meant for use only within the path-based import subsystem provided
    by :class:`PathFinder`. This ABC is a subclass of :class:`Finder` for
-   compatibility.
+   compatibility reasons only.
 
    .. versionadded:: 3.3
 
@@ -194,9 +205,12 @@ ABC hierarchy::
       package. The loader may be ``None`` while specifying ``portion`` to
       signify the contribution of the file system locations to a namespace
       package. An empty list can be used for ``portion`` to signify the loader
-      is not part of a package. If ``loader`` is ``None`` and ``portion`` is
-      the empty list then no loader or location for a namespace package were
-      found (i.e. failure to find anything for the module).
+      is not part of a namespace package. If ``loader`` is ``None`` and
+      ``portion`` is the empty list then no loader or location for a namespace
+      package were found (i.e. failure to find anything for the module).
+
+      .. versionchanged:: 3.4
+         Returns ``(None, [])`` instead of raising :exc:`NotImplementedError`.
 
    .. method:: find_module(fullname):
 
@@ -249,21 +263,29 @@ ABC hierarchy::
         - :attr:`__package__`
             The parent package for the module/package. If the module is
             top-level then it has a value of the empty string. The
-            :func:`importlib.util.set_package` decorator can handle the details
-            for :attr:`__package__`.
+            :func:`importlib.util.module_for_loader` decorator can handle the
+            details for :attr:`__package__`.
 
         - :attr:`__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 :term:`loader` is used.)
+            The loader used to load the module. The
+            :func:`importlib.util.module_for_loader` decorator can handle the
+            details for :attr:`__package__`.
+
+        .. versionchanged:: 3.4
+           Raise :exc:`ImportError` when called instead of
+           :exc:`NotImplementedError`.
 
     .. method:: module_repr(module)
 
-        An abstract method which when implemented calculates and returns the
-        given module's repr, as a string.
+        An optional method which when implemented calculates and returns the
+        given module's repr, as a string. The module type's default repr() will
+        use the result of this method as appropriate.
 
         .. versionadded: 3.3
 
+        .. versionchanged:: 3.4
+        Made optional instead of an abstractmethod.
+
 
 .. class:: ResourceLoader
 
@@ -281,6 +303,9 @@ ABC hierarchy::
         be found. The *path* is expected to be constructed using a module's
         :attr:`__file__` attribute or an item from a package's :attr:`__path__`.
 
+        .. versionchanged:: 3.4
+           Raises :exc:`IOError` instead of :exc:`NotImplementedError`.
+
 
 .. class:: InspectLoader
 
@@ -297,6 +322,9 @@ ABC hierarchy::
         .. index::
            single: universal newlines; importlib.abc.InspectLoader.get_source method
 
+        .. versionchanged:: 3.4
+           Raises :exc:`ImportError` instead of :exc:`NotImplementedError`.
+
     .. method:: get_source(fullname)
 
         An abstract method to return the source of a module. It is returned as
@@ -305,12 +333,18 @@ ABC hierarchy::
         if no source is available (e.g. a built-in module). Raises
         :exc:`ImportError` if the loader cannot find the module specified.
 
+        .. versionchanged:: 3.4
+           Raises :exc:`ImportError` instead of :exc:`NotImplementedError`.
+
     .. method:: is_package(fullname)
 
         An abstract method to return a true value if the module is a package, a
         false value otherwise. :exc:`ImportError` is raised if the
         :term:`loader` cannot find the module.
 
+        .. versionchanged:: 3.4
+           Raises :exc:`ImportError` instead of :exc:`NotImplementedError`.
+
 
 .. class:: ExecutionLoader
 
@@ -328,6 +362,9 @@ ABC hierarchy::
         the source file, regardless of whether a bytecode was used to load the
         module.
 
+        .. versionchanged:: 3.4
+           Raises :exc:`ImportError` instead of :exc:`NotImplementedError`.
+
 
 .. class:: FileLoader(fullname, path)
 
@@ -392,10 +429,13 @@ ABC hierarchy::
         - ``'size'`` (optional): the size in bytes of the source code.
 
         Any other keys in the dictionary are ignored, to allow for future
-        extensions.
+        extensions. If the path cannot be handled, :exc:`IOError` is raised.
 
         .. versionadded:: 3.3
 
+        .. versionchanged:: 3.4
+           Raise :exc:`IOError` instead of :exc:`NotImplementedError`.
+
     .. method:: path_mtime(path)
 
         Optional abstract method which returns the modification time for the
@@ -404,7 +444,10 @@ ABC hierarchy::
         .. deprecated:: 3.3
            This method is deprecated in favour of :meth:`path_stats`.  You don't
            have to implement it, but it is still available for compatibility
-           purposes.
+           purposes. Raise :exc:`IOError` if the path cannot be handled.
+
+          .. versionchanged:: 3.4
+             Raise :exc:`IOError` instead of :exc:`NotImplementedError`.
 
     .. method:: set_data(path, data)
 
@@ -415,6 +458,9 @@ ABC hierarchy::
         When writing to the path fails because the path is read-only
         (:attr:`errno.EACCES`), do not propagate the exception.
 
+        .. versionchanged:: 3.4
+           No longer raises :exc:`NotImplementedError` when called.
+
     .. method:: source_to_code(data, path)
 
         Create a code object from Python source.

Lib/importlib/_bootstrap.py

@@ -893,8 +893,10 @@ class SourceLoader(_LoaderBasics):
     def path_mtime(self, path):
         """Optional method that returns the modification time (an int) for the
         specified path, where path is a str.
+
+        Raises IOError when the path cannot be handled.
         """
-        raise NotImplementedError
+        raise IOError
 
     def path_stats(self, path):
         """Optional method returning a metadata dict for the specified path
@@ -905,6 +907,7 @@ class SourceLoader(_LoaderBasics):
         - 'size' (optional) is the size in bytes of the source code.
 
         Implementing this method allows the loader to read bytecode files.
+        Raises IOError when the path cannot be handled.
         """
         return {'mtime': self.path_mtime(path)}
 
@@ -922,9 +925,7 @@ class SourceLoader(_LoaderBasics):
         """Optional method which writes data (bytes) to a file path (a str).
 
         Implementing this method allows for the writing of bytecode files.
-
         """
-        raise NotImplementedError
 
 
     def get_source(self, fullname):
@@ -973,7 +974,7 @@ class SourceLoader(_LoaderBasics):
         else:
             try:
                 st = self.path_stats(source_path)
-            except NotImplementedError:
+            except IOError:
                 pass
             else:
                 source_mtime = int(st['mtime'])

Lib/importlib/abc.py

@@ -37,9 +37,8 @@ class Finder(metaclass=abc.ABCMeta):
     def find_module(self, fullname, path=None):
         """An abstract method that 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 or None.
         """
-        raise NotImplementedError
 
 
 class MetaPathFinder(Finder):
@@ -49,16 +48,14 @@ class MetaPathFinder(Finder):
     @abc.abstractmethod
     def find_module(self, fullname, path):
         """Abstract method which, when implemented, should find a module.
-        The fullname is a str and the path is a str or None.
-        Returns a Loader object.
+        The fullname is a str and the path is a list of strings or None.
+        Returns a Loader object or None.
         """
-        raise NotImplementedError
 
     def invalidate_caches(self):
         """An optional method for clearing the finder's cache, if any.
         This method is used by importlib.invalidate_caches().
         """
-        return NotImplemented
 
 _register(MetaPathFinder, machinery.BuiltinImporter, machinery.FrozenImporter,
           machinery.PathFinder, machinery.WindowsRegistryFinder)
@@ -70,13 +67,14 @@ class PathEntryFinder(Finder):
 
     @abc.abstractmethod
     def find_loader(self, fullname):
-        """Abstract method which, when implemented, returns a module loader.
+        """Abstract method which, when implemented, returns a module loader or
+        a possible part of a namespace.
         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 and the loader may be
         None.
         """
-        raise NotImplementedError
+        return None, []
 
     find_module = _bootstrap._find_module_shim
 
@@ -84,25 +82,34 @@ class PathEntryFinder(Finder):
         """An optional method for clearing the finder's cache, if any.
         This method is used by PathFinder.invalidate_caches().
         """
-        return NotImplemented
 
 _register(PathEntryFinder, machinery.FileFinder)
 
 
 class Loader(metaclass=abc.ABCMeta):
 
-    """Abstract base class for import loaders."""
+    """Abstract base class for import loaders.
+
+    The optional method module_repr(module) may be defined to provide a
+    repr for a module when appropriate (see PEP 420). The __repr__() method on
+    the module type will use the method as appropriate.
+
+    """
 
     @abc.abstractmethod
     def load_module(self, fullname):
         """Abstract method which when implemented should load a module.
-        The fullname is a str."""
-        raise NotImplementedError
+        The fullname is a str.
+
+        ImportError is raised on failure.
+        """
+        raise ImportError
 
-    @abc.abstractmethod
     def module_repr(self, module):
-        """Abstract method which when implemented calculates and returns the
-        given module's repr."""
+        """Return a module's repr.
+
+        Used by the module type when implemented without raising an exception.
+        """
         raise NotImplementedError
 
 
@@ -119,7 +126,7 @@ class ResourceLoader(Loader):
     def get_data(self, path):
         """Abstract method which when implemented should return the bytes for
         the specified path.  The path must be a str."""
-        raise NotImplementedError
+        raise IOError
 
 
 class InspectLoader(Loader):
@@ -134,20 +141,29 @@ class InspectLoader(Loader):
     @abc.abstractmethod
     def is_package(self, fullname):
         """Abstract method which when implemented should return whether the
-        module is a package.  The fullname is a str.  Returns a bool."""
-        raise NotImplementedError
+        module is a package.  The fullname is a str.  Returns a bool.
+
+        Raises ImportError is the module cannot be found.
+        """
+        raise ImportError
 
     @abc.abstractmethod
     def get_code(self, fullname):
         """Abstract method which when implemented should return the code object
-        for the module.  The fullname is a str.  Returns a types.CodeType."""
-        raise NotImplementedError
+        for the module.  The fullname is a str.  Returns a types.CodeType.
+
+        Raises ImportError if the module cannot be found.
+        """
+        raise ImportError
 
     @abc.abstractmethod
     def get_source(self, fullname):
         """Abstract method which should return the source code for the
-        module.  The fullname is a str.  Returns a str."""
-        raise NotImplementedError
+        module.  The fullname is a str.  Returns a str.
+
+        Raises ImportError if the module cannot be found.
+        """
+        raise ImportError
 
 _register(InspectLoader, machinery.BuiltinImporter, machinery.FrozenImporter,
             machinery.ExtensionFileLoader)
@@ -165,8 +181,11 @@ class ExecutionLoader(InspectLoader):
     @abc.abstractmethod
     def get_filename(self, fullname):
         """Abstract method which should return the value that __file__ is to be
-        set to."""
-        raise NotImplementedError
+        set to.
+
+        Raises ImportError if the module cannot be found.
+        """
+        raise ImportError
 
 
 class FileLoader(_bootstrap.FileLoader, ResourceLoader, ExecutionLoader):
@@ -198,7 +217,7 @@ class SourceLoader(_bootstrap.SourceLoader, ResourceLoader, ExecutionLoader):
     def path_mtime(self, path):
         """Return the (int) modification time for the path (str)."""
         if self.path_stats.__func__ is SourceLoader.path_stats:
-            raise NotImplementedError
+            raise IOError
         return int(self.path_stats(path)['mtime'])
 
     def path_stats(self, path):
@@ -209,7 +228,7 @@ class SourceLoader(_bootstrap.SourceLoader, ResourceLoader, ExecutionLoader):
         - 'size' (optional) is the size in bytes of the source code.
         """
         if self.path_mtime.__func__ is SourceLoader.path_mtime:
-            raise NotImplementedError
+            raise IOError
         return {'mtime': self.path_mtime(path)}
 
     def set_data(self, path, data):
@@ -220,8 +239,6 @@ class SourceLoader(_bootstrap.SourceLoader, ResourceLoader, ExecutionLoader):
         Any needed intermediary directories are to be created. If for some
         reason the file cannot be written because of permissions, fail
         silently.
-
         """
-        raise NotImplementedError
 
 _register(SourceLoader, machinery.SourceFileLoader)

Misc/NEWS

@@ -30,6 +30,13 @@ Core and Builtins
 Library
 -------
 
+- Issue #17093: Make the ABCs in importlib.abc provide default values or raise
+  reasonable exceptions for their methods to make them more amenable to super()
+  calls.
+
+- Issue #17566: Make importlib.abc.Loader.module_repr() optional instead of an
+  abstractmethod and raising NotImplementedError so as to be ignored by default.
+
 - Issue #17678: Remove the use of deprecated method in http/cookiejar.py.
   Changing the usage of get_origin_req_host() to origin_req_host.