Description

johnslavik

opened on Jan 8, 2026

Issue body actions

In this usage of singledispatchmethod:

from functools import singledispatchmethod

class C:
    @singledispatchmethod
    def d(self, x):
        return "base!"

    @d.register(int)
    def _(self, x):
        return "int!"

print(C.d(C(), 1))

This is what I expected in the output:

int!

But instead this was printed out:

base!

singledispatch and singledispatchmethod don't analyze and adapt argument semantics to the registered callable type, which makes them very nice and simple.

_singledispatchmethod_get always dispatches on args[0] regardless of whether it's bound or unbound, and if it didn't, that would break descriptor support; this facet is crucial for staticmethod (and classmethod) dispatches to work:

from functools import singledispatchmethod

class C:
    @singledispatchmethod
    @staticmethod
    def d(x):
        return "base!"

    @d.register(int)
    @staticmethod
    def s(x):
        return "int!"
    
print(C.d(1))  # int!

Our tests rely on this and possibly millions of projects rely on this.

I think we could document this explicitly (and backport to 3.13 and 3.14).

---

Found this while working on GH-143465.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-143535: Document dispatch behavior of singledispatchmethod #143592
• gh-143535: Dispatch on the second argument if generic method is instance-bindable #144615

Activity

1. johnslavik

   added

   docsDocumentation in the Doc dirDocumentation in the Doc dir

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

   on Jan 8, 2026
2. github-project-automation

   added this to docs issueson Jan 8, 2026
3. github-project-automation

   moved this to Todo in docs issueson Jan 8, 2026
4. johnslavik

   changed the title [-]`singledispatchmethod` dispatches on the `self` argument what invoked from class level[/-] [+]`singledispatchmethod` dispatches on the `self` argument when invoked from class level[/+] on Jan 8, 2026
5. johnslavik

   changed the title [-]`singledispatchmethod` dispatches on the `self` argument when invoked from class level[/-] [+]Document that `singledispatchmethod` dispatches on the `self` argument when invoked from class level[/+] on Jan 9, 2026
6. priyanshu2282-cyber

   added a commit that references this issue on Jan 9, 2026

   pythongh-143535: Document dispatch behavior of singledispatchmethod

   666a2a8
7. priyanshu2282-cyber commented on Jan 9, 2026

   

   priyanshu2282-cyber

   on Jan 9, 2026

   Contributor

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   I have prepared a doc fix and raising a PR in which I have documented the behaviour that singledispatchmethod always dispatches first argument during call, which fails the regular behaviour of x.f(a)=c.f(a).
8. priyanshu2282-cyber

   added 3 commits that reference this issue on Jan 9, 2026

   pythongh-143535: Move singledispatchmethod note outside example block

   ea2fda1

   pythongh-143535: Clarify dispatch semantics of singledispatchmethod

   2834c45

   pythongh-143535: Fix indentation in singledispatchmethod docs

   e033b66
9. johnslavik

   changed the title [-]Document that `singledispatchmethod` dispatches on the `self` argument when invoked from class level[/-] [+]`singledispatchmethod` dispatches on bound argument when invoked from class level[/+] on Feb 8, 2026
10. johnslavik commented on Feb 8, 2026

    

    johnslavik

    on Feb 8, 2026

    MemberAuthor

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    I was wrong. This can be fixed, 100% low-risk.
11. johnslavik

    changed the title [-]`singledispatchmethod` dispatches on bound argument when invoked from class level[/-] [+]`singledispatchmethod` dispatches on first argument to instance-bindable method called from class level[/+] on Feb 8, 2026
12. johnslavik

    added

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

    3.13only security fixesonly security fixes

    3.14bugs and security fixesbugs and security fixes

    and removed

    docsDocumentation in the Doc dirDocumentation in the Doc dir

    on Feb 8, 2026
13. ambv

    added a commit that references this issue on Feb 24, 2026

    gh-143535: Dispatch on the second argument if generic method is insta…

    97181bb
14. ambv

    added

    type-featureA feature request or enhancementA feature request or enhancement

    and removed

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

    on Feb 24, 2026
15. ambv commented on Feb 24, 2026

    

    ambv

    on Feb 24, 2026

    Contributor

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    Reclassifying as feature for backward compat safety. Fixed by #144615.

    Reacted by Bartosz Sławecki
16. ambv

    closed this as completedon Feb 24, 2026
17. brijkapadia

    added a commit that references this issue on Feb 28, 2026

    pythongh-143535: Dispatch on the second argument if generic method is…

    1d94bd7
18. ljfp

    added a commit that references this issue on Apr 25, 2026

    pythongh-143535: Dispatch on the second argument if generic method is…

    50b2f3a

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