mypy plugin incorrectly computes return type for some parameters

[anlambert]

After upgrading to luigi 3.7.2, mypy reports a lot of errors related to luigi task parameters in some of our projects, see swh-export.log, swh-datasets.log or swh-graph.log.

While some of these errors must be fixed from our side, I noticed the luigi mypy plugin does not correctly compute type for task parameters that use a TypeVar in their definition. The concerned parameters types are: ChoiceParameter, ChoiceListParameter, EnumParameter and EnumListParameter.

You can reproduce the issue using that test_mypy_luigi_parameters.py file.

from enum import Enum
from typing import Tuple

import luigi


class MyEnum(Enum):
    A = 1
    B = 2
    C = 3

class MyTask(luigi.Task):
    choice_int_p: int = luigi.parameter.ChoiceParameter(choices=[1, 2, 3])
    choice_list_int_p: Tuple[int, ...] = luigi.parameter.ChoiceListParameter(choices=[1, 2, 3])
    choice_list_str_p: Tuple[str, ...] = luigi.parameter.ChoiceListParameter(choices=["foo", "bar", "baz"])
    choice_str_p: str = luigi.parameter.ChoiceParameter(choices=["foo", "bar", "baz"])
    enum_p: MyEnum = luigi.parameter.EnumParameter(enum=MyEnum)
    enums_p: Tuple[MyEnum, ...] = luigi.parameter.EnumListParameter(enum=MyEnum)

MyTask(
    choice_int_p=1,
    choice_list_int_p=(1, 3),
    choice_list_str_p=("foo", "bar"),
    choice_str_p="toto",
    enum_p=MyEnum.A,
    enums_p=(MyEnum.B, MyEnum.C)
)

$ mypy --version
mypy 1.19.1 (compiled: yes)

$ mypy --config-file pyproject.toml test_mypy_luigi_parameters 
luigi_mypy_choiceparameter_test.py:13: error: Incompatible types in assignment (expression has type "ChoiceType", variable has type "int")  [assignment]
luigi_mypy_choiceparameter_test.py:14: error: Incompatible types in assignment (expression has type "tuple[ChoiceType, ...]", variable has type "tuple[int, ...]")  [assignment]
luigi_mypy_choiceparameter_test.py:15: error: Incompatible types in assignment (expression has type "tuple[ChoiceType, ...]", variable has type "tuple[str, ...]")  [assignment]
luigi_mypy_choiceparameter_test.py:16: error: Incompatible types in assignment (expression has type "ChoiceType", variable has type "str")  [assignment]
luigi_mypy_choiceparameter_test.py:17: error: Incompatible types in assignment (expression has type "EnumParameterType", variable has type "MyEnum")  [assignment]
luigi_mypy_choiceparameter_test.py:18: error: Incompatible types in assignment (expression has type "tuple[EnumParameterType, ...]", variable has type "tuple[MyEnum, ...]")  [assignment]
Found 6 errors in 1 file (checked 1 source file)

I cooked a fix that I will submit as a pull request.

[anlambert]

Fixed in luigi 3.7.3 closing this.