PEP 604 and aliases

[asottile]

Feature

PEP 604 | operator should be sufficient to make an alias

Pitch

This is accepted:

from typing import Union
Alias = Union[int, str]
def f(x: Alias) -> None: ...
f(1)
f('2')

so the equivalent post-PEP-604 I believe should be allowed as well?

Alias = int | str
def f(x: Alias) -> None: ...
f(1)
f('2')

currently it produces this output:

$ mypy t.py --python-version 3.10
t.py:1: error: Unsupported left operand type for | ("Type[int]")
t.py:2: error: Variable "t.Alias" is not valid as a type
t.py:2: note: See https://mypy.readthedocs.io/en/latest/common_issues.html#variables-vs-type-aliases
Found 2 errors in 1 file (checked 1 source file)
$ cat t.py 
Alias = int | str
def f(x: Alias) -> None: ...
f(1)
f('2')
$ mypy --version
mypy 0.800

[hauntsaninja]

Yeah, the PEP 604 support in mypy (and typeshed) doesn't yet cover anything that would require runtime support. This is the same as #9880

[gvanrossum]

Probably we should first support PEP 613 (TypeAlias) and then you can write

A: TypeAlias = int | str

[asottile]

cool -- will follow #9880 instead 👍