如何正确地向 Mixin 类添加类型提示? [英] How do I correctly add type-hints to Mixin classes?

问题描述

考虑以下示例.这个例子是人为的，但在一个可运行的例子中说明了这一点:

Consider the following example. The example is contrived but illustrates the point in a runnable example:

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))

执行后将给出以下输出:

When executed this will give the following output:

12
20

代码有效.

但是在其上运行 mypy 会产生以下错误:

But running mypy on it, yields the following errors:

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

我明白为什么 mypy 会给出这个结果.但是 mixin 类从不单独使用.它们总是用作额外的超类.

I understand why mypy gives this result. But the mixin classes are never used by themselves. They are always used as additional superclasses.

对于上下文，这是一种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，错误是误报.我正在考虑使用 mixin 重写该部分，因为我不是特别喜欢它，同样可以通过重新组织类层次结构来完成.

For context, this is a pattern which has been used in an existing application and I am in the process of adding type-hints. And in this case, the errors are false-positives. I am thinking about rewriting the part using the mixins as I don't particularly like it and the same could probably be done with reorganising the class hierarchy.

但我仍然想知道如何正确暗示这样的事情.

But I still would like to know how something like this could be properly hinted.

推荐答案

我已经在我的机器上测试过了，希望它也适用于你:

I've tested it on my machine, hope it will also work for you:

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))

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