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Python Functions — Reusable Code Blocks

Learn how to create your own functions in Python — reusable blocks of code that make your programs organized, shorter, and easier to fix.

What Are Functions?

Imagine you make a peanut butter sandwich every day. The steps are always the same: get bread, spread peanut butter, close it. Instead of explaining those steps every single time, you just say 'make me a sandwich' — and everyone knows what you mean. Functions work the same way in code. You write a set of instructions once, give it a name, and then you can use that name to run those instructions whenever you want. No need to repeat yourself! You've already been using functions: print(), input(), len(), range() — these are all functions that someone else wrote for you. Now you'll learn to write your own.

Your First Function

python

# Define a function with 'def'
def say_hello():
    print("Hello there!")
    print("Welcome to Python!")

# Call (use) the function
say_hello()  # prints both lines
say_hello()  # you can call it as many times as you want!

# Functions with parameters (inputs)
def greet(name):
    print(f"Hello, {name}! Nice to meet you!")

greet("Aarav")   # Hello, Aarav! Nice to meet you!
greet("Priya")   # Hello, Priya! Nice to meet you!

Parameters vs Arguments

These two words confuse people, but it's simple: • Parameter — the placeholder name in the function definition. It's like a blank on a form. • Argument — the actual value you pass when calling the function. It's what you fill in. In 'def greet(name)', 'name' is the parameter. In 'greet("Aarav")', '"Aarav"' is the argument. Think of it like a vending machine: the button slot is the parameter, and the coin you insert is the argument.

Functions That Return Values

python

# Use 'return' to send a result back
def add(a, b):
    return a + b

result = add(5, 3)
print(result)  # 8

# A more useful example
def calculate_area(length, width):
    area = length * width
    return area

room = calculate_area(12, 10)
print(f"Room area: {room} sq ft")  # Room area: 120 sq ft

# Functions can return any type
def is_passing(score):
    return score >= 60  # returns True or False

print(is_passing(85))  # True
print(is_passing(42))  # False

Default Values and Multiple Parameters

python

# Default parameter values
def power(base, exponent=2):
    return base ** exponent

print(power(5))      # 25 (uses default exponent 2)
print(power(2, 10))  # 1024 (2 to the power of 10)

# Function with multiple parameters
def introduce(name, age, hobby):
    print(f"I'm {name}, {age} years old.")
    print(f"I love {hobby}!")

introduce("Meera", 11, "coding")

# Calling with named arguments (order doesn't matter)
introduce(hobby="drawing", name="Zain", age=13)

Pro Tip

A function should do ONE thing well. If your function is getting really long or doing many different things, break it into smaller functions. For example, instead of one big 'process_order()' function, have 'calculate_total()', 'apply_discount()', and 'print_receipt()'. This makes code way easier to read and fix.

Try It Yourself

Create these 3 functions: (1) celsius_to_fahrenheit(celsius) that converts Celsius to Fahrenheit using the formula F = C * 9/5 + 32, (2) is_even(number) that returns True if a number is even, False if odd, and (3) longest_word(words) that takes a list of words and returns the longest one. Test each function with at least 2 different inputs!

Python Lists — Collections of Data

Python Input & Output — Talking to Users

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