beginner⏱55 minLesson 5 of 10

Control Flow: Conditionals

Master Python conditionals: if, elif, else statements, nested conditions, ternary operator, and practical decision-making patterns.

Control Flow: Conditionals

Control flow determines the order in which statements execute. Conditionals allow your program to make decisions and execute different code based on conditions.

What is Control Flow?

By default, Python executes statements sequentially, one after another. Conditionals let you change this flow based on conditions.

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The if Statement

The if statement executes a block of code only when a condition is True.

Basic Syntax

python

if condition:
    # Code executes only if condition is True
    print("Condition is True!")

Simple if Example

python

# Check if a number is positive
number = 15

if number > 0:
    print(f"{number} is positive")

print("Program continues...")

Output:

15 is positive
Program continues...

python

# What if the condition is False?
number = -5

if number > 0:
    print(f"{number} is positive")  # This won't execute

print("Program continues...")

Output:

Program continues...

ℹ️Note

Python uses indentation (typically 4 spaces) to define code blocks. This is not optional - it's how Python knows which statements belong to the if block.

The if-else Statement

The else clause provides an alternative when the condition is False.

Syntax

python

if condition:
    # Executes when condition is True
    pass
else:
    # Executes when condition is False
    pass

if-else Example

python

# Age verification
age = 16

if age >= 18:
    print("You are an adult.")
    print("You can vote and drive.")
else:
    print("You are a minor.")
    print("You need parental consent.")

Output:

You are a minor.
You need parental consent.

Real-World Example: Login System

python

# login_system.py
def check_login(username, password):
    """Simple login verification."""
    
    # Valid credentials (in real apps, these would be in a database)
    valid_username = "admin"
    valid_password = "python123"
    
    if username == valid_username and password == valid_password:
        print("Login successful! Welcome back.")
        return True
    else:
        print("Invalid username or password.")
        return False

# Test the login system
print("=== Login System ===")
check_login("admin", "python123")   # Success
print()
check_login("admin", "wrongpass")   # Failure
print()
check_login("user", "python123")    # Failure

Output:

=== Login System ===
Login successful! Welcome back.

Invalid username or password.

Invalid username or password.

The if-elif-else Statement

When you have multiple conditions to check, use elif (else if).

Syntax

python

if condition1:
    # Executes if condition1 is True
    pass
elif condition2:
    # Executes if condition1 is False and condition2 is True
    pass
elif condition3:
    # Executes if condition1 and condition2 are False, and condition3 is True
    pass
else:
    # Executes if all conditions are False
    pass

Flowchart

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Grade Calculator Example

python

# grade_calculator.py
def get_letter_grade(score):
    """Convert numeric score to letter grade."""
    
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    elif score >= 60:
        return "D"
    else:
        return "F"

# Test with various scores
scores = [95, 87, 72, 65, 45]

for score in scores:
    grade = get_letter_grade(score)
    print(f"Score: {score:3d} → Grade: {grade}")

Output:

Score:  95 → Grade: A
Score:  87 → Grade: B
Score:  72 → Grade: C
Score:  65 → Grade: D
Score:  45 → Grade: F

⚠️Warning

Conditions are checked in order! Once a True condition is found, the rest are skipped. This means:

python

# WRONG ORDER - always returns "A"
if score >= 60:
    return "D"  # 95 >= 60 is True, so this executes!
elif score >= 90:
    return "A"  # Never reached
# CORRECT ORDER - check highest first
if score >= 90:
    return "A"
elif score >= 60:
    return "D"

Nested Conditionals

You can place if statements inside other if statements.

Nested if Example

python

# ticket_pricing.py
def calculate_ticket_price(age, is_student, is_weekend):
    """Calculate movie ticket price with discounts."""
    
    base_price = 15.00
    
    if age < 12:
        # Child pricing
        price = base_price * 0.5
        category = "Child"
    elif age >= 65:
        # Senior pricing
        price = base_price * 0.6
        category = "Senior"
    else:
        # Adult pricing
        if is_student:
            price = base_price * 0.8
            category = "Student"
        else:
            price = base_price
            category = "Adult"
    
    # Weekend surcharge
    if is_weekend:
        price += 2.00
    
    return category, price

# Test cases
test_cases = [
    (10, False, False),   # Child, weekday
    (70, False, True),    # Senior, weekend
    (25, True, False),    # Student, weekday
    (30, False, True),    # Adult, weekend
]

print("Movie Ticket Pricing")
print("=" * 45)
for age, student, weekend in test_cases:
    category, price = calculate_ticket_price(age, student, weekend)
    day = "Weekend" if weekend else "Weekday"
    print(f"Age {age:2d}, {category:8s}, {day}: R${price:.2f}")

Output:

Movie Ticket Pricing
=============================================
Age 10, Child   , Weekday: R$7.50
Age 70, Senior  , Weekend: R$11.00
Age 25, Student , Weekday: R$12.00
Age 30, Adult   , Weekend: R$17.00

Ternary Operator (Conditional Expression)

Python has a concise way to write simple if-else statements in one line.

Syntax

python

value = true_value if condition else false_value

Ternary Examples

python

# Basic usage
age = 20
status = "adult" if age >= 18 else "minor"
print(f"Status: {status}")  # adult

# With numbers
x = 10
y = 20
maximum = x if x > y else y
print(f"Maximum: {maximum}")  # 20

# Absolute value
number = -15
abs_value = number if number >= 0 else -number
print(f"Absolute value: {abs_value}")  # 15

# Even or odd
num = 7
result = "even" if num % 2 == 0 else "odd"
print(f"{num} is {result}")  # 7 is odd

Practical Example: Discount Calculator

python

# discount_calculator.py
def apply_discount(price, is_member, purchase_amount):
    """Apply discount based on membership and purchase amount."""
    
    # Member discount
    member_discount = 0.15 if is_member else 0.05
    
    # Bulk purchase bonus
    bulk_bonus = 0.10 if purchase_amount > 100 else 0.0
    
    # Total discount (capped at 20%)
    total_discount = min(member_discount + bulk_bonus, 0.20)
    
    final_price = price * (1 - total_discount)
    
    return final_price, total_discount * 100

# Test cases
print("Discount Calculator")
print("=" * 50)
print(f"{'Price':>8} {'Member':>8} {'Amount':>8} {'Discount':>10} {'Final':>8}")
print("-" * 50)

tests = [
    (50.00, True, 50),
    (120.00, True, 120),
    (80.00, False, 80),
    (200.00, False, 200),
]

for price, member, amount in tests:
    final, disc_pct = apply_discount(price, member, amount)
    member_str = "Yes" if member else "No"
    print(f"R${price:7.2f} {member_str:>8} R${amount:7.2f} {disc_pct:9.1f}% R${final:7.2f}")

Output:

Discount Calculator
==================================================
   Price   Member   Amount   Discount    Final
--------------------------------------------------
R$  50.00      Yes  R$  50.00      15.0% R$  42.50
R$ 120.00      Yes  R$ 120.00      20.0% R$  96.00
R$  80.00       No  R$  80.00       5.0% R$  76.00
R$ 200.00       No  R$ 200.00      15.0% R$ 170.00

Multiple Conditions with Logical Operators

Combine conditions using and, or, and not.

Combining Conditions

python

# Weather activity recommender
def recommend_activity(temperature, is_raining, is_weekend):
    """Recommend an activity based on weather conditions."""
    
    if not is_raining and temperature >= 25:
        return "Go to the beach!"
    elif not is_raining and 15 <= temperature < 25:
        return "Go for a walk in the park."
    elif is_raining and is_weekend:
        return "Watch a movie at home."
    elif is_raining and not is_weekend:
        return "Take an umbrella to work."
    elif temperature < 15:
        return "Stay indoors with hot chocolate."
    else:
        return "Check local events."

# Test scenarios
scenarios = [
    (30, False, True),   # Hot, sunny, weekend
    (20, False, False),  # Mild, sunny, weekday
    (18, True, True),    # Mild, rainy, weekend
    (10, True, False),   # Cold, rainy, weekday
    (5, False, True),    # Cold, sunny, weekend
]

print("Activity Recommender")
print("=" * 60)
for temp, rain, weekend in scenarios:
    weather = "Rainy" if rain else "Sunny"
    day = "Weekend" if weekend else "Weekday"
    activity = recommend_activity(temp, rain, weekend)
    print(f"{temp:2d}°C, {weather:6s}, {day:7s} → {activity}")

Output:

Activity Recommender
============================================================
30°C, Sunny , Weekend → Go to the beach!
20°C, Sunny , Weekday → Go for a walk in the park.
18°C, Rainy, Weekend → Watch a movie at home.
10°C, Rainy, Weekday → Take an umbrella to work.
 5°C, Sunny , Weekend → Stay indoors with hot chocolate.

Truthy and Falsy Values in Conditions

Python allows any value in a condition, not just booleans.

Truthy and Falsy Values

python

# Falsy values (evaluate to False)
falsy_values = [
    False,
    None,
    0,
    0.0,
    "",           # Empty string
    [],           # Empty list
    {},           # Empty dict
    set(),        # Empty set
]

print("Falsy values:")
for val in falsy_values:
    print(f"  bool({repr(val):10s}) = {bool(val)}")

# Truthy values (evaluate to True)
print("\nTruthy values:")
print(f"  bool(1) = {bool(1)}")
print(f"  bool('hello') = {bool('hello')}")
print(f"  bool([1, 2, 3]) = {bool([1, 2, 3])}")

Output:

Falsy values:
  bool(False    ) = False
  bool(None     ) = False
  bool(0        ) = False
  bool(0.0      ) = False
  bool(''       ) = False
  bool([]       ) = False
  bool({}       ) = False
  bool(set()    ) = False

Truthy values:
  bool(1) = True
  bool('hello') = True
  bool([1, 2, 3]) = True

Idiomatic Python: Checking for Empty Values

python

# Instead of this:
if len(my_list) > 0:
    print("List has items")

# Write this (more Pythonic):
if my_list:
    print("List has items")

# Instead of this:
if my_string != "":
    print("String is not empty")

# Write this:
if my_string:
    print("String is not empty")

Real-World Example: ATM Withdrawal System

python

# atm_system.py
def atm_withdrawal(balance, amount, pin, daily_limit):
    """Simulate an ATM withdrawal with multiple checks."""
    
    # Check 1: PIN validation
    if pin != 1234:
        return "ERROR: Invalid PIN"
    
    # Check 2: Amount must be positive
    if amount <= 0:
        return "ERROR: Amount must be positive"
    
    # Check 3: Amount must be multiple of 10
    if amount % 10 != 0:
        return "ERROR: Amount must be a multiple of 10"
    
    # Check 4: Sufficient balance
    if amount > balance:
        return f"ERROR: Insufficient funds. Balance: R${balance:.2f}"
    
    # Check 5: Daily limit
    if amount > daily_limit:
        return f"ERROR: Exceeds daily limit of R${daily_limit:.2f}"
    
    # All checks passed - process withdrawal
    new_balance = balance - amount
    return f"SUCCESS: Withdrawn R${amount:.2f}. New balance: R${new_balance:.2f}"

# Test the ATM system
print("=== ATM Withdrawal System ===\n")

test_cases = [
    (500.00, 100, 1234, 300),   # Valid withdrawal
    (500.00, 100, 5678, 300),   # Wrong PIN
    (500.00, -50, 1234, 300),   # Negative amount
    (500.00, 55, 1234, 300),    # Not multiple of 10
    (500.00, 600, 1234, 300),   # Insufficient funds
    (500.00, 400, 1234, 300),   # Exceeds daily limit
]

for balance, amount, pin, limit in test_cases:
    result = atm_withdrawal(balance, amount, pin, limit)
    print(f"Balance: R${balance:7.2f}, Withdraw: R${amount:4d}, PIN: {pin}")
    print(f"  → {result}\n")

Output:

=== ATM Withdrawal System ===

Balance: R$ 500.00, Withdraw: R$ 100, PIN: 1234
  → SUCCESS: Withdrawn R$100.00. New balance: R$400.00

Balance: R$ 500.00, Withdraw: R$ 100, PIN: 5678
  → ERROR: Invalid PIN

Balance: R$ 500.00, Withdraw: R$ -50, PIN: 1234
  → ERROR: Amount must be positive

Balance: R$ 500.00, Withdraw: R$  55, PIN: 1234
  → ERROR: Amount must be a multiple of 10

Balance: R$ 500.00, Withdraw: R$ 600, PIN: 1234
  → ERROR: Insufficient funds. Balance: R$500.00

Balance: R$ 500.00, Withdraw: R$ 400, PIN: 1234
  → ERROR: Exceeds daily limit of R$300.00

Practice Exercises

Exercise 1: Positive, Negative, or Zero

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

Exercise 2: Number Comparator

Write a program that takes two numbers and prints which one is larger, or if they are equal.

Exercise 3: Leap Year Checker

Write a program that determines if a year is a leap year using nested conditionals.

Exercise 4: Grade Classifier

Create a program that takes a score (0-100) and returns:

"Excellent" for 90-100
"Good" for 75-89
"Average" for 60-74
"Needs Improvement" for below 60

Exercise 5: Simple Calculator

Write a calculator that takes two numbers and an operation (+, -, *, /) and performs the calculation. Handle division by zero.

Exercise 6: Ternary Practice

Rewrite these if-else statements as ternary expressions:

python

if x > 100:
    result = "high"
else:
    result = "low"

Exercise 7: Password Strength Checker

Write a function that checks password strength:

"Weak" if length < 6
"Medium" if length 6-11
"Strong" if length >= 12 and contains uppercase
"Very Strong" if length >= 12, contains uppercase, and contains a digit

Exercise 8: Rock Paper Scissors

Write a program that determines the winner of a Rock-Paper-Scissors round given two choices.

Summary

In this lesson, you learned:

How to use if statements for basic decision-making
How if-else provides two execution paths
How if-elif-else handles multiple conditions
How to nest conditionals for complex logic
How to use the ternary operator for concise expressions
How to combine conditions with logical operators
How truthy and falsy values work in Python
How to build real-world decision-making systems

Conditionals are essential for creating programs that respond to different situations. Practice writing conditionals to build your decision-making skills.

Progress50%

Operators & Expressions

Control Flow: Loops

Course Content

1. How Computers Work 2. Python Setup & First Program 3. Variables & Data Types 4. Operators & Expressions 5. Control Flow: Conditionals 6. Control Flow: Loops 7. Functions 8. Data Structures: Lists & Tuples 9. Data Structures: Dictionaries & Sets 10. Imperative Algorithm Projects