类装饰器不绑定自我

Posted 2023-02-23

【中文标题】类装饰器不绑定自我

【英文标题】：Class decorator not binding self

【发布时间】：2020-06-05 01:51:02

【问题描述】：

我已经定义了一个类，该类将装饰器定义为该类的方法。 装饰器本身会创建第二个类的可调用实例来替换被装饰的方法。 由于装饰方法现在实际上是一个类，我可以在它上面调用方法。 在我的（虚构的，最小的）示例中，我想使用每个方法的自定义最大回调数注册回调。

class CallbackAcceptor:

    def __init__(self, max_num_callbacks, func):
        self._func = func
        self._max_num_callbacks = max_num_callbacks
        self._callbacks = []

    def __call__(self, *args, **kwargs):
        # This ends up being called when the decorated method is called
        for callback in self._callbacks:
            print(f"Calling callback.__name__(args, kwargs)")
            callback(*args, **kwargs)
        return self._func(*args, **kwargs)  # this line is the problem, self is not bound

    def register_callback(self, func):
        # Here I can register another callback for the decorated function
        if len(self._callbacks) < self._max_num_callbacks:
            self._callbacks.append(func)
        else:
            raise RuntimeError(f"Can not register any more callbacks for self._func.__name__")
        return func


class MethodsWithCallbacksRegistry:

    def __init__(self):
        self.registry =   # Keep track of everything that accepts callbacks

    def accept_callbacks(self, max_num_callbacks: int = 1):

        def _make_accept_callbacks(func):
            # Convert func to an CallbackAcceptor instance so we can register callbacks on it
            if func.__name__ not in self.registry:
                self.registry[func.__name__] = CallbackAcceptor(max_num_callbacks=max_num_callbacks, func=func)
            return self.registry[func.__name__]

        return _make_accept_callbacks

函数的一切都按预期工作，但是当我装饰类方法时它会中断，因为类实例未绑定到装饰方法：

registry = MethodsWithCallbacksRegistry()

@registry.accept_callbacks(max_num_callbacks=1)
def bar(i):
    return i * 10

@bar.register_callback
def bar_callback(*args, **kwargs):
    print("Bar Callback")

print(bar(i=10))  # Works fine, prints "Bar Callback" and then 100

现在如果我定义一个接受回调的方法：

class Test:

    @registry.accept_callbacks(max_num_callbacks=1)
    def foo(self, i):
        return i * 2

@Test.foo.register_callback
def foo_callback(*args, **kwargs):
    print("Foo Callback")

如果我明确地传递 self ，它会起作用，但如果我只是假设实例已绑定，则不会：

t = Test()
# Note that I pass the instance of t explicitly as self
Test.foo(t, i=5)  # Works, prints "Foo Callback" and then 10
t.foo(t, i=5)  # Works, prints "Foo Callback" and then 10

t.foo(i=5)  # Crashes, because self is not passed to foo

这是回溯：

Traceback (most recent call last):
  File "/home/veith/.PyCharmCE2019.3/config/scratches/scratch_4.py", line 62, in <module>
    t.foo(i=5)
  File "/home/veith/.PyCharmCE2019.3/config/scratches/scratch_4.py", line 13, in __call__
    return self._func(*args, **kwargs)  # this line is the problem, self is not bound
TypeError: foo() missing 1 required positional argument: 'self'

我一直认为t.foo(i=5) 基本上是Test.foo(t, i=5) 通过描述符的语法糖，但似乎我错了。所以这是我的问题：

这不按预期工作的原因是什么？ 我必须做些什么才能让它发挥作用？

谢谢！

PS：我使用的是 python 3.8

【参考方案1】：

如果您将CallbackAcceptor 设为descriptor，它的工作方式如下：

class CallbackAcceptor:

    def __init__(self, max_num_callbacks, func):
        self._func = func
        self._max_num_callbacks = max_num_callbacks
        self._callbacks = []

    def __call__(self, *args, **kwargs):
        # This ends up being called when the decorated method is called
        for callback in self._callbacks:
            print(f"Calling callback.__name__(args, kwargs)")
            callback(*args, **kwargs)

        return self._func(*args, **kwargs)

    def register_callback(self, func):
        # Here I can register another callback for the decorated function
        if len(self._callbacks) < self._max_num_callbacks:
            self._callbacks.append(func)
        else:
            raise RuntimeError(f"Can not register any more callbacks for self._func.__name__")
        return func

    # Implementing __get__ makes this a descriptor
    def __get__(self, obj, objtype=None):
        if obj is not None:
            # the call is made on an instance, we can pass obj as the self of the function that will be called
            return functools.partial(self.__call__, obj)
        # Called on a class or a raw function, just return self so we can register more callbacks
        return self

现在调用按预期工作：

print(bar(i=10))
# Bar Callback
# 100

t = Test()
t.foo(i=5)
# Foo Callback
# 10

t.foo(t, i=5)
# TypeError: foo() got multiple values for argument 'i'