Module 4 - Control Flow

This module covers control flow statements in Python. You'll learn how to make decisions in your code using if/elif/else, match-case, and ternary operators.

---

1. If Statements

1.1 Basic If Statement

age = 18

if age >= 18:
    print("You are an adult")

1.2 If-Else Statement

temperature = 25

if temperature > 30:
    print("It's hot outside")
else:
    print("It's pleasant weather")

1.3 If-Elif-Else Statement

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"Your grade is: {grade}")

Best Practice

Use elif instead of multiple if statements when conditions are mutually exclusive. This improves performance and readability.

1.4 Nested If Statements

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive")
    else:
        print("You need a license")
else:
    print("You are too young to drive")

---

2. Comparison Operators

2.1 Basic Comparisons

Operator	Description	Example
==	Equal to	5 == 5 → True
!=	Not equal to	5 != 3 → True
>	Greater than	5 > 3 → True
<	Less than	3 < 5 → True
>=	Greater than or equal	5 >= 5 → True
<=	Less than or equal	3 <= 5 → True

x = 10
y = 20

print(x == y)   # False
print(x != y)   # True
print(x < y)    # True
print(x > y)    # False

2.2 Membership Operators

# in operator
fruits = ["apple", "banana", "orange"]
print("apple" in fruits)     # True
print("mango" in fruits)     # False

# not in operator
print("mango" not in fruits) # True

# Works with strings
text = "Python is awesome"
print("Python" in text)      # True

2.3 Identity Operators

a = [1, 2, 3]
b = [1, 2, 3]
c = a

# is operator (checks if same object)
print(a is c)      # True
print(a is b)      # False

# is not operator
print(a is not b)  # True

# == checks value equality
print(a == b)      # True

is vs ==

• == checks if values are equal
• is checks if objects are the same (same memory location)

---

3. Logical Operators

3.1 and, or, not

age = 25
has_job = True
has_degree = False

# and operator
if age > 18 and has_job:
    print("Eligible for loan")

# or operator
if has_degree or age > 30:
    print("Can apply for senior position")

# not operator
if not has_degree:
    print("Consider getting a degree")

# Combining operators
if (age > 21 and has_job) or has_degree:
    print("Qualified candidate")

3.2 Short-Circuit Evaluation

# and: Returns first falsy value or last value
result = 0 and 5        # 0
result = 5 and 10       # 10
result = None and "hi"  # None

# or: Returns first truthy value or last value
result = 0 or 5         # 5
result = 5 or 10        # 5
result = None or False  # False

# Practical use
name = input("Name: ") or "Guest"  # Default to "Guest" if empty

---

4. Ternary Operator (Conditional Expression)

4.1 Basic Syntax

# Syntax: value_if_true if condition else value_if_false

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)  # Adult

# Traditional if-else equivalent
if age >= 18:
    status = "Adult"
else:
    status = "Minor"

4.2 Practical Examples

# Finding max/min
a, b = 10, 20
maximum = a if a > b else b
print(maximum)  # 20

# Conditional assignment
score = 85
result = "Pass" if score >= 50 else "Fail"

# Nested ternary (use sparingly)
score = 95
grade = "A" if score >= 90 else ("B" if score >= 80 else "C")

# In function returns
def absolute(num):
    return num if num >= 0 else -num

print(absolute(-5))  # 5

Readability

Avoid complex nested ternary operators. Use regular if-elif-else for better readability.

---

5. Match-Case Statement (Python 3.10+)

5.1 Basic Match Statement

# Traditional if-elif-else
command = "start"

if command == "start":
    print("Starting...")
elif command == "stop":
    print("Stopping...")
elif command == "pause":
    print("Pausing...")
else:
    print("Unknown command")

# Using match-case
match command:
    case "start":
        print("Starting...")
    case "stop":
        print("Stopping...")
    case "pause":
        print("Pausing...")
    case _:
        print("Unknown command")

5.2 Pattern Matching with Multiple Values

status_code = 404

match status_code:
    case 200 | 201:
        print("Success")
    case 400 | 401 | 403:
        print("Client error")
    case 404:
        print("Not found")
    case 500 | 502 | 503:
        print("Server error")
    case _:
        print("Unknown status")

5.3 Matching with Conditions (Guards)

point = (0, 5)

match point:
    case (0, 0):
        print("Origin")
    case (0, y):
        print(f"On Y-axis at {y}")
    case (x, 0):
        print(f"On X-axis at {x}")
    case (x, y) if x == y:
        print(f"On diagonal at ({x}, {y})")
    case (x, y):
        print(f"Point at ({x}, {y})")

5.4 Matching Data Structures

# Matching lists
command = ["move", 10, 20]

match command:
    case ["quit"]:
        print("Exiting...")
    case ["move", x, y]:
        print(f"Moving to ({x}, {y})")
    case ["draw", color, shape]:
        print(f"Drawing {color} {shape}")
    case _:
        print("Unknown command")

# Matching dictionaries
user = {"name": "Alice", "role": "admin"}

match user:
    case {"role": "admin", "name": name}:
        print(f"Admin user: {name}")
    case {"role": "user", "name": name}:
        print(f"Regular user: {name}")
    case _:
        print("Unknown user type")

When to Use Match-Case

Use match-case when you have multiple specific values to check. It's more readable than long if-elif chains and supports advanced pattern matching.

---

6. Truthy and Falsy Values

6.1 Understanding Truthiness

# Falsy values in Python
bool(False)      # False
bool(None)       # False
bool(0)          # False
bool(0.0)        # False
bool("")         # False (empty string)
bool([])         # False (empty list)
bool({})         # False (empty dict)
bool(())         # False (empty tuple)
bool(set())      # False (empty set)

# Everything else is truthy
bool(True)       # True
bool(1)          # True
bool("hello")    # True
bool([1, 2, 3])  # True

6.2 Using Truthiness in Conditions

# Check if list is empty
numbers = []
if numbers:
    print("List has items")
else:
    print("List is empty")

# Check if string is empty
name = input("Enter name: ")
if name:
    print(f"Hello, {name}")
else:
    print("No name entered")

# Check for None
result = None
if result is not None:
    print(result)
else:
    print("No result")

---

7. Best Practices

7.1 Prefer Positive Conditions

# Less readable
if not is_invalid:
    process_data()

# More readable
if is_valid:
    process_data()

7.2 Avoid Redundant Comparisons

# Bad
if x == True:
    print("x is true")

# Good
if x:
    print("x is truthy")

# Bad
if len(items) > 0:
    process(items)

# Good
if items:
    process(items)

7.3 Use Early Returns

# Less readable
def process_user(user):
    if user is not None:
        if user.is_active:
            if user.has_permission:
                return do_something(user)
            else:
                return "No permission"
        else:
            return "User inactive"
    else:
        return "No user"

# More readable (early returns)
def process_user(user):
    if user is None:
        return "No user"
    
    if not user.is_active:
        return "User inactive"
    
    if not user.has_permission:
        return "No permission"
    
    return do_something(user)

---

8. Summary

Concept	Description	Example
if statement	Execute code if condition is True	if x > 0:
elif	Check additional condition	elif x == 0:
else	Execute if all conditions False	else:
Ternary operator	Inline if-else	result = a if x else b
match-case	Pattern matching (3.10+)	match value:
Comparison	Compare values	==, !=, <, >, <=, >=
Logical	Combine conditions	and, or, not
Membership	Check containment	in, not in
Identity	Check same object	is, is not

Key Takeaways

• Use proper indentation (4 spaces)
• Prefer elif over multiple if statements
• Use match-case for complex pattern matching (Python 3.10+)
• Leverage truthiness for cleaner conditions
• Use early returns to reduce nesting

---

9. What's Next?

In Module 5 - Loops, you'll learn:

• for loops for iteration
• while loops for conditional repetition
• break, continue, and else clauses
• List comprehensions
• Nested loops and loop optimization

---

10. Practice Exercises

Exercise 1: Grade Calculator

Write a program that takes a score (0-100) and prints the grade using match-case.

# Your code here

Exercise 2: Triangle Validator

Write a function that checks if three sides can form a valid triangle and determines its type (equilateral, isosceles, scalene).

def validate_triangle(a, b, c):
    # Your code here
    pass

Exercise 3: Login System

Create a simple login system that checks username and password with appropriate error messages.

def login(username, password):
    # Your code here
    pass

Exercise 4: Discount Calculator

Calculate discount based on purchase amount using ternary operators:

• = $100: 20% discount

• = $50: 10% discount

• < $50: No discount

# Your code here

Exercise 5: Menu System

Create a menu-driven calculator using match-case for operations: add, subtract, multiply, divide.

# Your code here

Solutions

Try solving these exercises on your own first. Solutions will be provided in the practice section.