🌱

Basic

def · return · Arguments · Defaults · Scope

Week 9 — Functions

Functions let you name and reuse blocks of logic. Learn def, return, default arguments, keyword arguments, and variable scope (LEGB rule).

functiondefreturnscopearguments

Duration

⏱ 2.5 hours

Level

📊 Beginner

Prerequisite

🎯 Week 5

OUTCOME

Refactor a calculator into well-named functions with docstrings

What you'll learn

1Define and call functions with def
2Return single and multiple values
3Use positional, keyword, and default arguments
4Understand the LEGB scope rule
5Write docstrings to document functions

1. Defining Functions

python

def greet(name):
    """Return a greeting string."""
    return f"Hello, {name}!"

print(greet("Alice"))   # Hello, Alice!
print(greet("Bob"))     # Hello, Bob!

💡

Always write a docstring (triple-quoted string right after def) to document what the function does.

2. Return Values

python

def divide(a, b):
    if b == 0:
        return None, "Division by zero"
    return a / b, None

result, error = divide(10, 2)
if error:
    print(error)
else:
    print(result)  # 5.0

3. Arguments

python

def power(base, exp=2):          # default argument
    return base ** exp

print(power(3))       # 9  (exp defaults to 2)
print(power(3, 3))    # 27
print(power(exp=3, base=2))  # keyword arg: 8

4. Scope (LEGB)

Python looks up variable names in this order: Local → Enclosing → Global → Built-in.

python

x = 10          # global

def func():
    x = 20      # local — does NOT change the global x
    print(x)    # 20

func()
print(x)        # 10

# To modify global x inside a function:
def func2():
    global x
    x = 99
func2()
print(x)        # 99

5. Common Mistakes

A function without return returns None implicitly.
Mutable default arguments (def f(lst=[])) are shared between calls — use def f(lst=None): lst = lst or [].
Calling a function before defining it → NameError. Define first, call second.

💻 Examples

Run these examples and check the output yourself.

01_calculator_funcs.py— Calculator refactored into functions

CODE

def add(a, b): return a + b
def sub(a, b): return a - b
def mul(a, b): return a * b
def div(a, b): return a / b if b else "Error: division by zero"

a = float(input("a: "))
b = float(input("b: "))
print(f"  +: {add(a,b)}")
print(f"  -: {sub(a,b)}")
print(f"  *: {mul(a,b)}")
print(f"  /: {div(a,b)}")

02_stats.py— Statistical functions

CODE

def mean(lst):
    return sum(lst) / len(lst)

def variance(lst):
    m = mean(lst)
    return sum((x - m)**2 for x in lst) / len(lst)

def std_dev(lst):
    return variance(lst) ** 0.5

data = [10, 20, 30, 40, 50]
print(f"Mean:    {mean(data):.2f}")
print(f"Std Dev: {std_dev(data):.2f}")

▶ Output

Mean:    30.00
Std Dev: 14.14

03_recursive_factorial.py— Factorial using recursion

CODE

def factorial(n):
    """Return n! recursively."""
    if n <= 1:
        return 1
    return n * factorial(n - 1)

for i in range(1, 8):
    print(f"{i}! = {factorial(i)}")

▶ Output

1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040

📝 Exercises

Try them yourself first, then open the solution to compare.

Exercise 1

Is Prime?

Goal: Write is_prime(n) that returns True if n is prime.

Requirements

Handle n < 2 → False
Check divisors only up to sqrt(n)
Test for n = 1, 2, 17, 100

▶Toggle solution

SOLUTION

def is_prime(n):
    if n < 2: return False
    for i in range(2, int(n**0.5)+1):
        if n % i == 0: return False
    return True

for n in [1, 2, 17, 100]:
    print(f'is_prime({n}) = {is_prime(n)}')

▶ Output

is_prime(1) = False
is_prime(2) = True
is_prime(17) = True
is_prime(100) = False

Exercise 2

Caesar Cipher

Goal: Write encrypt(text, shift) that shifts each letter by shift positions.

Requirements

Preserve case (upper/lower)
Non-alpha characters pass through unchanged
Write a matching decrypt(text, shift)

Sample I/O

encrypt('Hello, World!', 3) → 'Khoor, Zruog!'

▶Toggle solution

SOLUTION

def encrypt(text, shift):
    result = []
    for c in text:
        if c.isalpha():
            base = ord('A') if c.isupper() else ord('a')
            result.append(chr((ord(c) - base + shift) % 26 + base))
        else:
            result.append(c)
    return ''.join(result)

def decrypt(text, shift):
    return encrypt(text, -shift)

print(encrypt('Hello, World!', 3))
print(decrypt('Khoor, Zruog!', 3))

▶ Output

Khoor, Zruog!
Hello, World!

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗