Cannot assign an `Optional[float]` to a `float`

[fpdotmonkey]

I have the class below which can optionally take a tuple, which itself optionally has float members, so (1.0, 2.0), None, (1.0, None), etc are all valid inputs.

I do checking in the class to see if None has been input, and ignore the input in those cases, but mypy is saying Optional[float] and float are incompatible for these assignments.

How can I specify to mypy that I've done my due diligence in type checking the input?

# foo.py
from typing import Optional, Tuple


class Foo:
    def __init__(
        self, bounds: Optional[Tuple[Optional[float], Optional[float]]] = None
    ):
        self._lower_bound: float = float("-inf")
        self._upper_bound: float = float("inf")
        if bounds:
            if bounds[0]:
                self._lower_bound = bounds[0]
            if bounds[1]:
                self._upper_bound = bounds[1]


Foo((1, 2))

$ mypy foo.py
foo.py:12: error: Incompatible types in assignment (expression has type "Optional[float]", variable has type "float")
foo.py:14: error: Incompatible types in assignment (expression has type "Optional[float]", variable has type "float")
Found 2 errors in 1 file (checked 1 source file)

I'm running mypy 0.812 with Python 3.8.5

[JelleZijlstra]

The problem here is that mypy doesn't do type narrowing on bounds[0]. Here's a shorter example to demonstrate that:

from typing import Optional
tpl: tuple[Optional[int]]
reveal_type(tpl[0])  # int | None
if tpl[0]:
    reveal_type(tpl[0])  # int | None

There are some similar existing issues like #7339, but Ivan's comments on that issue suggest that tpl[0] should work for type narrowing.

[hauntsaninja]

If you rephrase your check as bounds[0] is not None, it works

[fpdotmonkey]

If you rephrase your check as bounds[0] is not None, it works

Ok this works. It does seem unpythonic to disallow truthiness, but whatever works.

[hauntsaninja]

Yeah, definitely a bug / am sort of surprised by this

[Akuli]

No longer happens on master:

(env310) akuli@akuli-desktop:~/mypy$ python3 -m mypy a.py
Success: no issues found in 1 source file

(env310) akuli@akuli-desktop:~/mypy$ cat a.py
from typing import Optional, Tuple
class Foo:
    def __init__(
        self, bounds: Optional[Tuple[Optional[float], Optional[float]]] = None
    ):
        self._lower_bound: float = float("-inf")
        self._upper_bound: float = float("inf")
        if bounds:
            if bounds[0]:
                self._lower_bound = bounds[0]
            if bounds[1]:
                self._upper_bound = bounds[1]
Foo((1, 2))

[hauntsaninja]

Thanks, I think this was fixed by #11202