Lesson 4.3: Logical Operators

Why Logical Operators?

Sometimes a single comparison is not enough to make a decision. For example, to ride a roller coaster you might need to be tall enough AND old enough. Logical operators let you combine multiple conditions into one.

age = 15
height = 140  # in cm

# Must be at least 12 AND at least 120cm tall
if age >= 12 and height >= 120:
    print("You can ride the roller coaster!")
else:
    print("Sorry, you can't ride yet.")

The and Operator

and returns True only when both conditions are True. If either one is False, the whole expression is False.

# Both conditions must be True
username = "admin"
password = "secret123"

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

# Check if a number is between 1 and 100 (inclusive)
number = 42

if number >= 1 and number <= 100:
    print(f"{number} is in the valid range.")
else:
    print(f"{number} is out of range.")

The or Operator

or returns True when at least one condition is True. It is only False when both conditions are False.

# At least one condition must be True
day = "Saturday"

if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")
else:
    print("It's a weekday.")

age = 70
is_student = False
is_veteran = True

# Discount if any of these conditions are met
if age >= 65 or is_student or is_veteran:
    print("You qualify for a discount!")
else:
    print("Regular price applies.")

The not Operator

not reverses a Boolean value. True becomes False, and False becomes True.

is_raining = False

if not is_raining:
    print("It's a nice day for a walk!")
else:
    print("Better bring an umbrella.")

# not is useful for checking "is NOT something"
user_role = "guest"

if not user_role == "admin":
    print("Access denied. Admin only.")

# Equivalent (and often preferred):
if user_role != "admin":
    print("Access denied. Admin only.")

Combining Logical Operators

You can combine and, or, and not in the same expression. Python follows this order of precedence: not first, then and, then or. Use parentheses to make your intent clear.

# Without parentheses - and is evaluated before or
print(True or True and False)    # True or (True and False) = True or False = True

# With parentheses - changes the order
print((True or True) and False)  # (True or True) and False = True and False = False

age = 25
has_license = True
has_insurance = True
has_violations = False

# Can rent a car: must be 21+, have license, have insurance, no violations
if (age >= 21) and has_license and has_insurance and (not has_violations):
    print("You can rent a car!")
else:
    print("Sorry, you cannot rent a car.")

Short-Circuit Evaluation

Python is smart about evaluating logical expressions. It stops as soon as it knows the result:

• and: If the first condition is False, Python does not bother checking the second (the result must be False)
• or: If the first condition is True, Python does not check the second (the result must be True)

# Short-circuit with and
x = 0

# Python checks x != 0 first. Since it's False,
# it skips the division (which would cause an error!)
if x != 0 and 10 / x > 2:
    print("Result is greater than 2")
else:
    print("x is zero or result is not greater than 2")

This is a useful pattern! By checking x != 0 first, you prevent a ZeroDivisionError from the 10 / x part.