Consider the following:

from functools import singledispatch

class Dispatch:
    @singledispatch
    def foo(self, a):
        return a

    @foo.register(int)
    def _(self, a):
        return "int"

    @foo.register(str)
    def _(self, a):
        return "str"

cls = Dispatch()
cls.foo(3)  # 3
cls.foo('hm')  # 'hm'

I find this quite unintuitive. Essentially, since singledispatch dispatches based solely on a functions first argument, it is useless on methods unless one wraps it and modifies how it uses the internal wrapper function. I believe this should be relatively easy to fix with adding a check of inspect.ismethod and then modifying the number of the checked argument where appropriate.

I'm happy to write a patch if this change is seen as a good idea.

I have also noticed this problem and I like the idea. It appeared some time ago on python-ideas, but no one has written a patch.

Added Łukasz to the nosy list, as I'd like his +1 before accepting this change (I do still think it's a good idea, for the same reasons we added partialmethod).

+1

New changeset c651275afe8515b2cf70b8152e19ce39df88f0dd by Łukasz Langa (Ethan Smith) in branch 'master':
bpo-32380: Create functools.singledispatchmethod (#6306)
https://github.com/python/cpython/commit/c651275afe8515b2cf70b8152e19ce39df88f0dd

This was fixed, so I think it can be closed.

New changeset bc284f0c7a9a7a9a4bf12c680823023a6770ce06 by Inada Naoki in branch 'master':
bpo-32380: add "versionadded: 3.8" to singledispatchmethod (GH-12580)
https://github.com/python/cpython/commit/bc284f0c7a9a7a9a4bf12c680823023a6770ce06