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Python Decorators

Video link: https://youtu.be/8hWIWyBfdQE

In this video, we learned how to create decorators and why we should use them.

Programs in the Video

Prerequisites for Decorators

Before we learn about decorators, we must first understand a few basic things in Python.

We must be comfortable with the fact that everything in Python is an object, including classes as well as functions. Variables are simply identifiers bound to these objects.

Since functions are also objects, we can also pass functions as arguments to other functions:

def inc(x):
    return x + 1

def operate(func, x):
    result = func(x)
    return result

print(operate(inc, 3))

Output

4

In Python, we can also define a function inside a function:

def print_msg(message):
    greeting = "Hello"
    def printer():
        print(greeting, message)

    printer()


print_msg("Python is awesome")

Output

Hello Python is awesome

Functions is that they can also return a function as a value. Let's modify our previous code:

def print_msg(message):
    greeting = "Hello"
    def printer():
        print(greeting, message)

    return printer

func = print_msg("Python is awesome")
func()

Output

Hello Python is awesome

Do you notice something weird here? Even though print_msg() function is done executing, the returned inner printer() function can still access the message and greeting variables. Such a function is called a closure function.

A closure is simply an inner function that remembers the values and variables in its enclosing scope even if the outer function is done executing.

Python Decorators make an extensive use of closures.

Python Decorators

A Python decorator is a function that takes in a function, adds some functionality to it and returns the original function.

Let's try to build a decorator function that prints out some information before and after executing another function.

def printer():
    print("Hello, World!")


def display_info(func):
    def inner():
        print("Executing",func.__name__,"function")
        func()
        print("Finished execution")
    return inner

Let's call printer() normally first.

def printer():
    print("Hello, World!")


def display_info(func):
    def inner():
        print("Executing",func.__name__,"function")
        func()
        print("Finished execution")
    return inner

printer()

Output

Hello, World!

Now, let's use the decorator function to run the same printer function.

def printer():
    print("Hello, World!")


def display_info(func):
    def inner():
        print("Executing",func.__name__,"function")
        func()
        print("Finished execution")
    return inner

decorated_func = display_info(printer)
decorated_func()

Output

Executing printer function
Hello, World!
Finished execution

In python, we have a much more elegant way of to achieve this functionality using the @ symbol.

def display_info(func):
    def inner():
        print("Executing",func.__name__,"function")
        func()
        print("Finished execution")
    return inner

@display_info
def printer():
    print("Hello, World!")

printer()

Output

Executing printer function
Hello, World!
Finished execution

Decorating Functions with Parameters

Suppose we have a simple divide function.

def divide(a, b):
    return a/b

We know this code will throw an exception if we pass the value for b as 0.

Let's make a decorator function called smart_divide to prevent this.

def smart_divide(func):
    def inner(a, b):
        print("Dividing", a, "by", b)
        if b == 0:
            print("Cannot divide by 0!")
            return

        return func(a, b)
    return inner


@smart_divide
def divide(a, b):
    return a/b

value1 = divide(15, 3)
print(value1)

value2 = divide(5, 0)
print(value2)

Output

Dividing 15 by 3
5.0
Dividing 5 by 0
Cannot divide by 0!
None

Chaining Decorators in Python

In Python, a function can be decorated multiple times with different or the same decorator.

Here, are two decorator functions called star and percent. These functions print a series of star and percentage symbols before and after executing the function

def star(func):
    def inner(arg):
        print("*" * 30)
        func(arg)
        print("*" * 30)
    return inner

def percent(func):
    def inner(arg):
        print("%" * 30)
        func(arg)
        print("%" * 30)
    return inner

@star
@percent
def printer(msg):
    print(msg)

printer("Decorators are wonderful")

Output

******************************
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Decorators are wonderful
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
******************************

As you can see, these decorators are chained and they wrap the original function.

Here, we have first called the star function, and then the percent function. So the star function wraps the percent function which in turn wraps the printer function.