Using class as decorator for another class's method

Question

I have an issue with using a class to decorate another class' method. Code is as follows:

class decorator(object):
    def __init__(self, func):
        self.func = func

    def __call__(self, *args):
        return self.func(*args)

class test(object):
    @decorator
    def func(self, x, y):
        print x, y

t = test()
t.func(1, 2)

It shows this error

TypeError: func() takes exactly 3 arguments (2 given).

If called using:

t.func(t, 1, 2)

then it passes. But then if the decorator is taken away, then this line will have issue again.

Why this is happening and how to solve it?

Edit: second version of the code to show the self in decorator.__call__ should be different than the self in test.func:

class decorator(object):
    def __init__(self, func):
        self.func = func

    def __call__(self, *args):
        return self.func(*args)

class test(object):
    def __init__(self):
        self.x = 1
        self.y = 2
    @decorator
    def func(self):
        print self
        print self.x, self.y

t = test()
t.func()

This shows the same error. But

t.func(t)

works but not ideal.

Blckknght · Accepted Answer · 2018-03-07 07:05:54Z

5

To work as a method, an object in a class needs to implement part of the descriptor protocol. That is, it should have a __get__ method that returns a callable object which has been "bound" to the instance the method was looked up on.

Here's one way that you could make that work, using a wrapper function:

class decorator(object):
    def __init__(self, func):
        self.func = func

    def __get__(self, instance, owner):
        def wrapper(*args):
            return self.func(instance, *args) # note, self here is the descriptor object
        return wrapper

You could instead return an instance of some other class from __get__, rather than a function, and use the __call__ method of that other class to implement the wrapper. If you're not using a closure though, you'd need to pass the instance to the wrapper class explicitly (as well as the function, since self.func won't work outside the descriptor class).

edited Mar 7, 2018 at 7:05

answered Mar 7, 2018 at 2:14

Blckknght

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1 Comment

Jacky Over a year ago

Thanks for the descriptor suggestion. I found a related question in stackoverflow.com/questions/2365701/…. As you suggest, either getting the descriptor working or using closure function works. This code is clear for the descriptor solution stackoverflow.com/a/45361673. I ended up using closure function for easier maintenance.

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