bpo-35673: Add a public alias for namespace package __loader__ attribute (#29049)

Rename namespace package __loader__ class to be public.

Make the old name, i.e. _NamespaceLoader, an alias for the public name, for backward compatibility.

Lib/importlib/_bootstrap.py

@@ -507,9 +507,9 @@ def _init_module_attrs(spec, module, *, override=False):
             if spec.submodule_search_locations is not None:
                 if _bootstrap_external is None:
                     raise NotImplementedError
-                _NamespaceLoader = _bootstrap_external._NamespaceLoader
+                NamespaceLoader = _bootstrap_external.NamespaceLoader
 
-                loader = _NamespaceLoader.__new__(_NamespaceLoader)
+                loader = NamespaceLoader.__new__(NamespaceLoader)
                 loader._path = spec.submodule_search_locations
                 spec.loader = loader
                 # While the docs say that module.__file__ is not set for

Lib/importlib/_bootstrap_external.py

@@ -1279,8 +1279,10 @@ class _NamespacePath:
         self._path.append(item)
 
 
-# We use this exclusively in module_from_spec() for backward-compatibility.
-class _NamespaceLoader:
+# This class is actually exposed publicly in a namespace package's __loader__
+# attribute, so it should be available through a non-private name.
+# https://bugs.python.org/issue35673
+class NamespaceLoader:
     def __init__(self, name, path, path_finder):
         self._path = _NamespacePath(name, path, path_finder)
 
@@ -1291,7 +1293,7 @@ class _NamespaceLoader:
         The method is deprecated.  The import machinery does the job itself.
 
         """
-        _warnings.warn("_NamespaceLoader.module_repr() is deprecated and "
+        _warnings.warn("NamespaceLoader.module_repr() is deprecated and "
                        "slated for removal in Python 3.12", DeprecationWarning)
         return '<module {!r} (namespace)>'.format(module.__name__)
 
@@ -1327,6 +1329,10 @@ class _NamespaceLoader:
         return NamespaceReader(self._path)
 
 
+# We use this exclusively in module_from_spec() for backward-compatibility.
+_NamespaceLoader = NamespaceLoader
+
+
 # Finders #####################################################################
 
 class PathFinder:

Lib/importlib/abc.py

@@ -213,7 +213,7 @@ class InspectLoader(Loader):
     exec_module = _bootstrap_external._LoaderBasics.exec_module
     load_module = _bootstrap_external._LoaderBasics.load_module
 
-_register(InspectLoader, machinery.BuiltinImporter, machinery.FrozenImporter)
+_register(InspectLoader, machinery.BuiltinImporter, machinery.FrozenImporter, machinery.NamespaceLoader)
 
 
 class ExecutionLoader(InspectLoader):

Lib/importlib/machinery.py

@@ -12,6 +12,7 @@ from ._bootstrap_external import FileFinder
 from ._bootstrap_external import SourceFileLoader
 from ._bootstrap_external import SourcelessFileLoader
 from ._bootstrap_external import ExtensionFileLoader
+from ._bootstrap_external import NamespaceLoader
 
 
 def all_suffixes():