Logical Operators

Introduction

Logical operators let us combine or invert conditions. They are widely used in decision-making (e.g., if statements, loops). Logical expressions evaluate to True or False but can also take advantage of Python's truthy and falsy values.

Python treats values like (0, "", [], {}, None, False) as falsy and everything else is truthy. This makes logical operators powerful and intuitive. Python's logical operators use English words (and, or, not) making code more readable than symbols.

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Concepts

• AND (and)
  • True if both conditions are True.
  • Example: True and False → False.
• OR (or)
  • True if at least one condition is True.
  • Example: True or False → True.
• NOT (not)
  • Inverts the condition.
  • Example: not True → False.
• Short-circuiting:
  • Stops evaluating when result is already known
    • a and b → if a is False, b is never checked.
    • a or b → if a is True, b is never checked.

This is often used for default values and safe operations in Python.

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Examples

age = 20
has_id = True
print(age >= 18 and has_id)  # True
print(age < 18 or has_id)    # True
print(not has_id)            # False

# Using or for default values
username = ""
display_name = username or "Guest"
print(display_name)  # Guest

# Short-circuiting with and
data = None
data and print("Processing data...")  # Nothing happens

# Chaining comparisons (Python-specific feature)
score = 85
print(0 <= score <= 100)  # True (elegant range check)

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Challenges

1. Check if number between 10 and 20

• Write a condition that checks if a number is between 10 and 20 (inclusive).

💡 Show Answer

num = 15
print(10 <= num <= 20)  # True (Python's chained comparison)

# Alternative with and
print(num >= 10 and num <= 20)  # True

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2. Driving check

• Check if a person can drive: must be at least 18 and have a driving license.

💡 Show Answer

age = 18
has_license = True
can_drive = age >= 18 and has_license
print(f"Can drive: {can_drive}")  # True

# As a function
def can_drive(age, has_license):
    return age >= 18 and has_license

print(can_drive(17, True))   # False
print(can_drive(20, False))  # False
print(can_drive(18, True))   # True

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3. Welcome if logged in or guest

• Write a condition that prints "Welcome" if a user has logged in or is a guest.

💡 Show Answer

is_logged_in = False
is_guest = True

if is_logged_in or is_guest:
    print("Welcome")

# One-liner using conditional expression
message = "Welcome" if is_logged_in or is_guest else "Access denied"
print(message)

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4. Flip boolean

• Use not to flip the result of a boolean variable is_active.

💡 Show Answer

is_active = True
print(not is_active)  # False

# Toggle a boolean
is_active = not is_active
print(f"is_active is now: {is_active}")  # False

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5. Secure data access

• Demonstrate short-circuiting with and by writing a condition that only prints "Accessing secure data..." if is_logged_in is True.

💡 Show Answer

is_logged_in = True
is_logged_in and print("Accessing secure data...")

# More Pythonic approach
if is_logged_in:
    print("Accessing secure data...")

# With function call
def access_secure_data():
    print("Accessing secure data...")

is_logged_in and access_secure_data()

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6. Default with OR

• Use or to assign a default value to a variable if it is None or empty.

💡 Show Answer

username = ""
display_name = username or "Guest"
print(display_name)  # Guest

# More examples
config_file = None
config = config_file or "default_config.txt"
print(config)

# With various falsy values
values = ["", 0, [], {}, None, False]
for val in values:
    result = val or "default"
    print(f"{val} or 'default' = {result}")

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7. Predict outputs

• Predict the output of these expressions:

💡 Show Answer

print(True or False and False)  # True (and has higher precedence)
print(not (False or True))      # False
print("" or "Hello")            # Hello
print("Hi" and "Bye")           # Bye (and returns the last truthy value)

# More complex examples
print(0 or 5 or 10)            # 5 (first truthy value)
print(1 and 2 and 3)           # 3 (last value if all truthy)
print(1 and 0 and 3)           # 0 (first falsy value)
print(not not "hello")         # True (double negation)

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8. Valid grade

• Write a function is_valid_grade(grade) that returns True if grade is between 0 and 100.

💡 Show Answer

def is_valid_grade(grade):
    return 0 <= grade <= 100

print(is_valid_grade(85))   # True
print(is_valid_grade(-5))   # False
print(is_valid_grade(105))  # False

# Test with various inputs
test_grades = [0, 50, 100, -10, 150, 99.5]
for grade in test_grades:
    valid = is_valid_grade(grade)
    print(f"Grade {grade}: {'Valid' if valid else 'Invalid'}")

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9. Simple login

• Build a simple login system where the user must have the right username and password.

💡 Show Answer

def login(username, password):
    return username == "admin" and password == "1234"

# Test the login
print(login("admin", "1234"))     # True
print(login("user", "1234"))      # False
print(login("admin", "wrong"))    # False

# More complete login system
def secure_login(username, password):
    valid_users = {
        "admin": "1234",
        "user": "password",
        "guest": "guest123"
    }
    return username in valid_users and valid_users[username] == password

print(secure_login("admin", "1234"))      # True
print(secure_login("user", "password"))   # True
print(secure_login("hacker", "1234"))     # False

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10. Majority vote

• Write a function majority_vote(a, b, c) that returns True if at least two of the three boolean inputs are True.

💡 Show Answer

def majority_vote(a, b, c):
    return (a + b + c) >= 2  # Works because True = 1, False = 0

print(majority_vote(True, True, False))   # True
print(majority_vote(True, False, False))  # False
print(majority_vote(False, False, False)) # False

# Alternative implementation
def majority_vote_alt(a, b, c):
    return (a and b) or (a and c) or (b and c)

# More explicit implementation
def majority_vote_explicit(a, b, c):
    count = sum([a, b, c])  # Count True values
    return count >= 2

# Test all implementations
test_cases = [
    (True, True, True),
    (True, True, False),
    (True, False, False),
    (False, False, False)
]

for a, b, c in test_cases:
    result1 = majority_vote(a, b, c)
    result2 = majority_vote_alt(a, b, c)
    result3 = majority_vote_explicit(a, b, c)
    print(f"({a}, {b}, {c}): {result1} {result2} {result3}")