Python Decorator in Inheritance

Question

I have the following code:

class Foo:
    iterations = 3

class Bar(Foo):
    @test_decorator(<????>)
    def hello(self):
        print("Hello world!")

def test_decorator(input):
    def my_decorator(func):
        def wrapper(*args, **kwargs):
            print("Something is happening before the function is called.")
            for _ in range(input):
                func(*args, **kwargs)
            print("Something is happening after the function is called.")
        return wrapper
    return my_decorator

I would like to pass my iterations variable which is in the parent class to the decorator test_decorator which is in my child class, instead of <????>

I tried the following ways:

• self.iteration doesn't work since we don't have access to self
• Foo.iterations doesn't work because it will act as a constant, if we change iterations "hello world" will be displayed only 3 times instead of 5 (as in the example below)

Example:

b = Bar()
b.iterations = 5
b.hello()

# "hello world" will be displayed 3 times

Is there a way to do this or is it anti pattern to python ?

Answer 1

I found a solution to your problem. The idea is to write your own decorator.

Since that decorator is a function wrapping your method, it has access to the class instance using the *args first index. From there, you can access the iterations variable:

def decorator(iterations, source):
    def inner_transition(func):
        return func
    return inner_transition

def custom_transition(source):
    def inner_custom_transition(func):
        def wrapper(*args, **kwargs):
            iterations = args[0].iterations
            return decorator(iterations=iterations, source=source)(func)(*args, **kwargs)
        return wrapper
    return inner_custom_transition

class Foo:
    iterations = 3

class Bar(Foo):
    @custom_transition(source="my_source")
    def hello(self, string_to_show, additional_string = "default"):
        print(string_to_show)
        print(additional_string)

bar = Bar()
bar.hello("hello", additional_string="++++")

Result:

hello
++++