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
    • @[email protected]OPEnglish
      2•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!

    • @[email protected]
      1•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