Decorators in Python

 Decorators to add new functionality to the existing code. Basically a decorator takes in a function, add functionality and return it.

Before understanding Decorators, we need to get some knowledge on Nested functions, Non-local variables and closures.

Nested Functions: A function is defined inside a function is called Nested function.

Program: 

def outer_function():

    # This is a outer function

    def inner_function():

        # This is a inner function

        print("This is Inner Function")

    inner_function()

    # inner function called

outer_function()

# outer function called

Output:

This is a Inner Function

Non-local Variables: Nested functions (Inner function) can access variables of enclosing scope (Outer Function)

Program:

def outer_function(message):

    # This is a outer function

    def inner_function():

        # Accessing outer function variable

        print(message)

        # This is a inner function

        print("This is Inner Function")

    inner_function()

    # inner function called

outer_function("Hello,")

# outer function called

Output:

Hello,

This is Inner Function

Closures: Closure is a function object that remembers values (inner function) in the enclosing scopes (outer function)

Program:

1. def calculate_squares(exponent):
2.     def squares(base):
3.         return pow(base, exponent)
4.     return squares # In closures we need to return the function without parenthesis 
5. 
   
6. square = calculate_squares(2)
7. print(square(2))
8. print(square(3))

Output:

4

9

Will try to understand the above program. We called calculate_squares(2) in line6 and it returned a function squares in line4. The returned function squares bound to the name square in line6. In simple terms squares function is stored in square variable with calculate_squares function value 2 (non-local variable).

In line7 and 8, we are calling square(2) and square(3) that means square variable having inner function squares() with values of outer function calculate_squares. Here the value is exponent = 2 which is non local variable.

Decorators: As mentioned above decorator takes in a function, add functionality and return it.

Program:

1. def decorator_func(func):
2.     def wrapper_func():
3.         print("Hi")
4.         func()
5.     return wrapper_func
6. 
   
7. def greeting_func():
8.     print("How are you")
9. 
   
10. hi = decorator_func(greeting_func)
11. hi()

Output:

Hi

How are you

Will try to understand above program, greeting_func() function only prints "How are you", we feel the greeting is incomplete, we want to add "Hi" before "How are you", we need add some functionality to greeting_func() function to complete our requirement.

As mentioned in the definition decorator takes in a function and adds functionality and return it. So in line10 decorator_func() takes greeting_func() as input. So before calling the greeting_func() we added a print statement in line3. So we got the above ouput. Here we didn't touch greeting_func() but we added some functionality to it.

The above program is equivalent to the below program.

Program:

1. def decorator_func(func):
2.     def wrapper_func():
3.         print("Hi")
4.         func()
5.     return wrapper_func
6. @decorator_func  # This line is equivalent to the lines 10 and 11 in the above program
7. def greeting_func():
8.     print("How are you")
9. greeting_func()

Sample Program: 1

def decorator_func(func):

    def wrapper_func(x,y):

        print("division between", x,"and", y)

        if y == 0:

            print("Divisible by zero is not possible")

            return

        return x/y

    return wrapper_func

@decorator_func

def division(x,y):

    return x/y

print(division(10,2))

print(division(10,5))

print(division(10,0))

output:

division between 10 and 2

5.0

division between 10 and 5

2.0

division between 10 and 0

Divisible by zero is not possible

None