gh-86298: Ensure that __loader__ and __spec__.loader agree in warnings.warn_explicit()

Doc/reference/import.rst

@@ -558,6 +558,11 @@ listed below.
    It is **strongly** recommended that you rely on :attr:`__spec__`
    instead instead of this attribute.
 
+   .. versionchanged:: 3.12
+      The value of ``__loader__`` is expected to be the same as
+      ``__spec__.loader``.  The use of ``__loader__`` is deprecated and slated
+      for removal in Python 3.14.
+
 .. attribute:: __package__
 
    The module's ``__package__`` attribute may be set.  Its value must
@@ -568,6 +573,9 @@ listed below.
    submodules, to the parent package's name.  See :pep:`366` for further
    details.
 
+   This attribute is used instead of ``__name__`` to calculate explicit
+   relative imports for main modules, as defined in :pep:`366`.
+
    It is **strongly** recommended that you rely on :attr:`__spec__`
    instead instead of this attribute.
 

Lib/importlib/_bootstrap_external.py

@@ -862,6 +862,52 @@ def spec_from_file_location(name, location=None, *, loader=None,
     return spec
 
 
+def _bless_my_loader(module_globals):
+    """Helper function for _warnings.c
+
+    See GH#97850 for details.
+    """
+    # 2022-10-06(warsaw): For now, this helper is only used in _warnings.c and
+    # that use case only has the module globals.  This function could be
+    # extended to accept either that or a module object.  However, in the
+    # latter case, it would be better to raise certain exceptions when looking
+    # at a module, which should have either a __loader__ or __spec__.loader.
+    # For backward compatibility, it is possible that we'll get an empty
+    # dictionary for the module globals, and that cannot raise an exception.
+    if not isinstance(module_globals, dict):
+        return None
+
+    missing = object()
+    loader = module_globals.get('__loader__', None)
+    spec = module_globals.get('__spec__', missing)
+
+    if loader is None and spec is missing:
+        #raise AttributeError('Module globals is missing a __spec__')
+        return None
+    if loader is None and spec is None:
+        raise ValueError('Module globals is missing a __spec__.loader')
+
+    spec_loader = getattr(spec, 'loader', missing)
+
+    if spec_loader in (missing, None):
+        if loader is None:
+            exc = AttributeError if spec_loader is missing else ValueError
+            raise exc('Module globals is missing a __spec__.loader')
+        _warnings.warn(
+            'Module globals is missing a __spec__.loader',
+            DeprecationWarning)
+        spec_loader = loader
+
+    assert spec_loader is not None
+    if loader is not None and loader != spec_loader:
+        _warnings.warn(
+            'Module globals; __loader__ != __spec__.loader',
+            DeprecationWarning)
+        return loader
+
+    return spec_loader
+
+
 # Loaders #####################################################################
 
 class WindowsRegistryFinder:

Lib/test/test_importlib/import_/test_helpers.py

@@ -2,7 +2,9 @@
 
 from importlib import _bootstrap_external, machinery
 import os.path
+from types import ModuleType, SimpleNamespace
 import unittest
+import warnings
 
 from .. import util
 
@@ -67,5 +69,114 @@ def test_no_loader_no_spec_but_source(self):
 
 FrozenFixUpModuleTests, SourceFixUpModuleTests = util.test_both(FixUpModuleTests)
 
+
+class TestBlessMyLoader(unittest.TestCase):
+    # GH#86298 is part of the migration away from module attributes and toward
+    # __spec__ attributes.  There are several cases to test here.  This will
+    # have to change in Python 3.14 when we actually remove/ignore __loader__
+    # in favor of requiring __spec__.loader.
+
+    def test_gh86298_no_loader_and_no_spec(self):
+        bar = ModuleType('bar')
+        del bar.__loader__
+        del bar.__spec__
+        # 2022-10-06(warsaw): For backward compatibility with the
+        # implementation in _warnings.c, this can't raise an
+        # AttributeError.  See _bless_my_loader() in _bootstrap_external.py
+        ## self.assertRaises(
+        ##     AttributeError, _bootstrap_external._bless_my_loader,
+        ##     bar.__dict__)
+        self.assertIsNone(_bootstrap_external._bless_my_loader(bar.__dict__))
+
+    def test_gh86298_loader_is_none_and_no_spec(self):
+        bar = ModuleType('bar')
+        bar.__loader__ = None
+        del bar.__spec__
+        # 2022-10-06(warsaw): For backward compatibility with the
+        # implementation in _warnings.c, this can't raise an
+        # AttributeError.  See _bless_my_loader() in _bootstrap_external.py
+        ## self.assertRaises(
+        ##     AttributeError, _bootstrap_external._bless_my_loader,
+        ##     bar.__dict__)
+        self.assertIsNone(_bootstrap_external._bless_my_loader(bar.__dict__))
+
+    def test_gh86298_no_loader_and_spec_is_none(self):
+        bar = ModuleType('bar')
+        del bar.__loader__
+        bar.__spec__ = None
+        self.assertRaises(
+            ValueError,
+            _bootstrap_external._bless_my_loader, bar.__dict__)
+
+    def test_gh86298_loader_is_none_and_spec_is_none(self):
+        bar = ModuleType('bar')
+        bar.__loader__ = None
+        bar.__spec__ = None
+        self.assertRaises(
+            ValueError,
+            _bootstrap_external._bless_my_loader, bar.__dict__)
+
+    def test_gh86298_loader_is_none_and_spec_loader_is_none(self):
+        bar = ModuleType('bar')
+        bar.__loader__ = None
+        bar.__spec__ = SimpleNamespace(loader=None)
+        self.assertRaises(
+            ValueError,
+            _bootstrap_external._bless_my_loader, bar.__dict__)
+
+    def test_gh86298_no_spec(self):
+        bar = ModuleType('bar')
+        bar.__loader__ = object()
+        del bar.__spec__
+        with warnings.catch_warnings():
+            self.assertWarns(
+                DeprecationWarning,
+                _bootstrap_external._bless_my_loader, bar.__dict__)
+
+    def test_gh86298_spec_is_none(self):
+        bar = ModuleType('bar')
+        bar.__loader__ = object()
+        bar.__spec__ = None
+        with warnings.catch_warnings():
+            self.assertWarns(
+                DeprecationWarning,
+                _bootstrap_external._bless_my_loader, bar.__dict__)
+
+    def test_gh86298_no_spec_loader(self):
+        bar = ModuleType('bar')
+        bar.__loader__ = object()
+        bar.__spec__ = SimpleNamespace()
+        with warnings.catch_warnings():
+            self.assertWarns(
+                DeprecationWarning,
+                _bootstrap_external._bless_my_loader, bar.__dict__)
+
+    def test_gh86298_loader_and_spec_loader_disagree(self):
+        bar = ModuleType('bar')
+        bar.__loader__ = object()
+        bar.__spec__ = SimpleNamespace(loader=object())
+        with warnings.catch_warnings():
+            self.assertWarns(
+                DeprecationWarning,
+                _bootstrap_external._bless_my_loader, bar.__dict__)
+
+    def test_gh86298_no_loader_and_no_spec_loader(self):
+        bar = ModuleType('bar')
+        del bar.__loader__
+        bar.__spec__ = SimpleNamespace()
+        self.assertRaises(
+            AttributeError,
+            _bootstrap_external._bless_my_loader, bar.__dict__)
+
+    def test_gh86298_no_loader_with_spec_loader_okay(self):
+        bar = ModuleType('bar')
+        del bar.__loader__
+        loader = object()
+        bar.__spec__ = SimpleNamespace(loader=loader)
+        self.assertEqual(
+            _bootstrap_external._bless_my_loader(bar.__dict__),
+            loader)
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2022-10-04-14-04-40.gh-issue-86298.QVM7G1.rst

@@ -0,0 +1,3 @@
+In cases where ``warnings.warn_explicit()`` consults the module's loader, an
+``DeprecationWarning`` is issued when ``m.__loader__`` differs from
+``m.__spec__.loader``.

Python/_warnings.c

@@ -977,19 +977,26 @@ warnings_warn_impl(PyObject *module, PyObject *message, PyObject *category,
 static PyObject *
 get_source_line(PyInterpreterState *interp, PyObject *module_globals, int lineno)
 {
+    PyObject *external;
     PyObject *loader;
     PyObject *module_name;
     PyObject *get_source;
     PyObject *source;
     PyObject *source_list;
     PyObject *source_line;
 
-    /* Check/get the requisite pieces needed for the loader. */
-    loader = _PyDict_GetItemWithError(module_globals, &_Py_ID(__loader__));
+    /* stolen from import.c */
+    external = PyObject_GetAttrString(interp->importlib, "_bootstrap_external");
+    if (external == NULL) {
+        return NULL;
+    }
+
+    loader = PyObject_CallMethod(external, "_bless_my_loader", "O", module_globals, NULL);
+    Py_DECREF(external);
     if (loader == NULL) {
         return NULL;
     }
-    Py_INCREF(loader);
+
     module_name = _PyDict_GetItemWithError(module_globals, &_Py_ID(__name__));
     if (!module_name) {
         Py_DECREF(loader);