How to Use Def Function in Python?

A function is an integral part of programming as it promotes modularity and makes the job of developers worldwide easier. Functions are omnipresent across almost all programming languages. In Python, def is used to create a function and remains a necessity to use it while defining a function.

Why a Function Need?

A few of the important reasons to use a function in programming is that it promotes reusability where the same modules can be reused without making us rewrite the same function in our codebase and this practice makes projects or tasks more manageable and achievable.

The reason for the application of python def is that it is used to create user-defined functions in python. Unlike low-level languages like C, where a function must be declared first and defined in order to use it, Python offers the flexibility to directly define and use it accordingly using Python Def.

Working with Python Def

In order to write functions in python, we are aware by how to write a def keyword, and how does it work in simple computing terms and what variation it can have, we will study this in this section.

Syntax of user-defined function

def function_name(parameters):
    """Optional documentation string (docstring)"""
    code to be executed
    return [expression]

Components of a Function

A function comprises the parts as discussed below:-

  1. def keyword
  2. Function name
  3. Function statements
  4. Parameters
  5. Return statement (optional)

1️. Def Keyword
It is written at the start of the function indicating a user-defined function is being created to be put into use for calling in the later stages of the program.

2️. Function Name
It is the name of the function. Advised to name the functions meaningful, accordingly with its purpose, in order to make the program more readable.

3️. Function Parameters
Passed arguments upon the calling of functions are set as parameters on defining a function using Python Def. The positioning of the parameter depends upon the position or keyword of the arguments.

Let us have a look a how positional and keyword arguments fare among each other.

Position Argument:
The parameters are executed in the order assigned to them at the time of calling the function. Such that an argument at the first position will be ordered first in the parameter.

def print_name(first_name, last_name):
  print("First name:", first_name)
  print("Last name:", last_name)

print_name("PrepBytes", "Learning")


First name: PrepBytes
Last name: Learning

Keyword Argument:
The parameters are executed in the keyword assigned to them at the time of calling the function. Such that an argument at the first_name will be passed to the parameter named first_name

def print_name(first_name, last_name):
  print("First name:", first_name)
  print("Last name:", last_name)

print_name(first_name="PrepBytes", last_name="Learning")


First name: PrepBytes
Last name: Learning

4️. Function Statements
These are the actual statements to be executed within the function. Function statements are indented two spaces or four spaces, as per the preference of the user.

def greeting(name):
    print("Hello, " + name)


In the above code, Hello John will be printed as our function statement has a print function with name as the function parameter holding the name value.

5️. Docstrings
Strings that are kept as a commented code inside the function created for the sake of accessibility of the developer to know and store important documentation related to the function. It is another optional preference to keep docstrings inside the function. Accessed using doc.

def exp(a, b):
    """Returns the power of a raised to the power b"""

    return a**b


6️. Return
Return is another optional but useful statement that returns the value back to the called function that requires an output generated from the set of statements generated inside the function.

In given example, we return the square of the number that is passed as an argument.

def sq(x):
    return x * x

result = sq(5)



Among the discussed above, Python Def is the most used keyword in Python Programming Language.

Implementing Programs using Python Def

With some solid knowledge of functional programming in python, let us hop on to see how python def can prove to be a handy tool in different scenarios of programming.

In the first of the two examples, we will be finding the multiples of a number up to ten.

def print_multiples(n):
    for i in range(1, 11):
        print(n * i)

# Call the function with a number


3 6 9 12 15 18 21 24 27 30

A function named print_multiples is defined using the python def keyword that takes in an input i.e. 3 passed and it runs a loop from 1 to 10 as 11 won’t be considered in for loop such that we will get the first 10 multiples of 3 as our output.

In the second example, we will be printing the first n prime numbers by defining a user-defined function using the python def keyword.

def generate_prime_numbers(limit):
    This function generates and prints the first `limit` prime numbers.
    number = 2
    prime_count = 0
    while prime_count < limit:
        for divisor in range(2, number):
            if number % divisor == 0:
            print(number,end=" ")
            prime_count += 1
        number += 1

# Driver code
limit = 10

# Printing the first 10 prime numbers
print("The first 10 prime numbers are:")


The first 10 prime numbers are:
2 3 5 7 11 13 17 19 23 29

We define a function with Python def that finds out the first n number of prime numbers, a prime count object will check for the number of the prime numbers printed and once it reaches the threshold, the program is terminated with results.

Applications of Def Keyword

As we approach the completion of the article on Python Def discussing the theoretical, syntactical and program implementation, let us discuss the most frequent use cases of Python Def Keyword.

  1. Recursion
    Recursion is one of the important parts of functional programming and the python def keyword is the initial step to get started with the practice of writing a recursive function.

  2. Object-Oriented Programming
    Right from making an init constructor work to implementing the practices of abstraction. Def remains to be essential in constructive object-oriented programming paradigms.

  3. Testing
    Once modularity is achieved, components can be tested efficiently using user-defined functions.

  4. Reusability
    Once created, the function is ever-accessible without any hassle, hence reusability is achieved.

In this article, we discussed the Python Def Keyword, how it can be used to define functions from the perspective of a user and the correct syntax to define one. We studied examples by coding a multiplication and prime number program and as we progressed towards the end of the article, we studied the major applications of the def keyword. We hope you liked this article on How to use the def function in Python and hope to see you again with another informative article from our side.

Frequently Asked Questions

1. What is a def statement in Python?
The def statement is used to define a function in Python. It starts with the keyword def followed by the name of the function and a set of parentheses that may include parameters.

2. What is the need of using a function in Python?
Functions in Python are statements grouped together to perform a specific task. Functions help in breaking down complex programs into simpler and reusable pieces of code.

3. What are the parameters of a Python function?
Parameters are variables that are used to pass values into a function. They are defined within the parentheses of a function definition and can be referenced within the function body.

4. What is the return statement in Python functions?
The return statement is used to return a specific value from a function. When the return statement is executed, the function terminates and the value is passed back to the calling state..

5. Can a Python function return more than one value?
Yes, a Python function can return more than one value using tuple packing and unpacking or by using the return statement multiple times.

Leave a Reply

Your email address will not be published. Required fields are marked *