Improve typing for Results Backend. about dramatiq

[orsinium]

In your example, MResult is a Union of Missing and Result which means you have to return an instance of Missing, not the type itself. But then refining the type on the caller side will be hard.

The scenario you showed is called a "sentinel value", and there is an open issue for supporting it in python typing: python/typing#689 . The current approach is to use an enum with one value:

class Missing(Enum):
    MISSING = 0

def foo(value: bool) -> int | Literal[Missing.MISSING]:
    if value:
        return Missing.MISSING
    return 5

[jenstroeger]

Oh sorry, I think in that case

MResult = typing.Union[type(Missing), Result]

should work.

[orsinium]

type[Missing] kinda would but then you can't use it with Literal, and literal types allow better type refining, especially for override. FOr instance, your example can be rewritten as:

@override
def foo(value: Literal[True] -> Literal[Missing.MISSING]: ...
def foo(value: Literal[False] -> int: ...

def foo(value: bool) -> int | Literal[Missing.MISSING]:
    if value:
        return Missing.MISSING
    return 5

In that case, if I call foo(False), mypy will know that I won't get missing result, only int. Maybe this example doesn't give enough justice to that scenario, though, it plays a bit better with argument types rather than return types. You can find more examples in the issue I linked above.

[jenstroeger]

I just wanted to suggest a minor adjustment to the existing code, and tried to be as compatible with the existing approach as possible.

Unfortunately I don’t have the bandwidth at the moment to refactor more of the code to add proper typing, and there are other issues which discuss that, e.g.

• #305
• #521