自定义鸭子类型的Python类型注释 [英] Python type annotation for custom duck type

问题描述

Python的typing模块定义了许多鸭子类型，例如typing.SupportsAbs代表任何实现__abs__特殊方法的类型.

Python's typing module defines a number of duck types, e.g., typing.SupportsAbs to represent any type that implements the __abs__ special method.

是否有可能以一种自定义的鸭子类型定义方式，以便可以将它们用作有效的类型注释?

Is it possible to define custom duck types in a way such that I can use them as valid type annotations?

例如，我希望能够注释一个参数应该是threading.Lock的鸭子类型等效项，即，任何实现acquire和release方法的对象.理想情况下，我可以注释诸如SupportsAcquireAndRequire或DuckLock这样的参数，而不是object.

For example, I would like to be able to annotate that an argument should be a duck-type equivalent of a threading.Lock, i.e., any object that implements acquire and release methods. Ideally, I could annotate such an argument as SupportsAcquireAndRequire or DuckLock, rather than object.

推荐答案

您可以定义抽象基类(ABC)以指定接口:

You can define an abstract base class (ABC) to specify the interface:

from abc import ABCMeta, abstractmethod

class SupportsAcquireAndRequire(metaclass=ABCMeta):
    @abstractmethod
    def acquire(self):
        pass

    @abstractmethod
    def release(self):
        pass

    @classmethod
    def __subclasshook__(cls, C):
        for method in ('release', 'acquire'):
            for B in C.__mro__:
                if method in B.__dict__:
                    if B.__dict__[method] is None:
                        return NotImplemented
                    break
            else:
                return NotImplemented
        return True

基本上，这是协议(如typing.SupportsAbs)的实现方式，尽管不直接使用ABCMeta.

This is basically how the protocols (like typing.SupportsAbs) are implemented, albeit without directly using ABCMeta.

通过为ABC提供 __subclasshook__方法，您可以在isinstance()和issubclass()测试中使用它，对于 mypy :

By giving the ABC a __subclasshook__ method, you can use it in isinstance() and issubclass() tests, which is more than good enough for tools like mypy:

>>> from threading import Lock
>>> isinstance(Lock(), SupportsAcquireAndRequire)
True

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