this post was submitted on 23 Sep 2023
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Python

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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
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[–] [email protected] 13 points 1 year ago (1 children)

from typing import Union is probably what you're looking for, but yes, I'd argue you should try to avoid that kind of pattern, even if it's convenient.

Sorry for the triple(?) notifications. Trying out the beta version of the boost app and it's still a bit buggy.

[–] [email protected] 1 points 1 year ago (1 children)

I thought about it, but it isn't as expressive as I wished.

Meaning if I do

a = foo(return_more=True)
or
a, b = foo(return_more=False)

it doesn't catch these errors for me.

In comparison, the other suggested solution does catch these.

[–] [email protected] 1 points 1 year ago

Yeah, good point, the linked answer seems better suited (even if I would still recommended not having a variable return). I appreciate the feedback!