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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，以替换被装饰的方法。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来注册回调。

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，但是当我装饰一个类方法时它会中断，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'

I always though that t.foo(i=5) is basically syntactic sugar for Test.foo(t, i=5) via descriptors, but it seems I am wrong. So here are my questions:

  1. What is the reason that this does not work as expected?
  2. What do I have to do to make it work?

Thank you!

PS: I am using python 3.8

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1 回答 1

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If you make CallbackAcceptor a descriptor it works as follows:

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

Calling now works as expected:

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'
于 2020-02-20T17:31:29.997 回答