Description

vodik

opened on Dec 8, 2024

Issue body actions

Bug report

Bug description:

Also reported upstream to Django here.

The following code works on Python 3.12 and I believe #85160 is the cause.

When traversing a relationship and then calling a singledispatchmethod method registered on a model, the reference to self seems to get cached and locked to the first created model.

class Test(models.Model):
    ...

class Test2(models.Model):
    relationship = models.ForeignKey("Test", on_delete=models.CASCADE)

    @singledispatchmethod
    def bug(self, x):
        print(">", id(self))

    @bug.register
    def _(self, x: int):
        print(">", id(self))

and then it's pretty easy to trigger:

for t in test.test2_set.all():
    print("id:", id(t))  # always changes
    t.bug(4)             # only ever prints a single id

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

• gh-127750: Improve caching in singledispatchmethod #127751
• gh-127750: Fix singledispatchmethod caching #127839
• gh-127750: Fix singledispatchmethod caching (v2) #128648
• gh-127750: Fix and optimize functools.singledispatchmethod() #130008
• [3.13] gh-127750: Fix functools.singledispatchmethod()  #130029
• gh-127750: Improve repr of functools.singledispatchmethod #130220
• gh-127750: Restore inspect and pydoc support of singledispatchmethod #130309
• [3.13] gh-127750: Backport some tests for singledispatchmethod (GH-130309) #130340
• [3.12] gh-127750: Backport some tests for singledispatchmethod (GH-130309) (GH-130340) #130341
• gh-127750: Fix annotations in singledispatchmethod signature tests #143571
• [3.14] gh-127750: Fix annotations in singledispatchmethod signature tests (GH-143571) #143707
• [3.13] gh-127750: Fix annotations in singledispatchmethod signature tests (GH-143571) #143708

Activity

1. vodik

   added

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

   on Dec 8, 2024
2. picnixz

   added

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   3.12only security fixesonly security fixes

   3.13only security fixesonly security fixes

   3.14bugs and security fixesbugs and security fixes

   and removed

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   3.12only security fixesonly security fixes

   on Dec 8, 2024
3. vodik commented on Dec 9, 2024

   

   vodik

   on Dec 9, 2024

   Author

   More actions

   I see what the issue is now: https://github.com/AlexWaygood/cpython/blob/3d8d4ba5f5367f2d87488b00afd55f75401c2b7d/Lib/functools.py#L950

   Since dictionaries use __hash__/__eq__ for determining if a key is present, we can have collisions. Django hashs models based on the primary key. Here's a way to reproduce the same bug with just the stdlib:

   from functools import singledispatchmethod
   from dataclasses import dataclass
   
   @dataclass(frozen=True)  # or (unsafe_hash=True)
   class Test:
       pk: int
   
       @singledispatchmethod
       def broken(self, x):
           raise NotImplemenetedError
   
       @broken.register
       def _(self, x: int):
           return id(self)
   
   
   t1 = Test(1)
   print(id(t1))
   
   t2 = Test(1)
   print(id(t2))
   
   assert id(t1) == t1.broken(2)  # both t1 and t2 will share the same dispatcher
   assert id(t2) == t2.broken(2)  # fails! 
4. vodik commented on Dec 9, 2024

   

   vodik

   on Dec 9, 2024

   Author

   More actions

   I have a fix, I think. Working on a PR.

   Reacted by Peter Bierma
5. vodik

   added 3 commits that reference this issue on Dec 9, 2024

   pythongh-127750: Improve singledispatchmethod caching

   2e08bd9

   pythongh-127750: Improve singledispatchmethod caching

   a81b83e

   pythongh-127750: Improve singledispatchmethod caching

   fe804ef
6. vodik commented on Dec 9, 2024

   

   vodik

   on Dec 9, 2024

   Author

   More actions

   I'm pretty sure the code in question is also leaking memory. See comment here.
7. eendebakpt commented on Dec 9, 2024

   

   eendebakpt

   on Dec 9, 2024

   Contributor

   More actions

   @vodik There is indeed a behavior change between 3.12 and 3.13, so perhaps that needs to be fixed. Another example showing the issue is:

   from functools import lru_cache
   from dataclasses import dataclass
   
   @dataclass(frozen=True)  # or (unsafe_hash=True)
   class Test:
       pk: int
   
   t1 = Test(1)
   t2 = Test(1)  
   
   @lru_cache
   def return_id(obj):
       return id(obj)
   
   print(return_id(t1))
   print(return_id(t2))
8. 9 remaining items
9. serhiy-storchaka

   added 3 commits that reference this issue on Feb 20, 2025

   [3.13] pythongh-127750: Restore inspect and pydoc support of singledi…

   e79ded4

   [3.13] pythongh-127750: Backport some tests for singledispatchmethod (p…

   c6c2d33

   [3.13] gh-127750: Backport some tests for singledispatchmethod (GH-13…

   68c57d6
10. miss-islington

    added a commit that references this issue on Feb 20, 2025

    [3.13] pythongh-127750: Backport some tests for singledispatchmethod (p…

    c160ee6
11. serhiy-storchaka

    added a commit that references this issue on Feb 20, 2025

    [3.12] gh-127750: Backport some tests for singledispatchmethod (GH-13…

    e63ca56
12. serhiy-storchaka

    added a commit that references this issue on Mar 5, 2025

    gh-127750: Improve repr of functools.singledispatchmethod (GH-130220)

    f33d21e
13. serhiy-storchaka

    closed this as completedon Mar 5, 2025
14. serhiy-storchaka

    added a commit that references this issue on Jan 11, 2026

    gh-127750: Fix annotations in singledispatchmethod signature tests (G…

    620a5b9
15. miss-islington

    added 2 commits that reference this issue on Jan 11, 2026

    pythongh-127750: Fix annotations in singledispatchmethod signature te…

    9c7f1fc

    pythongh-127750: Fix annotations in singledispatchmethod signature te…

    c9eccc7
16. serhiy-storchaka

    added 2 commits that reference this issue on Jan 11, 2026

    [3.13] gh-127750: Fix annotations in singledispatchmethod signature t…

    ea1ecf6

    [3.14] gh-127750: Fix annotations in singledispatchmethod signature t…

    ea26c88
17. reidenong

    added a commit that references this issue on Jan 12, 2026

    pythongh-127750: Fix annotations in singledispatchmethod signature te…

    13c9f56
18. thunder-coding

    added a commit that references this issue on Feb 15, 2026

    pythongh-127750: Fix annotations in singledispatchmethod signature te…

    5b0a063
19. rjpower

    added a commit that references this issue on Oct 26, 2026

    boil: test_functools drops the singledispatchmethod reference-loop test

    9eea555

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