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int and set subclass __sizeof__ under-reports the instance dictionary pointer #101266
Description
Bug report
Most built-in types, when subclassed, will have a __sizeof__ that is 8 (pointer size) larger than the parent object, due to having an instance dictionary.
class UserFloat(float): pass
assert UserFloat().__sizeof__() - (0.0).__sizeof__() == 8
class UserTuple(tuple): pass
assert UserTuple().__sizeof__() - ().__sizeof__() == 8
class UserList(list): pass
assert UserList().__sizeof__() - [].__sizeof__() == 8
class UserDict(dict): pass
assert UserDict().__sizeof__() - {}.__sizeof__() == 8This is unexpectedly, not the case for int and set, which exactly match the original __sizeof__
class UserSet(set): pass
print(UserSet().__sizeof__(), set().__sizeof__()) # 200 200
class UserInt(int): pass
print(UserInt().__sizeof__(), (0).__sizeof__()) # 24 24As a result this makes __dictoffset__ usages incorrect as well
from ctypes import py_object
class UserTuple(tuple): pass
x = UserTuple()
x.a = 5
print(x.__dict__)
>> {'a': 5}
py_object.from_address(id(x) + x.__sizeof__() + UserTuple.__dictoffset__)
>> py_object({'a': 5})from ctypes import py_object
class UserInt(int): pass
x = UserInt()
x.a = 5
print(x.__dict__)
>> {'a': 5}
py_object.from_address(id(x) + x.__sizeof__() + UserInt.__dictoffset__)
>> py_object(<NULL>)Your environment
- CPython versions tested on: 3.11, 3.12.0a3
Activity
mdickinson commentedon Jan 28, 2023
I'll leave the
setpart of this to someone more qualified. (@rhettinger?)For
int, I think this is an easy fix; I'll make a PR shortly.Though oddly enough, I believe the current behaviour in
mainis correct (though for the wrong reasons), sinceUserInt.__basicsize__now matchesint.__basicsize__.mdickinson commentedon Jan 28, 2023
Actually, I'm not sure there's any bug here for
set: the__dict__pointer for aUserSetinstance is stored in the preheader (the flags forUserSetincludePy_TPFLAGS_MANAGED_DICT), and is accounted for there when computing the size withsys.getsizeof.gh-101266: Fix __sizeof__ for subclasses of int (#101394)
[3.11] gh-101266: Fix __sizeof__ for subclasses of int (GH-101394) (#…
mdickinson commentedon Feb 5, 2023
The
intside of this is fixed in main, with the fix backported to 3.11 (but not to 3.10, since the "correct" behaviour is a bit less clear there, and since we didn't backport the previous fix to 3.10).I believe there's no actual issue on the
setside of things (@ionite34 please comment if you disagree), so I'm going to close here.Revert "[3.11] pythongh-101266: Fix __sizeof__ for subclasses of int (p…
[3.11] gh-101266: Revert fix __sizeof__ for subclasses of int (#101638)
Revert "pythongh-101266: Fix __sizeof__ for subclasses of int (python…