mypy_dependent_type.py

from typing import Literal, Optional, Tuple, overload

@overload
def f(with_extra_info: Literal[False]) -> Tuple[int, int]: ...

@overload
def f(with_extra_info: Literal[True]) -> Tuple[int, int, str]: ...

# This overload is necessafy for type checking the last `f(with_extra_info)`.
@overload
def f(with_extra_info: bool) -> Tuple[int, int] | Tuple[int, int, str]: ...

def f(with_extra_info: bool) -> Tuple[int, int] | Tuple[int, int, str]:
    if with_extra_info:
        return (0, 1, 'extra info')
    else:
        return (0, 1)

with_extra_info: bool = True
extra_info: Optional[str]

if with_extra_info:
    a, b, extra_info = f(with_extra_info)
else:
    a, b = f(with_extra_info)
    extra_info = None

f(with_extra_info)