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singledispatchmethod dispatches on first argument to instance-bindable method called from class level #143535
Description
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
Activity
[-]`singledispatchmethod` dispatches on the `self` argument what invoked from class level[/-][+]`singledispatchmethod` dispatches on the `self` argument when invoked from class level[/+][-]`singledispatchmethod` dispatches on the `self` argument when invoked from class level[/-][+]Document that `singledispatchmethod` dispatches on the `self` argument when invoked from class level[/+]pythongh-143535: Document dispatch behavior of singledispatchmethod
priyanshu2282-cyber commentedon Jan 9, 2026
I have prepared a doc fix and raising a PR in which I have documented the behaviour that
singledispatchmethodalways dispatches first argument during call, which fails the regular behaviour ofx.f(a)=c.f(a).pythongh-143535: Move singledispatchmethod note outside example block
pythongh-143535: Clarify dispatch semantics of singledispatchmethod
pythongh-143535: Fix indentation in singledispatchmethod docs
[-]Document that `singledispatchmethod` dispatches on the `self` argument when invoked from class level[/-][+]`singledispatchmethod` dispatches on bound argument when invoked from class level[/+]johnslavik commentedon Feb 8, 2026
I was wrong. This can be fixed, 100% low-risk.
[-]`singledispatchmethod` dispatches on bound argument when invoked from class level[/-][+]`singledispatchmethod` dispatches on first argument to instance-bindable method called from class level[/+]gh-143535: Dispatch on the second argument if generic method is insta…
ambv commentedon Feb 24, 2026
Reclassifying as feature for backward compat safety. Fixed by #144615.
pythongh-143535: Dispatch on the second argument if generic method is…
pythongh-143535: Dispatch on the second argument if generic method is…