fromtypingimportTypeVar,Generic,List,Iterable,IteratorimportattrT=TypeVar('T')defint_gen()->Iterator[int]:yield1deflist_converter(x:Iterable[T])->List[T]:returnlist(x)@attr.s(auto_attribs=True)classA(Generic[T]):x:List[T]=attr.ib(converter=list_converter)y:T=attr.ib()a1=A([1],2)# E: Argument 1 to "A" has incompatible type "Iterator[int]"; expected "Iterable[T]"

This MR fixes the bug by copying type vars from the field/attrib into the type extracted from the converter.

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chadrik force-pushed theattrs_converters branch 3 times, most recently from98fead7 tof33f58cCompare

March 16, 2023 05:00

Add support for generic attrs converters

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chadrik force-pushed theattrs_converters branch fromf33f58c to531f724Compare

March 16, 2023 05:22

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AlexWaygood added the topic-attrs label

Mar 16, 2023

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chadrik commentedMar 16, 2023

@t4lz @euresti Do either of you have a moment to review this attrs-related change, please?

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Mar 16, 2023

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This change looks good to me.

chadrik commented

Mar 16, 2023

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mypy/plugins/attrs.py

		variables= {
		binder.id:argforbinder,arginzip(converter_vars,init_vars)
		}
		init_type=expand_type(init_type,variables)

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chadrikMar 16, 2023•
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The idea behind this change is that a converter should ideally return the same type as the attrs field that it is added to. For example:

deflist_converter(x:Iterable[T])->List[T]:returnlist(x)@attr.s(auto_attribs=True)classA(Generic[T]):x:List[T]=attr.ib(converter=list_converter)

A.x is typeList[T] andlist_converter returnsList[T], and in theory this should always be the case for converters, since their purpose is to convert the init arg to the type of the field. In my changes I'm forgoing a complete comparison of the two types, but the assumption tells us that the number and order of TypeVars should be the same. We use that to generate a replacement map which will replace the converter's broken typevars, with typevars that work within the class's context.