TypeAlias cannot be indirectly imported?

Python Help
1

Dear all,

I received plenty of interesting comments regarding my previous question, so that I indulge myself in another one.

In file a.py, you import TypeAlias from module typing

from typing import TypeAlias

In file b.py, you import all of module a and use TypeAlias

from a import *
t:TypeAlias = int

Then Pyright raises an error:

error: Expression of type "Type[int]" cannot be assigned to declared type "TypeAlias"
    "Type[type]" is incompatible with "Type[TypeAlias]"

However, if I import TypeAlias direcly in module b, it works!

# from a import *
from typing import TypeAlias
t:TypeAlias = int

It also works if I use the extended name of TypeAlias, that is, typing.TypeAlias

# from a import *
import typing
t:typing.TypeAlias = int

In the typing.py source code, it is mentioned that TypeAlias is not a type-as-usual, but kind of a “Special marker”. It is emphasized that its use is strictly restricted to the form above, but nothing is specified about importation.

The official doc refers to PEP613, but I am not sure to understand all of it. At least, word “import” does not appear in the document.

Could anybody explain how it should be used properly?

Best regards,

Luc.

2

That looks like it might be a bug in pyright. I’d recommend filing a bug report at the issue tracker here, and seeing what Eric Traut thinks: Issues · microsoft/pyright · GitHub

3

Thanks, Alex. Will do. Luc

1 Like
4

Here is Eric Traut’s answer. He confirms that it is a design feature. I agree with him that the real solution is a specific syntax for type declaration, as proposed in his PEP 695.

Luc.

Copied from Pyright Issue #3941

You appear to be re-exporting a symbol called TypeAlias from your own module. TypeAlias is a special form that requires special-case handling in static type checkers. In all other languages that I know of, type alias declarations have their own syntax and keyword, but Python is a bit backward in this regard. I’ve authored PEP 695, which proposes to add such a facility to Python 3.12.

Pyright process the TypeAlias symbol in its “binder”, which runs prior to any type analysis. That explains the limitation you are seeing here.

Is there a reason you don’t want to import TypeAlias directly from typing or typing_extensions? I’ve never seen this done before.

The obvious workaround is to avoid doing this and import TypeAlias from typing directly.

1 Like
5

“from foo import *” is usually best avoided.