[Answered] Typehints for functions that have variable signatures - eviltoast

I know what I am asking is rather niche, but it has been bugging me for quite a while. Suppose I have the following function:

def foo(return_more: bool):
   ....
    if return_more:
        return data, more_data
   return data

You can imagine it is a function that may return more data if given a flag.

How should I typehint this function? When I use the function in both ways

data = foo(False)

data, more_data = foo(True)

either the first or the 2nd statement would say that the function cannot be assigned due to wrong size of return tuple.

Is having variable signature an anti-pattern? Is Python’s typehinting mechanism not powerful enough and thus I am forced to ignore this error?

Edit: Thanks for all the suggestions. I was enlightened by this suggestion about the existence of overload and this solution fit my requirements perfectly

from typing import overload, Literal

@overload
def foo(return_more: Literal[False]) -> Data: ...

@overload
def foo(return_more: Literal[True]) -> tuple[Data, OtherData]: ...

def foo(return_more: bool) -> Data | tuple[Data, OtherData]:
   ....
    if return_more:
        return data, more_data
   return data

a = foo(False)
a,b = foo(True)
a,b = foo(False) # correctly identified as illegal
  • InsecureSignpost@kbin.social
    5·
    1 year ago

    In my opinion, it doesn’t. I’d rather have foo() and detailed_foo() over foo(detailed: bool = False).

    Designing APIs can be hard at times. You have to shift your view to the person that will being using the code instead of the person implementing the code. There is also potential down side of returning a tuple or just a single thing if the single thing shares some of the same API as a tuple. Say the return type is Union[str, tuple[str, str]. Now result[0] can either be the first string or the first character of the returned string depending on how the function was called. This could lead to the failure happening farther away from where the bug is, which makes debugging harder. That being said, if you do want to proceed this way, overload with Literal[True] is the correct way to type this as mentioned in other comments.

    I also don’t think it’s overkill to extract functionality just for 2 functions. I often do that even when it is only used in one function. Maybe the number of lines to implement the block starts to make the primary function too long. Or the logic is a bit complicated, so it easier to give it a clearer name.