[Typing-sig] Re: Unions and initialization across type checkers

My 2 cents from a user's perspective.

Am 02.09.20 um 20:26 schrieb Guido van Rossum:

Consider these two similar functions:

from typing import Union

def f() -> int:
    x: Union[int, str] = 42
    return x  # Error here

def g() -> int:
    x: Union[int, str]
    x = 42
    return x  # But not here

The latter is very surprising to me. I would expect an error here, since Union[int, str] is not a sub-type of int. I would also expect it to be consistent with the first example.

There's another twist (note the return type is str here):

from typing import Any

def h() -> str:
    x: Any = 42
    return x

This is an error in pyre but not in mypy, pytype or pyright. And for good measure the other variant:

I wouldn't expect an error (especially not with default options). Any is compatible with all other types and is supposed to be a "free out of jail card" in case of type problems.

def i() -> str:
    x: Any
    x = 42
    return x

This is accepted by mypy and pyright, but an error to pyre and pytype.

Same here.

- Sebastian