False positive with attrs converter

[tsx]

Looks like mypy can't infer generic parameter properly when used in attrs converter.

from attr import attrs, attrib
from typing import FrozenSet, Text


@attrs
class Test(object):
    values = attrib(type=FrozenSet[Text], converter=frozenset)


Test(values={'a'})

Actual: reporting error

error: Argument 1 to <set> has incompatible type "str"; expected "_T"

Expected: no errors.

mypy version 0.770, python 3.5.2, no flags are used.
Latest master has basically the same error message except generic param has a different name:

error: Argument 1 to <set> has incompatible type "str"; expected "_T_co"

Possibly related: #5313

[msullivan]

Yeah, that does look wrong. Not sure how easy fixing it would be.

[markusschmaus]

This also happens in case of a generic function.

from attr import attrs, attrib
from typing import FrozenSet, Text, Iterable, TypeVar

_T = TypeVar('_T')
def converter(ar: Iterable[_T]) -> FrozenSet[_T]:
    return frozenset(ar)

@attrs
class Test(object):
    values = attrib(type=FrozenSet[Text], converter=converter)


Test(values={'a'})

results in an error:

error: Type variable "scratch_11._T" is unbound

I looked into how this could be fixed and here is what I found out for reference:
The attrs plugin gets called during

mypy/mypy/semanal_main.py

Line 78 in 41927cf

process_top_levels(graph, scc, patches)

At this point the type of the converter isn't fully processed so when attrs looks at its type at

mypy/mypy/plugins/attrs.py

Line 98 in 41927cf

converter_type = get_proper_type(converter_type)

converter.is_generic returns False even though the function is generic. The full type of this function is only established during

mypy/mypy/semanal_main.py

Line 79 in 41927cf

process_functions(graph, scc, patches)

Which the attrs plugin has already completed.

[markusschmaus]

I opened a PR which fixes this issue for builtin types, including frozenset. https://github.com/python/mypy/pull/8876/files

It doesn't fix the issue for generic functions, since their type still contains UnboundTypes when the attr plugin is run.

[ryansobol]

Just ran into this bug. Thank you so much for submitting a PR!