None is accepted as a type but it behaves erratically

[JukkaL]

The following fragment generates a confusing error message:

def f(x: None) -> None:
    x.f()   # Function does not return a value

[ddfisher]

After we add Optional checking, maybe we can make None more consistently mean "the type containing just the value None" instead of also "the uninhabited type" and "a value you can't use".

[ddfisher]

Fixed by #3024.

[orenbenkiki]

In mypy version 0.501, if you write:

from typing import List
from typing import Optional
from typing import Union
from typing import Generic
from typing import TypeVar

a: List[None] = [None]
a.append(None)

b: List[Optional[None]] = [None]
b.append(None)

c: List[Union[None]] = [None]
c.append(None)

T = TypeVar('T')

class Foo(Generic[T]):
    def __init__(self, value: T) -> None:
        return

    def append(self, value: T) -> None:
        return

f: Foo[None] = Foo(None)
f.append(None)

Then mypy will complain:

example.py:7: error: List item 0 has incompatible type None
example.py:8: error: Argument 1 to "append" of "list" has incompatible type None; expected None
example.py:10: error: List item 0 has incompatible type None
example.py:11: error: Argument 1 to "append" of "list" has incompatible type None; expected None
example.py:13: error: List item 0 has incompatible type None
example.py:14: error: Argument 1 to "append" of "list" has incompatible type None; expected None
example.py:25: error: Argument 1 to "Foo" has incompatible type None; expected None
example.py:26: error: Argument 1 to "append" of "Foo" has incompatible type None; expected None

So it seems that using None as a type still doesn't behave exactly as "the type containing just the value None" - more over, it seems like there's no way to trick mypy into declaring such a type.

[JukkaL]

@orenbenkiki 0.501 is pretty old. Have you a tried the latest versions? it works for me on git master.

[orenbenkiki]

Ah, it works in version 0.550. Thanks.

Version 0.501 was released in March this year, if that makes it "old.".. it is great to see mypy advancing so fast!

[JukkaL]

Yeah, 0.501 is already old in "mypy years" :-)