Class as decorator for class method

Question

I want to use a decorator to do some preparation job and record the status the function have, so I write something like that:

class Decorator: def __init__(self, func): self.count = 0 self.func = func def __call__(self, *args, **kwargs): self.count += 1 # Simply count the call times return self.func(self, *args, **kwargs) class Foo: def __init__(self): self.value = 0 @Decorator def test(self, value): self.value = value # change the value of instance print(self.value) f = Foo() f.test(1) print(f.value) print(f.test.value)

But it's obvious that self in __call__(self, *args, **kwargs) corresponds to instance of Decorator instead of the instance of Foo , which will make f.value unchanged but f.test.value increase .

Is there any way I can pass the instance of Foo to Decorator instead of Decorator itself?

Or is there any way to implement this function much more clear?

Marcy · Accepted Answer · 2016-06-13 15:41:43Z

As the decorator is only called once and replaces the method for all instance with one instance of the Decorator class. All it does is:

Foo.test = Decorator(Foo.test)

This makes it impossible to detect the instance called. One work-around would be to apply the decorator in the __init__ of Foo by hand:

class Foo: def __init__(self): self.value = 0 self.test = Decorator(self.test) def test(self, value): self.value = value # change the value of instance print(self.value)

This way the decorator wraps the instance method, so you do not need to pass self in the __call__ of Decorator:

class Decorator: def __init__(self, func): self.count = 0 self.func = func def __call__(self, *args, **kwargs): self.count += 1 # Simply count the call times return self.func(*args, **kwargs)

Now it works and you have to update you test method, as f.test.value no longer exists:

f = Foo() f.test(1) print(f.value)

It outputs two times a 1 as expected.

Community · Accepted Answer · 2017-05-23 10:33:58Z

I got this here

import functools class Decorator(object): def __init__(self, func): self.count = 0 self.func = func def __call__(self, *args, **kwargs): self.count += 1 # Simply count the call times return self.func( *args, **kwargs) def __get__(self, instance, instancetype): """Implement the descriptor protocol to make decorating instance method possible. """ # Return a partial function with the first argument is the instance # of the class decorated. return functools.partial(self.__call__, instance) class Foo: def __init__(self): self.value = 0 @Decorator def test(self, value): self.value = value # change the value of instance f = Foo() f.test(3) print(f.value) # prints 3 g = Foo() g.test(8) print(g.value) # prints 8

or May be this

def preJob(function): def updateToDo(self, *args, **kwargs): # do some recording function(self, *args, **kwargs) return updateToDo class Foo(object): def __init__(self): self.value = 0 @preJob def test(self, value): self.value = value f = Foo() f.test(3) print(f.value) # prints 3 g = Foo() g.test(8) print(g.value) # prints 8

nicosnow · Accepted Answer · 2019-09-20 22:22:11Z

class threadSafeGenerator(object): """docstring for threadSafeGenerator""" class SafeGenerator(object): """docstring for SafeGenerator""" def __init__(self, iterable): self.iterable = iterable self.lock = Lock() def __iter__(self): return self def __next__(self): with self.lock: return next(self.iterable) def __init__(self, func): super(threadSafeGenerator, self).__init__() self.func = func def __call__(self, *args, **kwargs): return self.SafeGenerator(self.func(self, *args, **kwargs))

I found using Priyesh Kumar's answer that you can simply pass the self argument from the call method to the function being decorated:

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

hope this helps!

EDIT: Never mind only works if the function being passed through the decorator does not call class variables defined in the init method

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