[Answered] Typehints for functions that have variable signatures (lemmy.cafe)

submitted 1 year ago* (last edited 1 year ago) by 0WN3D@lemmy.cafe to c/python@programming.dev

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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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[-] __init__@programming.dev 8 points 1 year ago

You may be able to achieve this using typing.Overload with typing.Literal for your argument. Check out this post about overload: https://adamj.eu/tech/2021/05/29/python-type-hints-how-to-use-overload/

[-] nikaro@jlai.lu 4 points 1 year ago

Nice! It looks like the best solution out there.

[-] 0WN3D@lemmy.cafe 2 points 1 year ago* (last edited 1 year ago)

yea, this is pretty close to what I'm looking for.

The only missing piece is the ability to define the overload methods on the bool

something like

@overload
def foo(return_more: True) -> (Data, Data)

@overload
def foo(return_more: False) -> Data

But I don't think such constructs are possible? I know it is possible in Typescript to define the types using constants, but I don't suppose Python allows for this?

EDIT: At first, when I tried the above, the typechecker said Literal[True] was not expected and I thought it was not possible. But after experimenting some, I figured out that it is actually possible. Added my solution to the OP

Thanks for the tip!

[-] eternacht@programming.dev 1 points 1 year ago

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