bpo-42134: Raise ImportWarning when calling find_module()

Doc/reference/import.rst

@@ -329,6 +329,10 @@ modules, and one that knows how to import modules from an :term:`import path`
    import machinery will try it only if the finder does not implement
    ``find_spec()``.
 
+.. versionchanged:: 3.10
+   Use of :meth:`~importlib.abc.MetaPathFinder.find_module` by the import system
+   now raises :exc:`ImportWarning`.
+
 
 Loading
 =======
@@ -470,6 +474,9 @@ import machinery will create the new module itself.
    An :exc:`ImportError` is raised when ``exec_module()`` is defined but
    ``create_module()`` is not.
 
+.. versionchanged:: 3.10
+   Use of ``load_module()`` will raise :exc:`ImportWarning`.
+
 Submodules
 ----------
 
@@ -896,6 +903,10 @@ a list containing the portion.
    exist on a path entry finder, the import system will always call
    ``find_loader()`` in preference to ``find_module()``.
 
+.. versionchanged:: 3.10
+    Calls to :meth:`~importlib.abc.PathEntryFinder.find_module` by the import
+    system will raise :exc:`ImportWarning`.
+
 
 Replacing the standard import system
 ====================================

Doc/whatsnew/3.10.rst

@@ -1004,6 +1004,15 @@ Deprecated
   :meth:`~importlib.abc.Loader.exec_module` is preferred.
   (Contributed by Brett Cannon in :issue:`26131`.)
 
+* The use of :meth:`importlib.abc.MetaPathFinder.find_module` and
+  :meth:`importlib.abc.PathEntryFinder.find_module` by the import system now
+  trigger an :exc:`ImportWarning` as
+  :meth:`importlib.abc.MetaPathFinder.find_spec` and
+  :meth:`importlib.abc.PathEntryFinder.find_spec`
+  are preferred, respectively. You can use
+  :func:`importlib.util.spec_from_loader` to help in porting.
+  (Contributed by Brett Cannon in  :issue:`42134`.)
+
 * The import system now uses the ``__spec__`` attribute on modules before
   falling back on :meth:`~importlib.abc.Loader.module_repr` for a module's
   ``__repr__()`` method. Removal of the use of ``module_repr()`` is scheduled

Lib/importlib/_bootstrap.py

@@ -903,8 +903,9 @@ def _resolve_name(name, package, level):
 
 
 def _find_spec_legacy(finder, name, path):
-    # This would be a good place for a DeprecationWarning if
-    # we ended up going that route.
+    msg = (f"{_object_name(finder)}.find_spec() not found; "
+                           "falling back to find_module()")
+    _warnings.warn(msg, ImportWarning)
     loader = finder.find_module(name, path)
     if loader is None:
         return None

Lib/importlib/_bootstrap_external.py

@@ -1324,6 +1324,9 @@ def _legacy_get_spec(cls, fullname, finder):
         if hasattr(finder, 'find_loader'):
             loader, portions = finder.find_loader(fullname)
         else:
+            msg = (f"{_bootstrap._object_name(finder)}.find_spec() not found; "
+                           "falling back to find_module()")
+            _warnings.warn(msg, ImportWarning)
             loader = finder.find_module(fullname)
             portions = []
         if loader is not None:

Lib/test/test_importlib/import_/test_path.py

@@ -123,12 +123,16 @@ def find_module(self, fullname):
         failing_finder.to_return = None
         path = 'testing path'
         with util.import_state(path_importer_cache={path: failing_finder}):
-            self.assertIsNone(
+            with warnings.catch_warnings():
+                warnings.simplefilter("ignore", ImportWarning)
+                self.assertIsNone(
                     self.machinery.PathFinder.find_spec('whatever', [path]))
         success_finder = TestFinder()
         success_finder.to_return = __loader__
         with util.import_state(path_importer_cache={path: success_finder}):
-            spec = self.machinery.PathFinder.find_spec('whatever', [path])
+            with warnings.catch_warnings():
+                warnings.simplefilter("ignore", ImportWarning)
+                spec = self.machinery.PathFinder.find_spec('whatever', [path])
         self.assertEqual(spec.loader, __loader__)
 
     def test_finder_with_find_loader(self):
@@ -248,7 +252,9 @@ def find_module(fullname):
 
         with util.import_state(path=[Finder.path_location]+sys.path[:],
                                path_hooks=[Finder]):
-            self.machinery.PathFinder.find_spec('importlib')
+            with warnings.catch_warnings():
+                warnings.simplefilter("ignore", ImportWarning)
+                self.machinery.PathFinder.find_spec('importlib')
 
     def test_finder_with_failing_find_module(self):
         # PathEntryFinder with find_module() defined should work.
@@ -266,7 +272,9 @@ def find_module(fullname):
 
         with util.import_state(path=[Finder.path_location]+sys.path[:],
                                path_hooks=[Finder]):
-            self.machinery.PathFinder.find_module('importlib')
+            with warnings.catch_warnings():
+                warnings.simplefilter("ignore", ImportWarning)
+                self.machinery.PathFinder.find_module('importlib')
 
 
 (Frozen_PEFTests,

Lib/test/test_importlib/test_api.py

@@ -151,6 +151,7 @@ def test_success(self):
             with test_util.import_state(meta_path=[self.FakeMetaFinder]):
                 with warnings.catch_warnings():
                     warnings.simplefilter('ignore', DeprecationWarning)
+                    warnings.simplefilter('ignore', ImportWarning)
                     self.assertEqual((name, None), self.init.find_loader(name))
 
     def test_success_path(self):
@@ -161,6 +162,7 @@ def test_success_path(self):
             with test_util.import_state(meta_path=[self.FakeMetaFinder]):
                 with warnings.catch_warnings():
                     warnings.simplefilter('ignore', DeprecationWarning)
+                    warnings.simplefilter('ignore', ImportWarning)
                     self.assertEqual((name, path),
                                      self.init.find_loader(name, path))
 

Misc/NEWS.d/next/Core and Builtins/2021-03-26-17-30-19.bpo-42134.G4Sjxg.rst

@@ -0,0 +1 @@
+Calls to find_module() by the import system now raise ImportWarning.