Conditionals (if/elif/else)

Conditionals in Python

Introduction

Conditionals allow your program to make decisions and execute different code paths based on conditions.

Syntax

if condition:
    # code to execute if condition is True
elif another_condition:
    # code to execute if another_condition is True
else:
    # code to execute if all conditions are False

Basic Examples

age = 18

if age >= 18:
    print("You can vote")
else:
    print("Too young to vote")

Multiple Conditions

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Grade: {grade}")  # Grade: B

Truthiness and Falsiness

# Falsy values: False, None, 0, 0.0, "", [], {}, set()
# Everything else is truthy

if []:        # ❌ — empty list is falsy
    print("Never runs")

if [1, 2]:    # ✅ — non-empty list is truthy
    print("Always runs")

if "":        # ❌ — empty string is falsy
    print("Never runs")

Ternary Expression

# Syntax: value_if_true if condition else value_if_false
age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)  # Adult

# With functions
result = "even" if x % 2 == 0 else "odd"

Nested Conditionals

user = {"role": "admin", "active": True}

if user["active"]:
    if user["role"] == "admin":
        print("Full access granted")
    elif user["role"] == "editor":
        print("Edit access granted")
    else:
        print("Read-only access")
else:
    print("Account deactivated")

Short-Circuit Evaluation

# and — stops at first False
def expensive():
    print("Expensive function called!")
    return True

if False and expensive():  # expensive() is NEVER called
    print("Won't print")

# or — stops at first True
if True or expensive():    # expensive() is NEVER called
    print("Short-circuited!")

Common Mistakes

# ❌ Wrong: Assignment instead of comparison
if x = 10:     # SyntaxError!

# ✅ Correct
if x == 10:
    pass

# ❌ Wrong: Comparing boolean with ==
if is_valid == True:  # Redundant!

# ✅ Correct
if is_valid:
    pass

Best Practices

Avoid nesting deeper than 3 levels — flatten with early returns
Use elif instead of nested if when conditions are mutually exclusive
Prefer positive conditions — if active: over if not inactive:
Use in for multiple comparisons — if x in (1, 2, 3): instead of if x == 1 or x == 2 or x == 3:
Keep conditions simple — extract complex logic into named variables

Practice Exercises

Exercise 1: Write a program that checks if a number is positive, negative, or zero.

Exercise 2: Create a simple calculator that takes two numbers and an operator (+, -, *, /) and performs the operation.

Exercise 3: Build a login system that checks username and password from a dictionary.