Issue #14938: importlib.abc.SourceLoader.is_package() now takes the

module name into consideration when determining whether a module is a
package or not. This prevents importing a module's __init__ module
directly and having it considered a package, which can lead to
duplicate sub-modules.

Thanks to Ronan Lamy for reporting the bug.

Doc/library/importlib.rst

@@ -351,8 +351,10 @@ are also provided to help in implementing the core ABCs.
     .. method:: is_package(self, fullname)
 
         Concrete implementation of :meth:`InspectLoader.is_package`. A module
-        is determined to be a package if its file path is a file named
+        is determined to be a package if its file path (as provided by
-        ``__init__`` when the file extension is removed.
+        :meth:`ExecutionLoader.get_filename`) is a file named
+        ``__init__`` when the file extension is removed **and** the module name
+        itself does not end in ``__init__``.
 
 
 .. class:: PyLoader

Lib/importlib/_bootstrap.py

@@ -578,7 +578,9 @@ class _LoaderBasics:
         """Concrete implementation of InspectLoader.is_package by checking if
         the path returned by get_filename has a filename of '__init__.py'."""
         filename = _path_split(self.get_filename(fullname))[1]
-        return filename.rsplit('.', 1)[0] == '__init__'
+        filename_base = filename.rsplit('.', 1)[0]
+        tail_name = fullname.rpartition('.')[2]
+        return filename_base == '__init__' and tail_name != '__init__'
 
     def _bytes_from_bytecode(self, fullname, data, bytecode_path, source_stats):
         """Return the marshalled bytes from bytecode, verifying the magic

Lib/importlib/test/source/test_abc_loader.py

@@ -602,10 +602,11 @@ class SourceOnlyLoaderTests(SourceLoaderTestHarness):
 
     def test_is_package(self):
         # Properly detect when loading a package.
-        self.setUp(is_package=True)
-        self.assertTrue(self.loader.is_package(self.name))
         self.setUp(is_package=False)
         self.assertFalse(self.loader.is_package(self.name))
+        self.setUp(is_package=True)
+        self.assertTrue(self.loader.is_package(self.name))
+        self.assertFalse(self.loader.is_package(self.name + '.__init__'))
 
     def test_get_code(self):
         # Verify the code object is created.

Misc/NEWS

@@ -24,6 +24,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #14938: importlib.abc.SourceLoader.is_package() will not consider a
+  module whose name ends in '__init__' a package (e.g. importing pkg.__init__
+  directly should be considered a module, not a package).
+
 - Issue #14982: Document that pkgutil's iteration functions require the
   non-standard iter_modules() method to be defined by an importer (something
   the importlib importers do not define).