Control Flow: if, elif, else

AiTechWorlds

Control flow is the set of statements — if, elif, else, match/case — that let a program choose which code runs based on a condition. Think of it like a fork in a hiking trail: the path (the outcome) depends entirely on which sign (condition) is true when you reach the junction.

Without control flow, code just executes line by line. With it, a program can make decisions, react to different inputs, and branch into completely different paths.

Basic if/elif/else

python

score = 78

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
elif score >= 60:
    print("Grade: D")
else:
    print("Grade: F")

# Output: Grade: C

Python uses indentation (4 spaces) to define blocks — no curly braces. This is not optional formatting; it's part of the syntax.

Truthiness: What Evaluates to False?

In Python, many values besides False are "falsy":

python

# These all evaluate to False:
bool(0)         # False
bool(0.0)       # False
bool("")        # False — empty string
bool([])        # False — empty list
bool({})        # False — empty dict
bool(None)      # False

# These all evaluate to True:
bool(1)         # True
bool(-1)        # True — any non-zero number
bool("hello")   # True
bool([0])       # True — non-empty list (even if it contains 0)

This means you can write very Pythonic conditions:

python

name = input("Enter your name: ")

# Instead of: if name != "":
if name:
    print(f"Hello, {name}!")
else:
    print("No name provided.")

# Instead of: if len(items) > 0:
items = []
if items:
    print(f"Processing {len(items)} items")
else:
    print("Nothing to process")

Comparison and Logical Operators

python

# Comparison
x = 10
print(x > 5)    # True
print(x == 10)  # True
print(x != 5)   # True
print(x >= 10)  # True

# Logical operators
age = 25
has_id = True

if age >= 18 and has_id:
    print("Entry allowed")

if age < 18 or not has_id:
    print("Entry denied")

# Chaining comparisons (Python-specific, very readable)
temperature = 22
if 18 <= temperature <= 26:
    print("Comfortable temperature")

The Ternary Operator

Python's one-line conditional:

python

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

# Another example
score = 75
result = "pass" if score >= 60 else "fail"

Use it sparingly — for simple cases only. If the logic is complex, use a regular if/else block for readability.

Nested if Statements

python

def classify_number(n):
    if n > 0:
        if n % 2 == 0:
            return "positive even"
        else:
            return "positive odd"
    elif n < 0:
        return "negative"
    else:
        return "zero"

print(classify_number(4))   # positive even
print(classify_number(-3))  # negative
print(classify_number(0))   # zero

Avoid deep nesting — more than 2-3 levels makes code hard to read. Use early returns instead:

python

# Deeply nested — hard to read
def process_order(user, order):
    if user is not None:
        if user.is_active:
            if order is not None:
                if order.total > 0:
                    # actual logic...
                    return "processed"

# Better: early returns (guard clauses)
def process_order(user, order):
    if user is None:
        return "error: no user"
    if not user.is_active:
        return "error: inactive user"
    if order is None:
        return "error: no order"
    if order.total <= 0:
        return "error: invalid total"
    # actual logic with no nesting
    return "processed"

Python 3.10+ Match/Case Statement

Python's match/case is similar to switch statements in other languages but more powerful:

python

command = "quit"

match command:
    case "quit":
        print("Exiting...")
    case "help":
        print("Commands: quit, help, start")
    case "start":
        print("Starting application...")
    case _:               # _ is the default/wildcard
        print(f"Unknown command: {command}")

Match with patterns:

python

point = (0, 1)

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

python

def authenticate(username, password, users_db):
    """Authenticate a user and return their role."""
    
    # Guard clauses first
    if not username or not password:
        return None, "Username and password required"
    
    if username not in users_db:
        return None, "Invalid credentials"  # Don't say "user not found" — security
    
    user = users_db[username]
    
    if not user['active']:
        return None, "Account is disabled"
    
    if user['password'] != hash_password(password):
        return None, "Invalid credentials"
    
    return user['role'], "Login successful"


users_db = {
    "alice": {"password": "hashed_pw_1", "role": "admin", "active": True},
    "bob":   {"password": "hashed_pw_2", "role": "user",  "active": True},
    "carol": {"password": "hashed_pw_3", "role": "user",  "active": False},
}

role, message = authenticate("alice", "correctpassword", users_db)
print(f"Role: {role}, Message: {message}")

Key Principles

Use if name: not if name != "" — Pythonic truthiness checks are cleaner
Use early returns to avoid deep nesting
Prefer elif over nested if when conditions are mutually exclusive
Ternary for simple one-liners, regular if/else for anything else

Next lesson: Loops — mastering for, while, and Python's powerful comprehensions.

Control Flow: if, elif, else