Week 4 — Conditionals

What you'll learn

• 1Use if / elif / else to branch on conditions
• 2Write nested if statements
• 3Use the ternary (conditional) expression
• 4Combine multiple conditions with and/or
• 5Understand truthiness of non-boolean values

1. if / elif / else

score = int(input("Score: "))
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(f"Grade: {grade}")

2. Ternary Expression

A concise single-line conditional: value_if_true if condition else value_if_false.

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

3. Truthiness

In Python, many non-boolean values evaluate as True or False in a boolean context.

4. Common Mistakes

1. Using = instead of == in a condition — this is a SyntaxError in Python (unlike C).
2. Forgetting the colon after if/elif/else — SyntaxError.
3. Overlapping conditions — put the most specific (highest/lowest) check first.

💻 Examples

Run these examples and check the output yourself.

score = int(input("Score (0-100): "))
if score >= 90:
    print("A — Excellent")
elif score >= 80:
    print("B — Good")
elif score >= 70:
    print("C — Average")
elif score >= 60:
    print("D — Below average")
else:
    print("F — Fail")

month = int(input("Month (1-12): "))
if month in [3, 4, 5]:
    season = "Spring"
elif month in [6, 7, 8]:
    season = "Summer"
elif month in [9, 10, 11]:
    season = "Autumn"
else:
    season = "Winter"
print(f"Month {month} is {season}.")

USER, PASS = "admin", "1234"
username = input("Username: ")
password = input("Password: ")
if username == USER and password == PASS:
    print("Login successful!")
else:
    print("Invalid credentials.")

📝 Exercises

Try them yourself first, then open the solution to compare.

a: 5
b: 5
c: 8
Isosceles triangle

a = float(input("a: "))
b = float(input("b: "))
c = float(input("c: "))
if a+b > c and b+c > a and a+c > b:
    if a == b == c:
        print("Equilateral triangle")
    elif a == b or b == c or a == c:
        print("Isosceles triangle")
    else:
        print("Scalene triangle")
else:
    print("Not a valid triangle")

computer = "rock"
user = input("rock / paper / scissors: ").lower()
if user == computer:
    print("Draw!")
elif (user == "paper" and computer == "rock") or \
     (user == "scissors" and computer == "paper") or \
     (user == "rock" and computer == "scissors"):
    print("You win!")
else:
    print("You lose!")