Day 16 – Master Defining and Calling Functions in Python | 60 Days Python Crash Course | ElectroLab

Defining and Calling Functions in Python
Introduction
Functions are one of the most important building blocks in Python. They allow us to organize our code into reusable pieces, making programs easier to write, maintain, and debug. Whether you’re building a small script or a large-scale application, Defining and Calling Functions in Python effectively is a must-have skill.
In this lesson, you’ll explore how to define your own functions and how to call them with and without arguments. You’ll also understand function parameters, return values, and the importance of modular programming. You’ll master the process of Defining and Calling Functions in Python through real-world examples and code snippets.
What You’ll Learn Today:
- What is a function in Python?
- How to define a function using
def
- Calling a function with or without arguments
- Using
return
statements effectively - Parameters vs arguments
- Scope of variables inside functions
- Function nesting and reusability
- Best practices while Defining and Calling Functions in Python
Defining and Calling Functions in Python

What is a Function?
A function is a block of organized, reusable code that performs a specific task. Python has many built-in functions (like print()
and len()
), but you can also create your own custom functions. Understanding Defining and Calling Functions in Python allows you to structure your code efficiently.
Why Use Functions?
- Reusability: Write once, use multiple times
- Maintainability: Easier to fix and update
- Modularity: Break complex problems into smaller, manageable tasks
Syntax to Define a Function
def function_name(parameters):
# code block
return result # optional
Example 1: Simple Function without Parameters
def greet():
print("Hello from ElectroLab!")
# Calling the function
greet()
Example 2: Function with Parameters
def add(a, b):
result = a + b
return result
print(add(10, 5))
Example 3: Function with Default Parameters
def greet_user(name="Guest"):
print(f"Hello, {name}!")
# With argument
greet_user("Jyoti")
# Without argument
greet_user()
Example 4: Return Statement
def square(x):
return x * x
print(square(4)) # Output: 16
Example 5: Nested Function
def outer():
print("Outer function")
def inner():
print("Inner function")
inner()
outer()
Example 6: Function Calling Inside a Loop
def multiply_by_2(num):
return num * 2
count = 0
while count < 5:
print(multiply_by_2(count))
count += 1
Quizzes
- What is the keyword used to define a function in Python?
- What will
return
do in a function? - What is the difference between a parameter and an argument?
- Can a function return multiple values?
- What happens if a function doesn’t have a return statement?
- Why is Defining and Calling Functions in Python useful in large programs?
FAQs
Q1: Can a function return multiple values?
Yes, using a tuple. Example:
def stats(a, b):
return a + b, a * b
Q2: What is the difference between return
and print()
?
return
sends a result back to the caller.print()
displays output on the screen.
Q3: Can you call a function inside another function?
Yes, Python supports nested function calls.
Q4: Is there a best practice for Defining and Calling Functions in Python?
Yes, always use meaningful function names and keep functions short and specific.
Summary
Recap:
- Functions help make code modular, clean, and reusable.
- Use
def
to define a function. - Use parentheses
()
to call a function. - Use parameters to accept input and
return
to give output. - Understand scope and local/global variables.
- Consistent practice of Defining and Calling Functions in Python leads to clean, readable, and maintainable code.
External Resources
Common Mistakes to Avoid
- Forgetting to use parentheses
()
when calling a function. - Not using indentation correctly inside a function.
- Confusing between
return
andprint()
. - Overwriting built-in function names like
sum
,max
, etc. - Using global variables unnecessarily inside functions.
- Inconsistent structure when Defining and Calling Functions in Python.
Real-Life Use Case Example
Imagine you are building a simple calculator app. Every operation like addition, subtraction, multiplication can be written as a separate function.
def add(a, b):
return a + b
def subtract(a, b):
return a - b
print("Sum:", add(10, 5))
print("Difference:", subtract(10, 5))
This modular approach makes the app easier to expand and debug. You’ve just applied Defining and Calling Functions in Python in a real-world project.
Learn More with ElectroLab.in
- What is Python & Install IDE
- First Python Program using print()
- Variables and Data Types
- Typecasting and Input Function
- Arithmetic & Comparison Operators
- String Manipulation
- String Formatting
- If-Elif-Else Statements
- For Loop in Python
- While Loop in Python
- Break, Continue, Pass
- Lists in Python
- Tuples in Python
- Sets in Python
- Dictionaries in Python
What’s Next?
In the next lesson, we’ll dive deeper into Arguments and Parameters in Python Functions where we’ll explore *args
, **kwargs
, and how to handle various types of function inputs efficiently. But before that, revise today’s topic to fully master Defining and Calling Functions in Python.