foo.py

from functools import wraps

class MyDecorator(object):
    """
    would normally be my_decorator because it's a function, and functions use underscore delimited lower-case.
    Used CamelCase to demo that it's really a class.
    """
    def __init__(multiplier):
        self.multiplier = multiplier

    def __call__(func):
        @wraps(func)
        def wrapper(*args, **kwargs):
            return self.multiplier * func(*args, **kwargs)
        return wrapper

@MyDecorator(3):
def foo():
    return 4

concrete_decorator=MyDecorator(4)
@concrete_decorator
def bar():
    return 5

assert(foo() == 12)
assert(bar() == 20)