Issue #23936: Clarify what finders are.

Thanks to Raúl Cumplido for the bug report and Thomas Kluyver for the
patch.

Doc/glossary.rst

@@ -308,10 +308,14 @@ Glossary
       A synonym for :term:`file object`.
 
    finder
-      An object that tries to find the :term:`loader` for a module. It must
-      implement either a method named :meth:`find_loader` or a method named
-      :meth:`find_module`. See :pep:`302` and :pep:`420` for details and
-      :class:`importlib.abc.Finder` for an :term:`abstract base class`.
+      An object that tries to find the :term:`loader` for a module that is
+      being imported.
+
+      Since Python 3.3, there are two types of finder: :term:`meta path finders
+      <meta path finder>` for use with :data:`sys.meta_path`, and :term:`path
+      entry finders <path entry finder>` for use with :data:`sys.path_hooks`.
+
+      See :pep:`302`, :pep:`420` and :pep:`451` for much more detail.
 
    floor division
       Mathematical division that rounds down to nearest integer.  The floor
@@ -593,10 +597,13 @@ Glossary
       :class:`collections.OrderedDict` and :class:`collections.Counter`.
 
    meta path finder
-      A finder returned by a search of :data:`sys.meta_path`.  Meta path
+      A :term:`finder` returned by a search of :data:`sys.meta_path`.  Meta path
       finders are related to, but different from :term:`path entry finders
       <path entry finder>`.
 
+      See :class:`importlib.abc.MetaPathFinder` for the methods that meta path
+      finders implement.
+
    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
@@ -630,7 +637,7 @@ Glossary
 
    module spec
       A namespace containing the import-related information used to load a
-      module.
+      module. An instance of :class:`importlib.machinery.ModuleSpec`.
 
    MRO
       See :term:`method resolution order`.
@@ -757,6 +764,9 @@ Glossary
       (i.e. a :term:`path entry hook`) which knows how to locate modules given
       a :term:`path entry`.
 
+      See :class:`importlib.abc.PathEntryFinder` for the methods that path entry
+      finders implement.
+
    path entry hook
       A callable on the :data:`sys.path_hook` list which returns a :term:`path
       entry finder` if it knows how to find modules on a specific :term:`path

Doc/library/sys.rst

@@ -774,19 +774,32 @@ always available.
 
 .. data:: meta_path
 
-    A list of :term:`finder` objects that have their :meth:`find_module`
-    methods called to see if one of the objects can find the module to be
-    imported. The :meth:`find_module` method is called at least with the
-    absolute name of the module being imported. If the module to be imported is
-    contained in package then the parent package's :attr:`__path__` attribute
-    is passed in as a second argument. The method returns ``None`` if
-    the module cannot be found, else returns a :term:`loader`.
-
-    :data:`sys.meta_path` is searched before any implicit default finders or
-    :data:`sys.path`.
-
-    See :pep:`302` for the original specification.
-
+    A list of :term:`meta path finder` objects that have their
+    :meth:`~importlib.abc.MetaPathFinder.find_spec` methods called to see if one
+    of the objects can find the module to be imported. The
+    :meth:`~importlib.abc.MetaPathFinder.find_spec` method is called with at
+    least the absolute name of the module being imported. If the module to be
+    imported is contained in a package, then the parent package's :attr:`__path__`
+    attribute is passed in as a second argument. The method returns a
+    :term:`module spec`, or ``None`` if the module cannot be found.
+
+    .. seealso::
+
+        :class:`importlib.abc.MetaPathFinder`
+          The abstract base class defining the interface of finder objects on
+          :data:`meta_path`.
+        :class:`importlib.machinery.ModuleSpec`
+          The concrete class which
+          :meth:`~importlib.abc.MetaPathFinder.find_spec` should return
+          instances of.
+
+    .. versionchanged:: 3.4
+
+        :term:`Module specs <module spec>` were introduced in Python 3.4, by
+        :pep:`451`. Earlier versions of Python looked for a method called
+        :meth:`~importlib.abc.MetaPathFinder.find_module`.
+        This is still called as a fallback if a :data:`meta_path` entry doesn't
+        have a :meth:`~importlib.abc.MetaPathFinder.find_spec` method.
 
 .. data:: modules