[Python] - Decorator

Decorator

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A decorator in Python is a function that takes another function as its argument, and returns yet another function

• Decorators can be extremely useful as they allow the extension of an existing function, without any modification to the original function source code.
• Decorators is similar to Closure , Decorator is same Closure which parameter is function and add logic before and after function

 

1.1. Extending the function with a decorator

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• We would like add_together to be able to take a list of 2 element tuples as its argument and return a list of integers which represents the summation of their values.

# Decorator
def decorator1(func):
    def wrapper():
        print('decorator1')
        func()
    return wrapper
 
def decorator2(func):
    def wrapper():
        print('decorator2')
        func()
    return wrapper

@decorator1
@decorator2
def hello():
    print('hello')
    
#decorated_hello = decorator1(decorator2(hello))
#decorated_hello()    
hello()
"""
decorator1
decorator2
hello
"""

def decorator_list(fnc):
    def inner(list_of_tuples):
        return [fnc(val[0],val[1]) for val in list_of_tuples]
    return inner

@decorator_list    
def add_together(x,y):
    return x+y

print(add_together([(1, 3), (3, 17), (5, 5), (6, 7)]))

 

1-2. Decorators that can take arguments themselves

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• pass an integer argument to the decorator
• To use arguments in decorators, we simply need to define a decorator itself.

def meta_decorator(power):
    def decorator_list(fnc):
        def inner(list_of_tuples):
            return [fnc(val[0],val[1])**power for val in list_of_tuples]
        return inner
    return decorator_list

@meta_decorator(2)    
def add_together(x,y):
    return x+y

print(add_together([(1, 3), (3, 17), (5, 5), (6, 7)]))
> [16, 400, 100, 169]

 

1-3. Default arguments in Decorators

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• callable function set default arg

def meta_decorator(arg):
    def decorator_list(fnc):
        def inner(list_of_tuples):
            return [fnc(val[0],val[1])**power for val in list_of_tuples]
        return inner

    if callable(arg):
        power=2
        return decorator_list(arg)
    else :
        power=arg
        return decorator_list

#default option
@meta_decorator
def add_together(x,y):
    return x+y

# @meta_decorator(3)
#def add_together(x,y):
#    return x+y

print(add_together([(1, 3), (3, 17), (5, 5), (6, 7)]))