Can't convert from Tuple of optional to Tuple of non-optional

[kventinel]

Code:

from typing import Tuple, Optional

a: Tuple[Optional[int], Optional[int]] = (1, 1)
b: Tuple[int, int]
if a[0] is not None and a[1] is not None:
    b = a

After that has error:

script.py:6: error: Incompatible types in assignment (expression has type "Tuple[Optional[int], Optional[int]]", variable has type "Tuple[int, int]")

Expected no error.
I have: mypy-0.770 and Python 3.7.7

[JelleZijlstra]

Did you mean to use and instead of or? (I haven't checked whether mypy handles that case correctly.)

[kventinel]

Ohhh. Sorry for wrong example. Yes, I mean and instead of or. Currently fix it and got same error.

[msullivan]

It works to write b = a[0], a[1]. I think propagating that fact up to the type of a itself is probably not something we're going to support.

[kventinel]

Why? Without this given error look strange. And tuple can have more than 2 element, in this case line with "crutch" will be too long.

Also I have example with cast list of optional to list of non-optional and double check on None.

from typing import Tuple, Optional, List
from typing import Tuple, Optional, List


def case0() -> None:
    a: Tuple[Optional[int], Optional[int]] = (1, 1)
    b: Tuple[int, int]
    if a[0] is not None and a[1] is not None:
        b = a  # wrong
        b = a[0], a[1]  # good


def case1() -> None:
    a: List[Optional[int]] = [1] * 100
    b: List[int]
    if all([i is not None for i in a]):
        b = a  # wrong
        b = [i for i in a]  # wrong
        b = [a[i] for i in range(len(a))]  # wrong
        b = [i for i in a if i is not None]  # good
    if all([a[i] is not None for i in range(len(a))]):
        b = a  # wrong
        b = [i for i in a]  # wrong
        b = [a[i] for i in range(len(a))]  # wrong
        b = [i for i in a if i is not None]  # good

Errors:

script.py:8: error: Incompatible types in assignment (expression has type "Tuple[Optional[int], Optional[int]]", variable has type "Tuple[int, int]")
script.py:16: error: Incompatible types in assignment (expression has type "List[Optional[int]]", variable has type "List[int]")
script.py:17: error: List comprehension has incompatible type List[Optional[int]]; expected List[int]
script.py:18: error: List comprehension has incompatible type List[Optional[int]]; expected List[int]
script.py:21: error: Incompatible types in assignment (expression has type "List[Optional[int]]", variable has type "List[int]")
script.py:22: error: List comprehension has incompatible type List[Optional[int]]; expected List[int]
script.py:23: error: List comprehension has incompatible type List[Optional[int]]; expected List[int]

[JukkaL]

@kventinel The reason why we are unlikely to support this is that the implementation looks non-trivial and this use case seems quite rare, relative some other use cases that we also don't support well. So at least the fix would be low priority.

About your other examples: you can create separate issues about these; we'll consider each case where type inference goes wrong separately.

[kventinel]

But what so non-trivial in the implementation? I think, it's not so difficult propagate new type to element of fixed size tuple as it's work with single value.

And I think, that my other examples even more non-trivial than first:)

And I think, this issue must be reopened as bug (even if it would be low priority).

[JukkaL]

Fair enough. If somebody comes up with a simple enough implementation, we will consider merging a PR.