python 3.x - How to get Mypy to realize that sorting two ints gives back two ints

Question

My code is as follows:

from typing import Tuple

a: Tuple[int, int] = tuple(sorted([1, 3]))

Mypy tells me:

Incompatible types in assignment (expression has type "Tuple[int, ...]", variable has type "Tuple[int, int]")

What am I doing wrong? Why can't Mypy figure out that the sorted tuple will give back exactly two integers?

Answer 1

The call to sorted produces a List[int] which carries no information about length. As such, producing a tuple from it also has no information about the length. The number of elements simply is undefined by the types you use.

You must tell your type checker to trust you in such cases. Use # type: ignore or cast to unconditionally accept the target type as valid:

# ignore mismatch by annotation
a: Tuple[int, int] = tuple(sorted([1, 3]))  # type: ignore
# ignore mismatch by cast
a = cast(Tuple[int, int], tuple(sorted([1, 3])))

Alternatively, create a lenght-aware sort:

 def sort_pair(a: T, b: T) -> Tuple[T, T]:
     return (a, b) if a < b else (b, a)