tox4: typing error when add_config's of_type is Optional[...]

[dcrosta]

With the following code:

class MyConfigSet(ConfigSet):
    def register_config(self) -> None:
        self.add_config(
            keys=["key"],
            of_type=Optional[str],
            default=None,
            desc="desc",
        )

I get the following mypy error:

error: Argument "of_type" to "add_config" of "ConfigSet" has incompatible type "object"; expected "Type[None]"

if I change the default to any string (eg ""), the error changes to:

error: Argument "of_type" to "add_config" of "ConfigSet" has incompatible type "object"; expected "Type[str]"

[gaborbernat]

The problem removed from the tox part, get this to work:

from typing import Optional, Type, TypeVar

V = TypeVar("V")


def call(of_type: Type[V], default: V) -> None:
    assert default is not None
    assert of_type is not None


call(str, "str")  # works
call(int, 1)  # works

call(Optional[str], None)  # Argument 1 to "call" has incompatible type "object"; expected "Type[None]"
call(Optional[str], "")  # Argument 1 to "call" has incompatible type "object"; expected "Type[str]"

[gaborbernat]

So seems for mypy Optional[str] is not a type; you'd need to define the type as type('', (type(None), str), {}) instead:

class MyConfigSet(ConfigSet):
    def register_config(self) -> None:
        self.add_config(
            keys=["key"],
            of_type=type("", (type(None), str), {}),
            default=None,
            desc="desc",
        )

Which while makes it work is not trivial or easy to guess. In light of this our options are:

1. Document using type("", (type(None), str), {})
2. Disable type checking here
3. Add a mypy plugin that handles this
4. First 2 and then later 3

I'm tempted to say to do option 4. @asottile @jugmac00 @dcrosta opinions?

[gaborbernat]

Seems no opinions, so I recommend disabling the type checker in these lines.