Issue #15502: Finish bringing importlib.abc in line with the current

state of the import system. Also make importlib.invalidate_caches()
work with sys.meta_path instead of sys.path_importer_cache to
completely separate the path-based import system from the overall
import system.

Patch by Eric Snow.

3 files changed, 41 insertions, 19 deletions

diff --git a/Lib/importlib/__init__.py b/Lib/importlib/__init__.py
index d30691a..6f40dac 100644
--- a/Lib/importlib/__init__.py
+++ b/Lib/importlib/__init__.py
@@ -38,9 +38,9 @@ from ._bootstrap import __import__
 
 
 def invalidate_caches():
-    """Call the invalidate_caches() method on all finders stored in
-    sys.path_importer_caches (where implemented)."""
-    for finder in sys.path_importer_cache.values():
+    """Call the invalidate_caches() method on all meta path finders stored in
+    sys.meta_path (where implemented)."""
+    for finder in sys.meta_path:
         if hasattr(finder, 'invalidate_caches'):
             finder.invalidate_caches()
 
diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index 4f61a5b..7b12ab0 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -1186,6 +1186,14 @@ class PathFinder:
     """Meta path finder for sys.path and package __path__ attributes."""
 
     @classmethod
+    def invalidate_caches(cls):
+        """Call the invalidate_caches() method on all path entry finders
+        stored in sys.path_importer_caches (where implemented)."""
+        for finder in sys.path_importer_cache.values():
+            if hasattr(finder, 'invalidate_caches'):
+                finder.invalidate_caches()
+
+    @classmethod
     def _path_hooks(cls, path):
         """Search sequence of hooks for a finder for 'path'.
 
@@ -1235,14 +1243,14 @@ class PathFinder:
                     portions = []
                 if loader is not None:
                     # We found a loader: return it immediately.
-                    return (loader, namespace_path)
+                    return loader, namespace_path
                 # This is possibly part of a namespace package.
                 #  Remember these path entries (if any) for when we
                 #  create a namespace package, and continue iterating
                 #  on path.
                 namespace_path.extend(portions)
         else:
-            return (None, namespace_path)
+            return None, namespace_path
 
     @classmethod
     def find_module(cls, fullname, path=None):
diff --git a/Lib/importlib/abc.py b/Lib/importlib/abc.py
index e8a0541..cd687e8 100644
--- a/Lib/importlib/abc.py
+++ b/Lib/importlib/abc.py
@@ -25,26 +25,22 @@ def _register(abstract_cls, *classes):
 
 class Finder(metaclass=abc.ABCMeta):
 
-    """Common abstract base class for import finders.
+    """Legacy abstract base class for import finders.
 
-    Finder implementations should derive from the more specific
-    MetaPathFinder or PathEntryFinder ABCs rather than directly from Finder.
+    It may be subclassed for compatibility with legacy third party
+    reimplementations of the import system.  Otherwise, finder
+    implementations should derive from the more specific MetaPathFinder
+    or PathEntryFinder ABCs.
     """
 
+    @abc.abstractmethod
     def find_module(self, fullname, path=None):
-        """An optional legacy method that should find a module.
+        """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.
-
-        The path finder will use this method only if find_loader() does
-        not exist. It may optionally be implemented for compatibility
-        with legacy third party reimplementations of the import system.
         """
         raise NotImplementedError
 
-    # invalidate_caches() is a completely optional method, so no default
-    # implementation is provided. See the docs for details.
-
 
 class MetaPathFinder(Finder):
 
@@ -52,12 +48,18 @@ class MetaPathFinder(Finder):
 
     @abc.abstractmethod
     def find_module(self, fullname, path):
-        """Abstract method which when implemented should find a module.
+        """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.
         """
         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)
 
@@ -68,13 +70,25 @@ 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.
         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.
+        a namespace package.  The sequence may be empty and the loader may be
+        None.
         """
         raise NotImplementedError
 
+    def find_module(self, fullname):
+        """Compatibility function which is the equivalent of
+        self.find_loader(fullname)[0]."""
+        return self.find_loader(fullname)[0]
+
+    def invalidate_caches(self):
+        """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)