Unsupported operand types for + ("list[None]" and "list[str]")

[randolf-scholz]

To Reproduce

from typing import Iterable

def prod[T](iter1: Iterable[T]) -> Iterable[tuple[T]]:
    return ((x,) for x in iter1)

x: list[str] = ["a", "b", "c"]
prod([None] + x)  # false positive [operator]

https://mypy-play.net/?mypy=master&python=3.12&gist=390f48f0541325d872d9cb419e3a5f20

[randolf-scholz]

Basically a duplicate of #19304, but the origin of the error is different (seeing how #19304 is fixed with #19399, but this one isn't)

[buildwithmobi]

Unsupported operand types for + ("list[None]" and "list[str]") [operator].

This happens in this line:
prod([None] + x)

In Python, None is not a list, so this:

is interpreted as trying to concatenate:

1. a list containing None: list[None]
2. with a list of strings: list[str]

✅ Solution:

from typing import Iterable, TypeVar

T = TypeVar("T")

def prod(iter1: Iterable[T]) -> Iterable[tuple[T]]:
return ((x,) for x in iter1)

x: list[str] = ["a", "b", "c"]
y: list[str | None] = [None] + x
list(prod(y)) # Example usage

💡 Bonus Tip:
If you're using Python 3.9+, you can write types like list[str | None] instead of List[Union[str, None]].

[randolf-scholz]

@buildwithmobi I do not think this AI slop adds anything to this issue. Meanwhile, I have discovered a few more issues that are essentially reporting the same bug.

• Spurious error when adding two list #15087
• Unsupported operand types for + (List[A] and List[B]) when B extends A #3933
• Mypy is still too eager about outer context #5874