Question

请考虑以下示例。该示例是人为设计的，但在可运行示例中说明了要点：

class MultiplicatorMixin:

    def multiply(self, m: int) -> int:
        return self.value * m


class AdditionMixin:

    def add(self, b: int) -> int:
        return self.value + b


class MyClass(MultiplicatorMixin, AdditionMixin):

    def __init__(self, value: int) -> None:
        self.value = value


instance = MyClass(10)
print(instance.add(2))
print(instance.multiply(2))

执行时将给出以下输出：

12
20

代码有效。

但是在其上运行mypy会产生以下错误：

example.py:4: error: "MultiplicatorMixin" has no attribute "value"
example.py:10: error: "AdditionMixin" has no attribute "value"

我了解为什么mypy会给出此结果。但是mixin类永远不会被自己使用。它们始终被用作其他超类。

对于上下文，这是一个已在现有应用程序中使用的模式，我正在添加类型提示。在这种情况下，错误是假阳性。我正在考虑使用mixins重写部分，因为我不太喜欢它，重组类层次结构可能也可以做到这一点。

但是我仍然想知道如何正确提示这样的事情。

Answer 1

我已经在机器上对其进行了测试，希望它也可以为您工作：

class MultiplicatorMixin:
    value = None # type: int

    def multiply(self, m: int) -> int:
        return self.value * m


class AdditionMixin:
    value = None # type: int

    def add(self, b: int) -> int:
        return self.value + b


class MyClass(MultiplicatorMixin, AdditionMixin):

    def __init__(self, value: int) -> None:
        self.value = value


instance = MyClass(10)
print(instance.add(2))
print(instance.multiply(2))

Answer 2

我在this question中看到的一种方法是通过类型提示self属性。与键入包中的Union一起使用，您可以使用与mixin一起使用的类中的属性，同时仍可以为自己的属性提供正确的类型提示：

from typing import Union

class AdditionMixin:

    def add(self: Union[MyBaseClass, 'AdditionMixin'], b: int) -> int:
        return self.value + b


class MyBaseClass:

    def __init__(self, value: int):
        self.value = value

缺点是必须将提示添加到每个方法中，这很麻烦。

Answer 3

作为参考，mypy建议通过协议（https://mypy.readthedocs.io/en/latest/more_types.html#advanced-uses-of-self-types）实现mixins。

它适用于mypy> = 750。

from typing_extensions import Protocol


class HasValueProtocol(Protocol):
    @property
    def value(self) -> int: ...


class MultiplicationMixin:

    def multiply(self: HasValueProtocol, m: int) -> int:
        return self.value * m


class AdditionMixin:

    def add(self: HasValueProtocol, b: int) -> int:
        return self.value + b


class MyClass(MultiplicationMixin, AdditionMixin):

    def __init__(self, value: int) -> None:
        self.value = value

Answer 4

尝试：

from typing import Type, TYPE_CHECKING, TypeVar

T = TypeVar('T')


def with_typehint(baseclass: Type[T]) -> Type[T]:
    """
    Useful function to make mixins with baseclass typehint

    ```
    class ReadonlyMixin(with_typehint(BaseAdmin))):
        ...
    ```
    """
    if TYPE_CHECKING:
        return baseclass
    return object

在Pyright中测试的示例：

class ReadOnlyInlineMixin(with_typehint(BaseModelAdmin)):
    def get_readonly_fields(self,
                            request: WSGIRequest,
                            obj: Optional[Model] = None) -> List[str]:

        if self.readonly_fields is None:
            readonly_fields = []
        else:
            readonly_fields = self.readonly_fields # self get is typed by baseclass

        return self._get_readonly_fields(request, obj) + list(readonly_fields)

    def has_change_permission(self,
                              request: WSGIRequest,
                              obj: Optional[Model] = None) -> bool:
        return (
            request.method in ['GET', 'HEAD']
            and super().has_change_permission(request, obj) # super is typed by baseclass
        )

>>> ReadOnlyAdminMixin.__mro__
(<class 'custom.django.admin.mixins.ReadOnlyAdminMixin'>, <class 'object'>)

Answer 5

除了上面提到的好答案。我的用例-用于测试的mixins。

如Guido van Rossum本人here所提议：

from typing import *
T = TypeVar('T')

class Base:
    fit: Callable

class Foo(Base):
    def fit(self, arg1: int) -> Optional[str]:
        pass

class Bar(Foo):
    def fit(self, arg1: float) -> str:
        pass

因此，当涉及混合时，它可能如下所示：


class UsefulMixin:

    assertLess: Callable
    assertIn: Callable
    assertIsNotNone: Callable

    def something_useful(self, key, value):
        self.assertIsNotNone(key)
        self.assertLess(key, 10)
        self.assertIn(value, ['Alice', 'in', 'Wonderland']


class AnotherUsefulMixin:

    assertTrue: Callable
    assertFalse: Callable
    assertIsNone: Callable

    def something_else_useful(self, val, foo, bar):
        self.assertTrue(val)
        self.assertFalse(foo)
        self.assertIsNone(bar)

我们的最后一堂课如下：

class TestSomething(unittest.TestCase, UsefulMixin, AnotherUsefulMixin):

    def test_something(self):
        self.something_useful(10, 'Alice')
        self.something_else_useful(True, False, None)

如何正确将类型提示添加到Mixin类？

5 个答案: