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
  • Dark ArcAEnglish
    1·
    1 year ago

    I think you stand on the former

    Absolutely.

    Suppose we have a function that calculates a price of an object. I feel it is agreeable for us to have compute_price(with_discount: bool), over compute_price_with_discount() + compute_price_without_discount()

    Well, presumably you’d also actually have some other inputs to a price compute function. In which case, I’d suggest bundling all that information into an Invoice type or something that includes whether or not discounts are applied…

    Maybe this is bandwagoning, but one of the reason for my stance is that there are quite a few examples of variable returns.

    getattr is really special, it’s basically a reflection operator, it shouldn’t be a model for how a normal function should behave.

    I’m not familiar with numpy. The linked function though looks like a true case of generic behavior where an input changes an output in a specified way for any number of values that meet its requirements. A boolean flag is never generic.