Control Flow: if, elif, else

Control flow lets your program make decisions and execute different code depending on conditions. Python uses if, elif, and else for branching logic.

The if Statement

The simplest form checks one condition:

temperature = 35

if temperature > 30:
    print("It's hot outside!")

The code inside the if block runs only when the condition is True. Python uses indentation (four spaces by convention) to define blocks — there are no curly braces.

if / else

Add an else branch for the case when the condition is false:

age = 16

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

if / elif / else

Use elif (short for "else if") to test multiple conditions:

score = 78

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}")

Python checks conditions from top to bottom and executes the first branch whose condition is True. Once a match is found, the rest are skipped.

Truthy and Falsy Values

In Python, values that are considered false in a boolean context (falsy) include:

• False
• 0 and 0.0
• Empty string ""
• Empty collections ([], {}, () )
• None

Everything else is truthy:

name = ""
if name:
    print(f"Hello, {name}")
else:
    print("No name provided")   # this runs

Nested if Statements

You can nest if statements inside other blocks:

is_logged_in = True
is_admin = False

if is_logged_in:
    if is_admin:
        print("Welcome, admin!")
    else:
        print("Welcome, user!")
else:
    print("Please log in.")

Ternary Expression

Python has a one-line conditional expression:

age = 20
status = "adult" if age >= 18 else "minor"
print(status)   # adult

Combining Conditions

username = "alice"
password = "secret123"

if username == "alice" and password == "secret123":
    print("Login successful")
else:
    print("Invalid credentials")

Mastering control flow is essential — virtually every real program makes decisions based on data. Practice writing conditions that are clear and readable.