python decorator for class method and static method?

I am trying to write a decorator for an instance method, as follows:

from functools import wraps

def plus_decorator(f):
    @wraps(f)
    def wrapper(*args, **kwargs):
        return 1 + f(*args, **kwargs)
    return wrapper

@plus_decorator
def return_i(i):
    return i

class A(object):
    @plus_decorator
    def return_i(self, i):
        return i

    @plus_decorator
    @classmethod
    def return_i_class(cls, i):
        return i

    @plus_decorator
    @staticmethod
    def return_i_static(i):
        return i

if __name__ == '__main__':
    print return_i(1)
    a = A()
    print a.return_i(1)
    print A.return_i_class(1)
    print A.return_i_static(1)

However, it pops up the error:

AttributeError: 'classmethod' object has no attribute '__module__'

I am wondering why the decorator does not work on classmethod and staticmethod. I think the decorator mostly just passes all the parameters it receives to the wrapper, and only modifies the result. How can I modify the decorator to make it work for classmethod and staticmethod?