Python · Quick Reference

Lesson 10 — Functions Cheat Sheet

Python

In one line: A function is a named, reusable block of code that performs a specific task. Without functions, every time you need to calculate a grade or validate a password, you'd copy-paste...

Key Ideas

1Why Functions?. A function is a named, reusable block of code that performs a specific task. Without functions, every time you need to calculate a grade or validate a password, you'd ...

2Parameters & Arguments. Parameters are the variable names in the function definition. Arguments are the actual values you pass when calling the function. Python has four parameter types — eac...

3Return Values. The return statement sends a value back to the caller and immediately exits the function. A function can return any Python object — a number, string, list, dict, tuple...

4Variable Scope — The LEGB Rule. When Python encounters a variable name, it searches for it in a specific order of scopes. This order is called the LEGB rule — Local → Enclosing → Global → Built-in.

5Lambda Functions & Higher-Order Functions. A lambda is a small anonymous function written in one line. It's syntactic sugar for a simple function — useful when you need a short function as an argument to anothe...

Code Examples

# Without functions — repeated code (bad)
tax1 = 1500 * 0.17
tax2 = 2800 * 0.17
tax3 = 950  * 0.17

# With a function — write once, use anywhere (DRY)
def calculate_tax(amount):
    """Calculate 17% sales tax on an amount."""
    return amount * 0.17

tax1 ...

# ── Positional and default parameters ────────
def describe_student(name, age, city="Lahore", grade="A"):
    return f"{name}, {age}, from {city}, Grade {grade}"

print(describe_student("Ali", 20))                    # uses defaults
print(describe_student(...

# Single return value
def square(n):
    return n ** 2

print(square(7))   # 49

# Early return — guard clause pattern
def divide(a, b):
    if b == 0:
        return None    # exit early on bad input
    return a / b

print(divide(10, 2))   # 5.0
print(div...