gh-135385: Fix memory regression for classes with both __slots__ and __dict__

[shuimu5418]

EDIT: Apologies, I commited my pr before seeing Eclips4's comment. I have since confirmed locally by compiling and testing the main branch that this issue is indeed fixed there.
This PR should NOT be merged or backported. It would fail to fix the bug on 3.13 and could potentially mask the real issue or introduce other instabilities.

---

This patch fixes a significant memory regression in Python 3.13 where classes that inherit __slots__ but also have a __dict__ consume ~4x more memory than necessary.

The root cause was that such subclasses incorrectly inherited a non-zero tp_itemsize from their base. This prevented the Py_TPFLAGS_INLINE_VALUES flag from being set, disabling the key-sharing dictionary optimization.

The fix, located in Objects/typeobject.c, adjusts the logic to ensure a subclass only inherits a non-zero tp_itemsize if it defines its own _slots_. Otherwise, tp_itemsize is correctly set to 0, re-enabling the memory optimization.

test:

PS F:\github\cpython001> .\python.bat -m test -j3 test_class test_types test_dict
Running Debug|x64 interpreter...
Using random seed: 1566459836
0:00:00 Run 3 tests in parallel using 3 worker processes
0:00:01 [1/3] test_class passed
0:00:02 [2/3] test_dict passed
0:00:03 [3/3] test_types passed

== Tests result: SUCCESS ==

All 3 tests OK.

Total duration: 3.5 sec
Total tests: run=273
Total test files: run=3/3
Result: SUCCESS

benchmark:

PS F:\github\cpython001> .\python.bat .\build\o-test.py  
Running Debug|x64 interpreter...
1M Point3D (only __slots__) instances:         64_448_944 bytes
1M Point3D (only __dict__) instances:          104_452_144 bytes
1M Point3D (__dict__ and __slots__) instances: 120_452_144 bytes

# o-test.py
import tracemalloc
import gc


class _Point2D:
    __slots__ = ("x", "y")


class Point3D_OnlySlots(_Point2D):
    __slots__ = ("z",)

    def __init__(self, x, y, z):
        self.x, self.y, self.z = x, y, z


class Point3D_DictAndSlots(_Point2D):
    def __init__(self, x, y, z):
        self.x, self.y, self.z = x, y, z


class Point3D_OnlyDict:
    def __init__(self, x, y, z):
        self.x, self.y, self.z = x, y, z


gc.collect()  # clear freelists
tracemalloc.start()
_ = [Point3D_OnlySlots(1, 2, 3) for _ in range(1_000_000)]
print(
    f"1M Point3D (only __slots__) instances:         {tracemalloc.get_traced_memory()[0]:_} bytes"
)

gc.collect()  # clear freelists
tracemalloc.start()
_ = [Point3D_OnlyDict(1, 2, 3) for _ in range(1_000_000)]
print(
    f"1M Point3D (only __dict__) instances:          {tracemalloc.get_traced_memory()[0]:_} bytes"
)

gc.collect()  # clear freelists
tracemalloc.start()
_ = [Point3D_DictAndSlots(1, 2, 3) for _ in range(1_000_000)]
print(
    f"1M Point3D (__dict__ and __slots__) instances: {tracemalloc.get_traced_memory()[0]:_} bytes"
)

• Issue: Objects with both __slots__ and __dict__ have much larger size than needed (up to 4x) on Python 3.13 #135385

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[ZeroIntensity]

No need to force-push, we squash at the end.

[ariebovenberg]

I'm not that familiar with the underlying type machinery. Does the fix also work for the opposite: __slots__ class inheriting from __dict__ class?

class _NoSlots:
    pass

class Point3D_BrokenSlots(_NoSlots):
    __slots__ = ("x", "y")

    def __init__(self, x, y, z):
        self.x, self.y, self.z = x, y, z

[AA-Turner]

Adding label per:

This PR should NOT be merged or backported. It would fail to fix the bug on 3.13 and could potentially mask the real issue or introduce other instabilities.