Description

JelleZijlstra

opened on Oct 12, 2025

Issue body actions

Bug report

Bug description:

TypeAliasType's traverse implementation does not visit its name. This means that we can leak memory if name is passed an instance of a mutable str subclass.

from typing import TypeAliasType
import weakref
import gc

class S(str):
    pass

def still_alive(ref):
    obj = ref()
    if not isinstance(obj, S):
        return False
    T = obj.T
    if not isinstance(T, TypeAliasType):
        return False
    return T

name = S("name")
T = TypeAliasType(name, int)
name.T = T
ref = weakref.ref(name)
assert still_alive(ref)
del name
del T
gc.collect()
gc.collect()
assert still_alive(ref)

del ref().T
assert not still_alive(ref)

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-140000: Traverse name attribute for TypeVar, TypeVarTuple, TypeAliasType, ParamSpec #140016
• [3.14] gh-140000: Traverse name attribute for TypeVar, TypeVarTuple, TypeAliasType, ParamSpec (GH-140016) #140063
• [3.13] gh-140000: Traverse name attribute for TypeVar, TypeVarTuple, TypeAliasType, ParamSpec (GH-140016) #140073

Activity

1. JelleZijlstra

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Oct 12, 2025
2. JelleZijlstra commented on Oct 12, 2025

   

   JelleZijlstra

   on Oct 12, 2025

   MemberAuthor

   More actions

   This also affects TypeVar, TypeVarTuple, and ParamSpec.
3. picnixz

   added

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   topic-typing

   on Oct 12, 2025
4. efimov-mikhail commented on Oct 12, 2025

   

   efimov-mikhail

   on Oct 12, 2025

   Member

   More actions

   Leak for all 4 types is confirmed on main, 3.14 and 3.13 branches. PR with fix is submitted.

   Reacted by Sergey Miryanov
5. JelleZijlstra

   added a commit that references this issue on Oct 13, 2025

   gh-140000: Traverse `name` attribute for `TypeVar`, `TypeVarTuple`, `…

   be60e4b
6. miss-islington

   added a commit that references this issue on Oct 13, 2025

   pythongh-140000: Traverse `name` attribute for `TypeVar`, `TypeVarTup…

   6e92029
7. JelleZijlstra

   added 2 commits that reference this issue on Oct 13, 2025

   [3.14] gh-140000: Traverse `name` attribute for `TypeVar`, `TypeVarTu…

   7a13953

   [3.13] gh-140000: Traverse `name` attribute for `TypeVar`, `TypeVarTu…

   cbcee9a
8. JelleZijlstra

   closed this as completedon Oct 14, 2025

Issue actions