Issue #25791: Warn when __package__ != __spec__.parent.

In a previous change, __spec__.parent was prioritized over
__package__. That is a backwards-compatibility break, but we do
eventually want __spec__ to be the ground truth for module details. So
this change reverts the change in semantics and instead raises an
ImportWarning when __package__ != __spec__.parent to give people time
to adjust to using spec objects.

1 files changed, 9 insertions, 3 deletions

diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index 9adcf7b..f0a4e1c 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -1032,11 +1032,17 @@ def _calc___package__(globals):
     to represent that its proper value is unknown.
 
     """
+    package = globals.get('__package__')
     spec = globals.get('__spec__')
-    if spec is not None:
+    if package is not None:
+        if spec is not None and package != spec.parent:
+            _warnings.warn("__package__ != __spec__.parent "
+                           f"({package!r} != {spec.parent!r})",
+                           ImportWarning, stacklevel=3)
+        return package
+    elif spec is not None:
         return spec.parent
-    package = globals.get('__package__')
-    if package is None:
+    else:
         _warnings.warn("can't resolve package from __spec__ or __package__, "
                        "falling back on __name__ and __path__",
                        ImportWarning, stacklevel=3)