New submission from Dave Evans <d...@evans.io>:
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-id...@python.org/thread/HF5HDXQ2SXZHO3TWODIRQATTE4TCAWPL/
https://stackoverflow.com/questions/61721761/type-hints-and-singledispatch-how-do-i-include-union-in-an-extensible-w
----------
components: Library (Lib)
messages: 408018
nosy: evansd
priority: normal
severity: normal
status: open
title: functools.singledispatch does not support Union types
type: enhancement
versions: Python 3.10, Python 3.11, Python 3.7, Python 3.8, Python 3.9
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46014>
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