Introduction to Functions

Why Functions?

Imagine you are writing a program that greets a user. You might write:

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print("Hello, Alice! Welcome to the Python course.")

If you have 100 users, you don’t want to write that line 100 times. Instead, you create a Function.

A function is a reusable block of code that performs a specific task. You define it once, and you can “call” it whenever you need it.

Defining a Function

In Python, we use the def keyword to define a function.

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def say_hello():
    print("Hello there!")
    print("Welcome to Day 8.")

# Calling the function
say_hello()
say_hello()

Key Rules:

1. Use the def keyword.
2. Give the function a name (use lowercase and underscores).
3. Add parentheses () and a colon :.
4. Indent the code block inside the function.

Parameters and Arguments

Functions become powerful when they can accept information. We use parameters to pass data into a function.

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def greet_user(username):
    print(f"Hello, {username}! Ready to code?")

greet_user("Alice")
greet_user("Bob")

• username is the parameter (the variable name inside the function).
• "Alice" is the argument (the actual value passed in).

Returning Values

Sometimes you want a function to calculate something and give the result back to you. We use the return statement for this.

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def add_numbers(a, b):
    result = a + b
    return result

sum_total = add_numbers(5, 10)
print(f"The total is: {sum_total}")

Interactive Practice

Try defining a function called multiply that takes two numbers and returns their product.