two-arg iter() with None sentinel does not remove ‘Optional’

[wbolster-eiq]

[moved from https://github.com/python/mypy/issues/7371]

The 2-arg form of iter() takes a callable and a sentinel value. Quoting its docstring:

iter(callable, sentinel) -> iterator

[...], the callable is called until it returns the sentinel.

When the sentinel is the literal None, this means that a callable that returns an Optional[T] is transformed into a Iterator[T]. However, mypy does not detect that, and reveal_type on the iter() result claims its Revealed type is 'typing.Iterator[Union[T, None]]', which allows for None values, which is impossible.

Here's a small example:

from typing import Optional

def maybe_int() -> Optional[int]:
    return 123  # or maybe None

for n in iter(maybe_int, None):
    print(n + n)

Even though n is always an int inside the loop (never None), mypy (version 0.720) is not aware of that, and complains:

error: Unsupported operand types for + ("int" and "None")
error: Unsupported operand types for + ("None" and "int")
error: Unsupported left operand type for + ("None")
Both left and right operands are unions

Since the code is perfectly safe, mypy should infer that n is of type int (not Optional[int]) and should not report any errors about adding int and None values.

[wbolster-eiq]

current definition:

@overload
def iter(__function: Callable[[], _T], __sentinel: _T) -> Iterator[_T]: ...

[srittau]

Changing the overload to use two type vars could work, but I haven't tested it:

@overload
def iter(__function: Callable[[], Union[_T, _U]], __sentinel: _U) -> Iterator[_T]: ...