[3.12] pythongh-130809: Fix PyFrame_LocalsToFast copying the wrong value (python#130816)

* pythongh-130809: Fix `PyFrame_LocalsToFast` copying the wrong value

* Skip hidden locals

* test, blurb

* Update Misc/NEWS.d/next/Core_and_Builtins/2025-03-04-12-52-21.gh-issue-130809.fSXq60.rst

Co-authored-by: Tian Gao <[email protected]>

* Update test

* PR feedback

* formatting

* comment

---------

Co-authored-by: Tian Gao <[email protected]>

Author: kycutler

Lib/test/test_listcomps.py

@@ -709,6 +709,23 @@ def test_multiple_comprehension_name_reuse(self):
         self._check_in_scopes(code, {"x": 2, "y": [3]}, ns={"x": 3}, scopes=["class"])
         self._check_in_scopes(code, {"x": 2, "y": [2]}, ns={"x": 3}, scopes=["function", "module"])
 
+    def test_name_collision_locals(self):
+        # GH-130809: The existence of a hidden fast from list comprehension
+        # should not cause frame.f_locals on module level to return a new dict
+        # every time it is accessed.
+
+        code = """
+            import sys
+            frame = sys._getframe()
+            f_locals = frame.f_locals
+            foo = 1
+            [foo for foo in [0]]
+            # calls _PyFrame_LocalsToFast which triggers the issue
+            from abc import *
+            same_f_locals = frame.f_locals is f_locals
+        """
+        self._check_in_scopes(code, {"foo": 1, "same_f_locals": True}, scopes=["module"])
+
     def test_exception_locations(self):
         # The location of an exception raised from __init__ or
         # __next__ should should be the iterator expression

Misc/NEWS.d/next/Core_and_Builtins/2025-03-04-12-52-21.gh-issue-130809.fSXq60.rst

@@ -0,0 +1,2 @@
+Fixed an issue where ``_PyFrame_LocalsToFast`` tries to write module level
+values to hidden fasts.

Objects/frameobject.c

@@ -1405,6 +1405,9 @@ _PyFrame_LocalsToFast(_PyInterpreterFrame *frame, int clear)
         if (kind & CO_FAST_FREE && !(co->co_flags & CO_OPTIMIZED)) {
             continue;
         }
+        if (kind & CO_FAST_HIDDEN) {
+            continue;
+        }
         PyObject *name = PyTuple_GET_ITEM(co->co_localsplusnames, i);
         PyObject *value = PyObject_GetItem(locals, name);
         /* We only care about NULLs if clear is true. */