Type aliases at function scope does not work as expected

[martin-ejdestig]

The following code:

from typing import Dict

def foo(text: str) -> str:
    Bar = str
    Baz = Dict[Bar, int]
    return ''

Results in mypy0.630 reporting 'error: Invalid type "Bar"'.

If Bar is moved to file scope, there are no warnings.

[ilevkivskyi]

This is actually by design. At nested scopes, such as class or function, assignments with a class object in the r.h.s. create a variable with type Type[...] instead of an alias. The reason is that this is much more typical use case. Note that a subscripted assignment like Baz = Dict[str, int] still creates an alias.

There is a long overdue documentation issue about this #3494, and also about the extra-terse error message #4030. I will raise priorities of both to high.

[martin-ejdestig]

It is very confusing though.

I am also wondering, is there a way to create an alias for str at local scope like in this example? Do not want to pollute the outer namespace.

[ilevkivskyi]

Why do you need create an alias to str? Maybe what you want is a NewType actually?

[martin-ejdestig]

For readability. Did not know about NewType, will try to use that instead.

[martin-ejdestig]

NewType worked. Thank you. (Had to convert str to Baz in a couple places but I guess I gained a bit of type safety.)

Would like to reiterate that it is still very confusing though. Especially since just moving Bar to file scope works. Documentation and better error message will help, but it is still not intuitive.

[ilevkivskyi]

Would like to reiterate that it is still very confusing though.

Just stating this is not very helpful, what is the solution that you propose?

[martin-ejdestig]

I know too little about how mypy relates to Python and the language specification. I assume it is too late to change so that "Bar = str" means the same thing at file and function scope? Or is that something that can be done?