[Python] How to annotate a function that preserves type but applies a transformation to...

Let's say I have a function that takes only an argument and is guaranteed to return a value of the same type as the argument. Inside, depending on the specific type of the argument, it will apply some transformations or others.

A minimal example could be something like this:

from typing import TypeVar

T = TypeVar("T")
S = TypeVar('S', bound=str)

def str_transform(s: S) -> S:
return s

def take_any_transform_only_str(obj: T) -> T:
if isinstance(obj, str):
return str_transform(obj)
return obj


It seems straightforward that take_any_transform_only_str will always return a value of the same type than the argument it was given. And yet, mypy (v1.15.0) complains with:

test.py:11: error: Incompatible return value type (got "str", expected "T") [return-value]


What would be the correct way to annotate this function?

(Probably I could use typing.overload, but I'd very much prefer a solution based on type variables than having to manually annotate the different cases)

Continue reading...