Support PEP 604 - union types as X | Y

[cdce8p]

Feature
Add support for PEP 604 to mypy. Since it has been officially accepted, should mypy add support for X | Y now? The syntax won't change anymore, is backwards compatible and there is an existing implementation https://github.com/pprados/mypy/tree/PEP604

Pitch
Adding this now would allow projects to already take advantage of the new syntax for type checking when using
from __future__ import annotations.

[JelleZijlstra]

We should definitely add support, and it's great there's already an implementation by @pprados. Looks like this feature will be available only in 3.10, so there's no great hurry. I'm guessing we'll also need typeshed changes to add type.__or__.

[cdce8p]

@JelleZijlstra That's not entirely correct. Although type.__or__ will only be added in 3.10, X | Y for typing is backwards compatible. The AST parser already knows how to handle it. I did some tests with the original implementation. Type comments work without issue. For type annotations from __future__ import annotations is necessary PEP 563.

from __future__ import annotations

t1: str | None = "Hello World"
t2 = "Hello World"  # type: str | None

What doesn't work are type aliases. The following would cause a type error:

from __future__ import annotations

T1 = str | None  # TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'
d: T1 = "Hello World"

[hauntsaninja]

@cdce8p it occurred to me that isinstance(x, int | str) won't yet work. As Jelle mentioned, that'll probably require typeshed changes, but thought I'd flag in case you want to try getting that working :-)

[cdce8p]

@hauntsaninja Thanks for letting me know. To be honest, I just wanted to replace Optional 😅
Although with the PR now finished, the Typeshed changes should be all to fully support PEP604 once 3.10 is released (if I didn't miss anything)

[hauntsaninja]

Would you be up for making the needed changes to typeshed / adding a test to mypy with an isinstance check (with --python-version 3.10, of course)? (no rush, just at some point in the next year 😊)

[cdce8p]

I can try. No promises :)
At the moment I'm still waiting for a review on open Typeshed PR. Would you mind taking a look? I don't seem to get any new responses. python/typeshed#4752

[matejcik]

What doesn't work are type aliases.

are type aliases supposed to work? ISTM from reading PEP0604 that they are, but mypy is still failing on them

[JelleZijlstra]

@matejcik yes, they should work (always in stubs and at runtime in Python >=3.10). If they don't, that's a bug in mypy.

[cdce8p]

Runtime uses of the PEP 604 aren't supported by mypy yet, see: #9880