Python @programming.dev 0WN3D @lemmy.cafe 1y ago

[Answered] Typehints for functions that have variable signatures

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

You're viewing a single thread.

18 comments

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/
- Nice! It looks like the best solution out there.
- 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!
- 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

18 comments