Chapter 6: Functions in Python

1. Function Fundamentals

Anatomy of a Python function

Defining and Calling Functions

# Function definition
def greet(name):
    """Returns a greeting message"""
    return f"Hello, {name}!"

# Function call
message = greet("Alice")
print(message)  # Output: Hello, Alice!

Exercise 1: Temperature Converter

Create a function that:

Converts Fahrenheit to Celsius
Formula: C = (F - 32) × 5/9
Test with 68°F (should return 20°C)

def fahrenheit_to_celsius(f_temp):
    return (f_temp - 32) * 5/9

print(fahrenheit_to_celsius(68))  # 20.0

2. Parameters & Arguments

Type	Description	Example
Positional	Matched by position	`func(1, 2)`
Keyword	Matched by name	`func(a=1, b=2)`
Default	Optional parameters	`def func(a=0)`

Example: Flexible Arguments

def describe_pet(pet_name, animal_type="dog"):
    print(f"I have a {animal_type} named {pet_name}")

# Different calling styles
describe_pet("Rover")
describe_pet("Fluffy", "cat")
describe_pet(animal_type="hamster", pet_name="Harry")

Output

I have a dog named Rover
I have a cat named Fluffy
I have a hamster named Harry

3. Return Values

How data flows through functions

def calculate_stats(numbers):
    """Returns tuple of (min, max, avg)"""
    return min(numbers), max(numbers), sum(numbers)/len(numbers)

stats = calculate_stats([10, 20, 30])
print(stats)  # (10, 30, 20.0)

Exercise 2: Password Validator

Create a function that:

Takes a password string
Returns True if length ≥ 8 and contains a digit
Returns False otherwise

def is_valid_password(password):
    return len(password) >= 8 and any(c.isdigit() for c in password)

print(is_valid_password("pass123"))  # True
print(is_valid_password("short"))    # False

4. Variable Scope

Global vs Local

count = 10  # Global variable

def increment():
    count = 5  # Local variable
    return count + 1

print(increment())  # 6
print(count)        # 10 (global unchanged)

Using global

total = 0

def add_to_total(n):
    global total
    total += n

add_to_total(5)
print(total)  # 5

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