Unable to assign type to generic (probably working as intended?)

[Sxderp]

I don't know the proper terminology to use to describe the issue so I'm having a hard time searching for posts.

I have the following code (simplification of actual code to demonstrate issue).

from typing import TypeVar

T = TypeVar('T')

def filter_unprintable(dfx: T) -> T:
    if isinstance(dfx, str):
        return dfx.translate({10: None})
    return dfx

This gives

Type "str" is not assignable to return type "T@filter_unprintable"
Type "str" is not assignable to type "T@filter_unprintable" (reportReturnType)

Basically, what can I do to type this without errors?

[erictraut]

The problem here is that str.translate always returns a str. That means if T is some subclass of str, calling translate will change it into a str rather than retaining its original type.

Your options include:

1. After calling translate, manually construct a new value that is guaranteed to have the same type as dfx:

return type(dfx)(dfx.translate({10: None}))

2. Use an overload:

@overload
def filter_unprintable(dfx: str) -> str: ...
@overload
def filter_unprintable(dfx: T) -> T: ...
def filter_unprintable(dfx: T | str) -> T | str:
    if isinstance(dfx, str):
        return dfx.translate({10: None})
    return dfx

3. Use a cast to tell the type checker to ignore the error. This is perhaps the easiest, but it also makes your code less robust.

def filter_unprintable(dfx: T) -> T:
    if isinstance(dfx, str):
        return cast(T, dfx.translate({10: None}))
    return dfx