Different behavior between `Union` and `|` in mypy when dealing with optionals

[jevandezande]

In the following two code examples, the first gives an error, while the latter does not. As I understand type1 | type2 is supposed to be an alias for Union[type1, type2] in python3.10 (hence the from __future__ import annotations).

from __future__ import annotations

def f(a: int | str = None):
    pass

error: Incompatible default for argument "a" (default has type "None", argument has type "Union[int, str]")

from typing import Union

def f(a: Union[int, str] = None):
    pass

As of PEP 484, non-explicit optional is discouraged. Is this the correct behavior, or a bug?

Your Environment

• python = 3.9.1
• mypy = 0.812

[JelleZijlstra]

This is related to the --no-implicit-optional flag. We're already planning to flip the default for this flag (#9091), so maybe it's a good thing that the special casing doesn't apply to the new | operator.

[hauntsaninja]

It wasn't intentional that implicit optionals don't work with PEP 604 syntax, but I'm not going to add the special casing for it. As you say, PEP 484 discourages this, it's not really supported by other type checkers, and the next release of mypy flips the default for --no-implicit-optional.