How to declare a forward type alias in Python 3.8 with recent `Union` change?

[Azureblade3808]

I have been working on Windows 7, which doesn't support Python over 3.8.

I used to write code like

from typing import Union

Alias = Union["Origin"]

def do_something_with_alias(x: Alias):
    ...

class Origin:
    ...

while using Union as a work-around for lack of TypeAlias.

But with the recent change of Union (probably since microsoft/pyright#2479), such usage would cause an error of "Union requires of two or more type arguments".

So, is there another way to accomplish a forward type alias in Python 3.8? Or is it possible to mute this error?

[erictraut]

A few suggestions:

1. Don't use a type alias. Just use "Origin" directly.
2. Use Alias: TypeAlias = "Origin" and import TypeAlias from typing_extension.
3. Use a # type: ignore statement on the line where you declare the alias using the single-argument Union.

[Azureblade3808]

Thank you for the suggestions. Good to know these ways, and the typing_extensions one best suits my need.

[jwannebo]

Unfortunately, support for PEP 613 and TypeAlias still isn't present in the latest mypy release (Though a PR has gone through to master python/mypy/pull/11305 ), and until it is, foo_alias = Union[foo] is still the idiomatic way to add type aliases as members of a class in mypy: python/mypy#7866 (comment)

Until either this change is toggleable in Pylance, or mypy issues a new release, the only way to have mypy and Pylance agree on the status of the following example is to # type: ignore which isn't ideal by any means.

class Foo():
    my_alias = Union[int] # Pylance: "Union requires two or more type arguments"
    my_alias: TypeAlias = int # MyPy 0.910:  'Variable "typing_extensions.TypeAlias" is not valid as a type'
    
    def bar() -> my_alias:
        return 0

[erictraut]

@jwannebo, I think that PEP 613 support is in the next release of mypy, which will be coming out very soon.

In the meantime, you can either use # type: ignore or supply the type twice in the Union, like this: my_alias = Union[int, int]. Both are somewhat ugly, but a better solution will arrive shortly.