The decorator on instance method as a descriptor

instance_method_decorator.py

import functools


class Increment(object):
    def __init__(self, wrapped, delta):
        self.wrapped = wrapped
        self.delta = delta

    def __get__(self, instance, owner=None):
        method = self.wrapped.__get__(instance, owner)
        if instance is None:
            return method  # unbound method

        @functools.wraps(method)
        def wrapper(*args, **kwargs):
            instance.n += self.delta  # no more inspect.getcallargs
            return method(*args, **kwargs)

        return wrapper


def increment(delta):
    def decorator(wrapped):
        return Increment(wrapped, delta)
    return decorator


class Foo(object):
    def __init__(self, n):
        self.n = n

    @increment(2)
    def foo(self):
        return self.n


foo = Foo(40)
print(foo.foo())