functools.singledispatch does not support Union types

[evansd]

BPO	46014
Nosy	@rhettinger, @ambv, @uriyyo, @AlexWaygood, @evansd
PRs	bpo-46014: Add ability to use typing.Union with singledispatch #30017 bpo-46014: Add docs regarding functools.singledispatch changes #32282

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-12-10.23:29:02.779>
created_at = <Date 2021-12-08.13:34:12.625>
labels = ['type-feature', 'library', '3.9', '3.10', '3.11']
title = 'functools.singledispatch does not support Union types'
updated_at = <Date 2022-04-03.08:22:50.956>
user = 'https://github.com/evansd'

bugs.python.org fields:

activity = <Date 2022-04-03.08:22:50.956>
actor = 'uriyyo'
assignee = 'none'
closed = True
closed_date = <Date 2021-12-10.23:29:02.779>
closer = 'lukasz.langa'
components = ['Library (Lib)']
creation = <Date 2021-12-08.13:34:12.625>
creator = 'evansd'
dependencies = []
files = []
hgrepos = []
issue_num = 46014
keywords = ['patch']
message_count = 6.0
messages = ['408018', '408261', '408262', '408272', '409366', '409369']
nosy_count = 6.0
nosy_names = ['rhettinger', 'wrobell', 'lukasz.langa', 'uriyyo', 'AlexWaygood', 'evansd']
pr_nums = ['30017', '32282']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46014'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[evansd]

It's not currently possible to use singledispatch with a function annotated with a Union type, although the desired behaviour is clear.

Example:

    from functools import singledispatch
    from typing import Union

    @singledispatch
    def test(arg):
        raise ValueError(arg)

    @test.register
    def _(arg: Union[int, float]):
        print(f"{arg} is a number")

    test(1)

This dies with:

TypeError: Invalid annotation for 'arg'. typing.Union[int, float] is not a class.

I've implemented a workaround which digs the types out of the union and registers them one-by-one:

    from functools import singledispatch
    from typing import Union, get_type_hints

    def register_for_union_type(target_fn):
        def decorator(fn):
            arg_type = list(get_type_hints(fn).values())[0]
            assert arg_type.__origin__ is Union
            for type_ in arg_type.__args__:
                fn = target_fn.register(type_)(fn)
            return fn

        return decorator

    @singledispatch
    def test(arg):
        raise ValueError(arg)

    @register_for_union_type(test)
    def _(arg: Union[int, float]):
        print(f"{arg} is a number")

    test(1)

Looking at the source for singledispatch is doesn't look like it would too difficult to add support for something like this, though of course there may be subtleties I've missed. Would you be open to a patch?

The only other mentions I've found of this online are:
https://mail.python.org/archives/list/python-ideas@python.org/thread/HF5HDXQ2SXZHO3TWODIRQATTE4TCAWPL/
https://stackoverflow.com/questions/61721761/type-hints-and-singledispatch-how-do-i-include-union-in-an-extensible-w