This is the real answer, overloads are meant for exactly this purpose.

It’ll be something like this:

from typing import Literal, overload

@overload
def foo() -> Data: …
@overload
def foo(return_more: Literal[True]) -> tuple[Data, Data]: …
def foo(return_more: bool = False) -> Data | tuple[Data, Data]
    ...
    if return_more:
        return data, more_data
   return data