[Introduction to Udemy Python 3 + Application] 57. Decorator

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■ Basics of decorators

◆ If you write without using a decorator

non_decorator

def add_num(a, b):
    return a + b

print('start')
r = add_num(10, 20)
print('end')

print(r)

result

start
end
30

You can use the "decorator" when you want to do something additional before and after calling ʻadd_num ()`.

◆ If you write using a decorator

decorator

def print_info(func):
    def wrapper(*args, **kwargs):
        print('start')
        result = func(*args, **kwargs)
        print('end')
        return result
    return wrapper

def add_num(a, b):
    return a + b

f = print_info(add_num)
r = f(10, 20)
print(r)

result

start
end
30

For * args, 52. Tupleization of positional arguments For ** kwargs, see 53. Dictionary of keyword arguments.

◆ General usage of decorators

If you write as above, it is difficult to understand that you are using it as a decorator. Therefore, in general, when using a decorator, describe as follows.

decorator

def print_info(func):
    def wrapper(*args, **kwargs):
        print('start')
        result = func(*args, **kwargs)
        print('end')
        return result
    return wrapper

@print_info
def add_num(a, b):
    return a + b

r = add_num(10, 20)
print(r)

result

start
end
30

By specifying a decorator function using @, you can decorate using that function.

■ When using multiple decorators

◆ Order to apply decorators

Write another decorator function and try using both.

two_decorators

def print_more(func):
    def wrapper(*args, **kwargs):
        print('func: ', func.__name__)
        print('args: ', args)
        print('kwargs: ', kwargs)
        result = func(*args, **kwargs)
        print('result: ', result)
        return result
    return wrapper

def print_info(func):
    def wrapper(*args, **kwargs):
        print('start')
        result = func(*args, **kwargs)
        print('end')
        return result
    return wrapper

@print_info
@print_more
def add_num(a, b):
    return a + b

r = add_num(10, 20)
print(r)

result

start
func: add_num
args: (10, 20)
kwargs: {}
result: 30
end
30

If you specify a decorator like this, You can see that print_more is included in print_info.

Now try swapping the order of the two decorators.

two_decorators

def print_more(func):
    def wrapper(*args, **kwargs):
        print('func: ', func.__name__)
        print('args: ', args)
        print('kwargs: ', kwargs)
        result = func(*args, **kwargs)
        print('result: ', result)
        return result
    return wrapper

def print_info(func):
    def wrapper(*args, **kwargs):
        print('start')
        result = func(*args, **kwargs)
        print('end')
        return result
    return wrapper

@print_more
@print_info
def add_num(a, b):
    return a + b

r = add_num(10, 20)
print(r)

result

func: add_num
args: (10, 20)
kwargs: {}
start
end
result: 30
30

This time, print_info is included in print_more.