Python Programming Defining And Calling Functions Complete Guide

Last Update:2025-06-23T00:00:00 .NET School AI Teacher - SELECT ANY TEXT TO EXPLANATION. 10 mins read Difficulty-Level: beginner

Understanding the Core Concepts of Python Programming Defining and Calling Functions

Python Programming: Defining and Calling Functions

Introduction

Why Use Functions?

Code Reusability: Once defined, a function can be used multiple times, reducing code duplication.
Modularity: Functions allow programs to be organized into smaller, understandable units.
Simplification: Complex problems can be solved by breaking them into smaller parts.
Abstraction: Functions provide a way to abstract code and hide complex implementations from users.

Defining a Function

In Python, you define a function using the def keyword followed by the function name and parentheses. Inside the parentheses, you can specify input parameters, and then you use a colon to introduce the function's body, indented under the def statement.

Here is the basic syntax:

def function_name(parameter1, parameter2, ...):
    """Docstring: Explanation"""
    # Function body
    # Perform operations using parameters
    return result

Function Name: Must be a valid identifier, and it's a good practice to use lowercase letters for function names separated by underscores if necessary.
Parameters: Not mandatory; functions can be called without parameters. Parameters are the variables listed within the parentheses after the function name.
Docstring: A string literal enclosed in triple quotes immediately after the function definition to describe what the function does.
Return Statement: Not mandatory; if the function is supposed to return data to the caller, the return statement followed by the value is used.

Example of Defining a Function

Let's define a function that adds two numbers and returns the result.

def add_numbers(a, b):
    """Add two numbers and return the result"""
    result = a + b
    return result

Here:

def add_numbers(a, b): defines a function add_numbers that takes two parameters, a and b.
"""Add two numbers and return the result""" is a docstring that describes what the function does.
result = a + b performs the addition operation.
return result sends the sum back to the caller.

Calling a Function

Once a function is defined, it can be called from anywhere in the program after the point of its definition. You simply type the function name followed by parentheses, including any required arguments.

Example of Calling a Function

Let's call the previously defined add_numbers function.

# Call the function and store the result in the variable sum_result
sum_result = add_numbers(5, 3)
print(sum_result)  # Output: 8

Here:

add_numbers(5, 3) calls the add_numbers function with arguments 5 and 3.
sum_result = add_numbers(5, 3) assigns the returned value from the function to the variable sum_result.

Default Parameters

Functions can be defined with default arguments. If a caller does not specify a value for a parameter with a default argument, the default value is used.

Example:

def greet(name, greeting="Hello"):
    """Greet the user with a personalized message"""
    return f"{greeting}, {name}!"

# Using both parameters
print(greet("Alice", "Hi"))  # Output: "Hi, Alice!"

# Using default value for greeting
print(greet("Bob"))           # Output: "Hello, Bob!"

Variable-Length Arguments

Sometimes, you may need a function that can accept any number of arguments, which Python allows through the use of variable-length arguments. These are often specified with an asterisk (*) before the parameter name.

Example:

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Step-by-Step Guide: How to Implement Python Programming Defining and Calling Functions

Example: Defining and Calling a Simple Function

Objective: This example will demonstrate how to define a simple function that takes two numbers as arguments, adds them, and returns the result. It will also show how to call this function and print its output.

Step 1: Define the Function

First, we need to define the function. In Python, we use the def keyword to start a function definition, followed by the function name and parentheses which may include parameters.

# Define a function named 'add_numbers'
def add_numbers(num1, num2):
    """This function takes two numbers and returns their sum."""
    result = num1 + num2
    return result

def: This tells Python that we are starting to define a function.
add_numbers: This is the name of our function. You can choose any valid identifier as the function name.
(num1, num2): These are Parameters or inputs to the function. You can pass values to these placeholders when you call the function.
"""...""": This is a docstring, which is a description of the function. It is optional but recommended for understanding what the function does.
result: This variable holds the sum of num1 and num2.
return result: The return statement sends the output (in this case, result) back to wherever the function was called from.

Step 2: Call the Function

Now that we have defined a function, the next step is to call it with appropriate values. Here's how we do it:

# Call the function with arguments 5 and 7
sum_result = add_numbers(5, 7)

# Print the result
print("The sum is:", sum_result)

add_numbers(5, 7): This calls the function add_numbers with the numbers 5 and 7 as arguments. The values 5 and 7 replace num1 and num2 inside the function.
sum_result: This variable captures the return value of the function add_numbers.
print(): This prints out the text followed by the value stored in sum_result.

Full Code

Combining both steps, here is the full code:

# Step 1: Define the function
def add_numbers(num1, num2):
    """This function takes two numbers and returns their sum."""
    result = num1 + num2  # Calculate the sum
    return result  # Return the result

# Step 2: Call the function
sum_result = add_numbers(5, 7)  # Call the function with arguments 5 and 7

# Print the result
print("The sum is:", sum_result)  # Output the result

Output

When you run the above code, the output will be:

The sum is: 12

Example 2: Function without Parameters

Let's define a function without any parameters that just prints a greeting message.

Step 1: Define the Function

# Define a function named 'greet'
def greet():
    """This function prints a greeting message."""
    print("Hello, welcome to my program!")

Step 2: Call the Function

# Call the function without any arguments
greet()

Full Code

Combining both steps, here is the full code:

# Step 1: Define the function
def greet():
    """This function prints a greeting message."""
    print("Hello, welcome to my program!")

# Step 2: Call the function
greet()  # Call the function without any arguments

Output

When you run the above code, the output will be:

Hello, welcome to my program!

Example 3: Function with Default Parameters

A function can have default values for some or all of its parameters. If no argument is passed, the default value will be used.

Step 1: Define the Function

# Define a function named 'greet_with_default'
def greet_with_default(name="Guest"):
    """This function prints a greeting message with the provided name or default if none is given."""
    print(f"Hello, {name}, welcome to my program!")

name="Guest": This sets a default parameter. If name is not provided when the function is called, it will use "Guest" as the name.

Step 2: Call the Function

You can call the function with a specific name or without any argument.

# Call the function with no argument (uses default value)
greet_with_default()

# Call the function with a specific name
greet_with_default("Alice")

Full Code

Combining both steps, here is the full code:

# Step 1: Define the function
def greet_with_default(name="Guest"):
    """This function prints a greeting message with the provided name or default if none is given."""
    print(f"Hello, {name}, welcome to my program!")

# Step 2: Call the function
greet_with_default()  # Call the function with no argument (uses default value)
greet_with_default("Alice")  # Call the function with a specific name

Output

When you run the above code, the output will be:

Hello, Guest, welcome to my program!
Hello, Alice, welcome to my program!

Example 4: Function Returning Multiple Values

A function can return multiple values by using a tuple.

Step 1: Define the Function

# Define a function named 'calculate_area_and_perimeter'
def calculate_area_and_perimeter(length, width):
    """This function calculates the area and perimeter of a rectangle and returns both values."""
    area = length * width  # Calculate area
    perimeter = 2 * (length + width)  # Calculate perimeter
    return area, perimeter  # Return both values

Step 2: Call the Function

# Call the function with arguments 5 and 3
area, perimeter = calculate_area_and_perimeter(5, 3)

# Print the results
print(f"The area of the rectangle is: {area}")
print(f"The perimeter of the rectangle is: {perimeter}")

Full Code

Combining both steps, here is the full code:

# Step 1: Define the function
def calculate_area_and_perimeter(length, width):
    """This function calculates the area and perimeter of a rectangle and returns both values."""
    area = length * width  # Calculate area
    perimeter = 2 * (length + width)  # Calculate perimeter
    return area, perimeter  # Return both values

# Step 2: Call the function
area, perimeter = calculate_area_and_perimeter(5, 3)  # Call the function with arguments 5 and 3

# Print the results
print(f"The area of the rectangle is: {area}")
print(f"The perimeter of the rectangle is: {perimeter}")

Output

When you run the above code, the output will be:

The area of the rectangle is: 15
The perimeter of the rectangle is: 16

Summary

In this chapter, you learned:

How to define a function using the def keyword.
How to pass arguments to a function and use them within the function.
How to return a value from a function using the return statement.
How to call a function with and without arguments.
How functions can handle default parameter values.
How a function can return multiple values.

Top 10 Interview Questions & Answers on Python Programming Defining and Calling Functions

1. What is a function in Python?

Answer: A function in Python is a block of organized, reusable code that is used to perform a single, related action. Functions provide better modularity for your application and a high degree of code reuse. You can define what a function does (using the def keyword) and then call it whenever you need it to execute those actions.

2. How do you define a function in Python?

Answer: In Python, a function is defined using the def keyword followed by the function name and parentheses (), which may include parameters. The statements that form the body of the function start at the next line and must be indented. By default, the indentation level is four spaces.

Here’s an example:

def my_function():
    print("Hello from a function")

3. Can a function have zero parameters?

Answer: Yes, a function can definitely have zero parameters. Functions with no parameters are useful when a certain task needs to be performed, but it doesn't require any inputs.

Example:

def greeting():
    print("Welcome to Python world!")
greeting()

4. How do you call a function in Python?

Answer: To call a function, just use its name followed by the parentheses (). If the function requires arguments, place them inside the parentheses.

For instance:

# Define a function with one parameter
def greet(name):
    print(f"Hello, {name}!")

# Call function with an argument
greet("Alice")  # Output: Hello, Alice!

5. What is meant by "argument" in a function call?

Answer: An argument refers to the value passed to a function while the function is being called. This value is used by the function to perform operations or calculations as specified in the function body.

Example:

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

result = add(5, 7)
print(result)  # Output: 12

In this example, 5 and 7 are arguments being passed to the add function.

6. Are parameters and arguments the same thing?

Answer: Parameters and arguments are closely related but represent different aspects of functions. Parameters are names listed in the function definition; they act as placeholders for values that will be passed into the function. Arguments are the actual values passed to the function when it's called.

Consider:

def multiply(x, y):  # x and y are parameters
    return x * y

product = multiply(3, 4)  # 3 and 4 are arguments
print(product)  # Output: 12

7. What is a return statement in Python?

Answer: A return statement is used within a function to send a value back to the caller. After return is executed, the function terminates immediately and returns control to the point in the code where the function call was made.

For example:

def square(number):
    return number ** 2

num_squared = square(4)
print(num_squared)  # Output: 16

8. How can default arguments be used in Python functions?

Answer: Default arguments allow a function to be called without specifying all of the mandatory arguments. These default arguments take a preset value when no argument value is provided during the function call.

def welcome_message(name="Guest"):
    print(f"Welcome, {name}!")

welcome_message()          # Output: Welcome, Guest!
welcome_message("Bob")     # Output: Welcome, Bob!

In the function welcome_message, name is set to a default value of "Guest".

9. Explain how to pass a variable number of arguments to a function in Python.

Answer: Python provides two ways to pass a variable number of arguments:

*args is used to accept a non-keyword, variable-length argument list. Inside the function, it appears as a tuple.
**kwargs allows passing of keyworded, variable-length argument lists as a dictionary.

Examples:

def sum_all_numbers(*args):
    return sum(args)

total = sum_all_numbers(1, 2, 3, 4)  # Passing multiple arguments
print(total)  # Output: 10


def person_details(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

person_details(name="Alice", age=30, city="London")
# Output: name: Alice
# Output: age: 30
# Output: city: London

10. What is a lambda function, and how is it different from a regular function?

Answer: A lambda function is a small anonymous function defined using the lambda keyword. It can have any number of input arguments but only one expression. The expression is evaluated and returned. Lambda functions are ideal for simple functionalities and can be defined in a single line.

Differences:

Syntax: Regular functions are defined using def with a name, whereas lambda functions are anonymous.
Structure: Lambda functions can't contain statements or annotations and are restricted to a single expression.
Complexity: Regular functions can handle more complex logic using multiple lines and statements, while lambda functions are typically limited to simple operations.

Example:

Python Programming Defining And Calling Functions Complete Guide