[Step-by-step] Python Decorators Explained with Examples for Beginners

Python decorators are considered as one of the very advanced and very useful concepts.

In this tutorial, I will explain Python decorators with examples. Going through this tutorial, you will learn to implement Python decorators and how it can be very useful in your project.

What is Python Decorators?

This is one of the very popular Python interview questions asked in the IT job interviews.

Python decorators is a technique for changing the behavior of an existing function without changing actual code inside the function.

Even though we are changing the behavior of the function, there should not have any change in the function definition and function call.

It means…

• The code inside the original function should not be changed.
• There should not have any change in the function call.

Python has a special mechanism called Python decorators to change the behavior of the existing function.

It might be a bit confusing to you. Let’s go with an example.

Python Decorators Explained

How does Python Decorator work?

Let’s start with a simple Python function code.

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

myFunc()

Output:

Hello, World!

This is the basic Python program to implement a Python function. If you are new to Python programming, check basic Python code syntax.

Now consider a function that you want to wrap inside another function (called decorator).

It is similar to the technique of wrapping gifts inside flashy decorative papers.

Let’s see how it can be implemented in Python.

We know, that everything in Python is an object including the Python function. It means, we can pass the function object as an argument to another function. By default, the function name works as a function object.

We can write one Python function inside another function.

def myWrapper(func):
  def myInnerFunc():
    print("Inside wrapper.")
    func()
  return myInnerFunc
 
def myFunc():
  print("Hello, World!")

c=myWrapper(myFunc)
c()

Output:

Inside wrapper.
Hello, World!

Here we are passing the function (myFunc) as a parameter to the wrapper function (myWapper).

This is how it works.

• The actual function is passed as a parameter to the wrapper function.
• myWrapper has an inner function which calls the actual function (myFunc).
• Inside Inner function, you can write any additional operation to perform along with calling the original function (myFunc).
• myWrapper returns the inner function object.

Note: When you write function name without brackets, it acts as a function object and it does not call function. The function gets called only when there is a function name (object) followed by brackets “()”.

If you compare the above two programs, you have changed the behavior of the original function. You can see the difference in the output.

We have fulfilled the first criteria of not changing code inside the function.

But the function call has changed

from

myFunc()

to

c=myWrapper(myFunc)
c()

This does not fulfill our second criteria.

Here is the use of Python decorators.

Python Decorator Basic Example

def myWrapper(func):
  def myInnerFunc():
    print("Inside wrapper.")
    func()
  return myInnerFunc
 
@myWrapper
def myFunc():
  print("Hello, World!")

myFunc()

Output:

Inside wrapper.
Hello, World!

Python decorator name (@myWrapper) is specified above the actual function definition. (2)

How does the Python decorator program execute?

Below are the steps of execution.

• The original function is called (1).
• There is a function wrapper name specified above the function definition (2). This indicates, that there is a function decorator assigned to the function.
• The decorator function gets called. The program controller passes the function object as a parameter to the decorator function (3).
• The function inside the decorator function gets executed (4).
• The inner function calls the actual function (5).
• The original function starts execution (6).

This is a simple Python decorator example. You can also pass the arguments to the Python decorators.

Many get confused decorators with the monkey patching techniques in Python. Two are the different things.

Python Decorators Example with Two Arguments

Original Function to add two numbers.

def addTwoNumbers(a, b):
    c=a+b
    return c

c=addTwoNumber(4, 5)
 
print("Addition of two numbers=", c)

Output:

Addition of two numbers=9

Now our aim is to modify the behavior of addTwoNumbers() without changing function definition and function call.

What function behavior do we want to change?

We want addTwoNumbers function should calculate the sum of the square of two numbers instead of the sum of two numbers.

Here is a simple decorator to change the behavior of the existing function.

def decorateFun(func): 
    def sumOfSquare(x, y): 
        return func(x**2, y**2) 
    return sumOfSquare 

@decorateFun
def addTwoNumbers(a, b): 
    c = a+b 
    return c 

c = addTwoNumbers(4,5) 
print("Addition of two numbers=", c)

Output:

Addition of two numbers=41

The below simple program is equivalent to the above decorator example. Here we are changing the function call.

def decorateFun(func): 
    def sumOfSquare(x, y): 
        return func(x**2, y**2) 
    return sumOfSquare 

def addTwoNumbers(a, b): 
    c = a+b 
    return c 

obj=decorateFun(addTwoNumbers) 
c=obj(4,5) 
print("Addition of square of two numbers=", c)

Output:

Addition of square of two numbers=41

Note: The number of arguments to the function inside the decorators should be the same as the number of arguments to the actual function.

When to Use Python Decorators?

Suppose you are working on a project. You are asked to make the changes in a certain complex function.

If you are making changes in already tested and robust functions, you can not deny the possibility of breaking functionality.

A better way is to use Python decorators.

• You don’t need to make changes in the already tested function.
• You don’t need to make any changes to the function call. This will be very useful if there are multiple function calls in your project source code.

You can also pass the arguments to the decorators. You can modify these arguments inside the decorators before passing them to the original function.

This is all about Python decorators explained with examples. If you have any questions, ask in the comment section below.

After decorators, learn lambda function which is another advance topic in Python programming.

Happy Pythoning!

Aniruddha Chaudhari

I am complete Python Nut, love Linux and vim as an editor. I hold a Master of Computer Science from NIT Trichy. I dabble in C/C++, Java too. I keep sharing my coding knowledge and my own experience on <b>CSEstack.org</b> portal.