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Cannot get attrs.Converter() with takes_self pass through mypy #1378

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nyoxi opened this issue Dec 5, 2024 · 3 comments · May be fixed by #1382
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Cannot get attrs.Converter() with takes_self pass through mypy #1378

nyoxi opened this issue Dec 5, 2024 · 3 comments · May be fixed by #1382

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@nyoxi
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nyoxi commented Dec 5, 2024

I've been playing with the attrs.Converter() but I am unable to get it type-checked with mypy when using takes_self. Now I am not considering (the obvious) Unsupported converter, only named functions, types and lambdas are currently supported error. What I am struggling with is how to annotate the self. The following code:

def str2int(s: str|int, self_: "C") -> int:
    return int(s)

@attrs.define
class C:
    x: int = attrs.field(
        converter=attrs.Converter(str2int, takes_self=True)
    )

Leads to the following error:

error: No overload variant of "Converter" matches argument types "Callable[[str | int, C], int]", "bool"  [call-overload]
note: Possible overload variants:
note:     def [In, Out] Converter(self, converter: Callable[[In], Out]) -> Converter[In, Out]
note:     def [In, Out] Converter(self, converter: Callable[[In, AttrsInstance, Attribute[Any]], Out], *, takes_self: Literal[True], takes_field: Literal[True]) -> Converter[In, Out]
note:     def [In, Out] Converter(self, converter: Callable[[In, Attribute[Any]], Out], *, takes_field: Literal[True]) -> Converter[In, Out]
note:     def [In, Out] Converter(self, converter: Callable[[In, AttrsInstance], Out], *, takes_self: Literal[True]) -> Converter[In, Out]

Is this just my own ignorance or an issue with attrs or mypy?

I am using:

  • attrs 24.2.0
  • mypy 1.13.0
@nyoxi
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nyoxi commented Dec 5, 2024 •
edited
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I just noticed that the check passes with mypy 1.12.1 which may suggest a regression in mypy.
Edit: Please ignore that.

@filbranden filbranden mentioned this issue Dec 13, 2024
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@nyoxi
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nyoxi commented Dec 13, 2024 •
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I forgot to circle back here. After playing a bit more with converters and validators I figured that the solution is actually pretty simple and straightforward:

def str2int(s: str|int, self_: attrs.AttrsInstance) -> int:
    if not isinstance(self_, C):
        raise TypeError(f"Converter does not know how to work with {type(self_)}")
    # Now the type of `self_` is properly inferred as `C`
    return int(s)

@attrs.define
class C:
    x: int = attrs.field(
        converter=attrs.Converter(str2int, takes_self=True)
    )

I am closing the issue. If anyone thinks this is worth documenting I can prepare a pull request for that.

@nyoxi nyoxi closed this as completed Dec 13, 2024
@filbranden
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Also possible to use typing.cast here.

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Fix type signature for callables passed to Converter with takes_self=True filbranden/attrs
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@filbranden @nyoxi