Comment by N1H1L

3 months ago

I have a problem with Python's `Optional` type. For example for this following code:

   from typing import Optional, Union
   def square(
      a: Union[int, float], 
      b: Optional[int] = 2
   ) -> float:
      c = a**b
      return c

Many type checkers throw an error because `Optional[int]` actually means `int | None` and you cannot square an `int` or a `float` with a `None`. Is there any plans for *ty* around this?

1 comment

N1H1L

dcreager 3 months ago

I think that's a genuine error, since as you say, `None` is a possible value for `b` according to your signature.

To handle this you would need to use "narrowing" to separately handle the case where `b` is `None`, and the case where it is not:

  def square(a: Union[int, float], b: Optional[int] = 2) -> float:
      if b is None:
          return 0
      else:
          c = a**b
          return c

https://play.ty.dev/97fe4a09-d988-4cc3-9937-8822e292f8d1

(This is not specific to ty, that narrowing check should work in most Python type checkers.)