[Question] Is it possible to signal a function returns a specific value

[sk-]

Hi everyone, I'm trying to add type annotations for a third_party module, namely more_itertools. The function I'm having issues with is first_true, which is defined as

def first_true(iterable, default=False, pred=None): ...

This means that the return type would be either Union[T, bool] or Union[T, U] when there's an explicit default.

The problem is that in the case of an implicit default, the pattern

element = find_true(elements, pred=....)
if element:
  # do something with the element

would not work as inside the if, element could still be True.

What I would like is to signal that the return type is either T or the value False, is there any idiom to do that?

Note that the same is true for builtin functions like next. See
https://github.com/python/typeshed/blob/master/stdlib/3/builtins.pyi#L862

ps: I know this would be solved if the API changed to have default=None instead.

[JelleZijlstra]

That's not currently supported, but the mypy team is planning to add support for a feature like this ("final types") soon.

[srittau]

I think the question has been answered.

[sk-]

@srittau It would be great if you could add a link to the tracking issue for "the mypy team is planning to add support for a feature like this ("final types") soon."

[srittau]

Dependent types (I think this is what @JelleZijlstra was referring to) are discussed in python/mypy#3062.