I've noticed a discrepancy in the value of thePy_TPFLAGS_MANAGED_DICT flag constant used inLib/test/test_class.py.

Summary

The test fileLib/test/test_class.py defines a local constantPy_TPFLAGS_MANAGED_DICT = (1 << 2). However, the canonical definition in the C headerInclude/object.h isPy_TPFLAGS_MANAGED_DICT = (1 << 4).

The existing tests that use this constant pass only coincidentally. They test plain classes which happen to have a flag set at the(1 << 2) position, but this is not the actual managed dictionary flag.

This can be proven by a test case that uses a__slots__ class. Such a class should not have a managed dictionary, and therefore, a test checking for theMANAGED_DICT flag should result in0. However, the test written to check for the incorrect(1 << 2) flag also results in0, which causes an assertion to fail, proving the constant is wrong.

Proof of Incorrect Constant

The following test case demonstrates that the(1 << 2) constant is incorrect. It uses a__slots__ class, which is known not to have a managed dictionary.

The test attempts to assert that this classhas the flag specified by the incorrect(1 << 2) constant. This assertion fails, providing evidence that the constant is wrong.

# In Lib/test/test_class.pyimportunittestclassTestMisleadingConstant(unittest.TestCase):deftest_slots_class_reveals_flaw_in_constant(self):# The incorrect constant as defined in test_class.pyPy_TPFLAGS_MANAGED_DICT_WRONG= (1<<2)# The correct constant from object.hPy_TPFLAGS_MANAGED_DICT_CORRECT= (1<<4)classNoManagedDict:__slots__= ('a',)# First, confirm that a __slots__ class correctly does NOT have# the true managed dict flag. This assertion passes.self.assertEqual(NoManagedDict.__flags__&Py_TPFLAGS_MANAGED_DICT_CORRECT,0,"A __slots__ class should not have the correct MANAGED_DICT flag (1<<4)"        )# Next, try to assert that this class HAS the flag at the (1<<2) position.# This assertion FAILS with "AssertionError: 0 != 4", because a __slots__ class# does not have this flag either.# This failure is the proof that the (1<<2) constant is wrong for this purpose.self.assertEqual(NoManagedDict.__flags__&Py_TPFLAGS_MANAGED_DICT_WRONG,Py_TPFLAGS_MANAGED_DICT_WRONG,"This fails, proving the constant is wrong."        )

Reference Links

1. Incorrect constant intest_class.py:

   • cpython/Lib/test/test_class.py

     Line 862 in49365bd

     Py_TPFLAGS_MANAGED_DICT= (1<<2)

2. Correct constant definition inobject.h:

   • cpython/Include/object.h

     Lines 532 to 535 in49365bd

     	/* Placement of dict (and values) pointers are managed by the VM, not by the type.
     	* The VM will automatically set tp_dictoffset. Implies Py_TPFLAGS_HAVE_GC.
     	*/
     	#definePy_TPFLAGS_MANAGED_DICT (1 << 4)

Proposed Fix

The constantPy_TPFLAGS_MANAGED_DICT inLib/test/test_class.py should be updated to(1 << 4) to match the C header definition.
Additionally, we could add a test case for a class that does not havePy_TPFLAGS_INLINE_VALUES.

If this analysis seems correct, I would be happy to open a Pull Request to update the constant and add a corresponding test case.

Linked PRs

• gh-136535: Tests: CorrectPy_TPFLAGS_MANAGED_DICT intest_class.py #136538