Lesson 2.2: Comparison and Logical Operators

Learning Objectives

By the end of this lesson, you will be able to:

• Understand and use all Python comparison operators
• Perform comparisons between different data types
• Use logical operators (and, or, not) effectively
• Understand operator precedence for comparison and logical operators
• Combine multiple conditions using logical operators
• Apply comparison and logical operators in conditional statements
• Avoid common pitfalls with comparison operators

Introduction to Comparison Operators

Comparison operators are used to compare two values and return a boolean result (True or False). These operators are fundamental for making decisions in your programs.

Comparison Operators Overview

Important: All comparison operators return boolean values (True or False).

Equal To (==)

The == operator checks if two values are equal.

Basic Equality

# Numeric equality
print(5 == 5)    # Output: True
print(5 == 3)    # Output: False

# String equality
print("hello" == "hello")  # Output: True
print("hello" == "Hello")  # Output: False (case-sensitive)

# Boolean equality
print(True == True)   # Output: True
print(True == False)  # Output: False

Equality with Variables

x = 10
y = 10
z = 5

print(x == y)  # Output: True
print(x == z)  # Output: False

Important: == vs =

Common Mistake: Using = instead of ==

# Wrong - this is assignment, not comparison!
# if x = 5:  # SyntaxError!

# Correct - this is comparison
if x == 5:
    print("x equals 5")

Type Coercion in Comparisons

Python automatically converts compatible types in some comparisons:

# Integer and float comparison
print(5 == 5.0)  # Output: True (5 is converted to 5.0)

# But be careful with strings and numbers
print("5" == 5)   # Output: False (different types)
print("5" == "5") # Output: True

Equality Examples

# Checking user input
user_input = "admin"
if user_input == "admin":
    print("Access granted")

# Comparing calculated values
result1 = 10 + 5
result2 = 3 * 5
if result1 == result2:
    print("Results are equal")

# String comparison (case-sensitive)
password = "Secret123"
if password == "Secret123":
    print("Password correct")
else:
    print("Password incorrect")

Not Equal To (!=)

The != operator checks if two values are NOT equal.

Basic Inequality

# Numeric inequality
print(5 != 3)    # Output: True
print(5 != 5)    # Output: False

# String inequality
print("hello" != "world")  # Output: True
print("hello" != "hello")  # Output: False

Inequality Examples

# Validating input
user_age = 17
if user_age != 18:
    print("You are not 18 years old")

# Checking for invalid values
status = "active"
if status != "inactive":
    print("System is operational")

# Filtering out specific values
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num != 3:
        print(num)  # Prints: 1, 2, 4, 5

Less Than (<)

The < operator checks if the left operand is less than the right operand.

Basic Less Than

# Numeric comparison
print(3 < 5)     # Output: True
print(5 < 3)     # Output: False
print(5 < 5)     # Output: False

# Float comparison
print(3.14 < 3.15)  # Output: True

String Comparison (Lexicographic)

Strings are compared lexicographically (alphabetically):

# String comparison
print("apple" < "banana")  # Output: True
print("zebra" < "apple")   # Output: False

# Case matters (uppercase comes before lowercase in ASCII)
print("Apple" < "apple")   # Output: True
print("Z" < "a")           # Output: True

Less Than Examples

# Age verification
age = 20
if age < 18:
    print("You are a minor")
else:
    print("You are an adult")

# Price comparison
price1 = 19.99
price2 = 24.99
if price1 < price2:
    print(f"Price 1 (${price1}) is cheaper")

# Sorting logic
scores = [85, 92, 78, 96, 88]
for score in scores:
    if score < 80:
        print(f"Score {score} is below 80")

Greater Than (>)

The > operator checks if the left operand is greater than the right operand.

Basic Greater Than

# Numeric comparison
print(5 > 3)     # Output: True
print(3 > 5)     # Output: False
print(5 > 5)     # Output: False

Greater Than Examples

# Score threshold
score = 85
if score > 80:
    print("You passed!")

# Temperature check
temperature = 32
if temperature > 0:
    print("Above freezing")

# Comparing values
a = 10
b = 5
if a > b:
    print(f"{a} is greater than {b}")

Less Than or Equal To (<=)

The <= operator checks if the left operand is less than or equal to the right operand.

Basic Less Than or Equal To

# Numeric comparison
print(5 <= 5)    # Output: True
print(3 <= 5)    # Output: True
print(5 <= 3)    # Output: False

Less Than or Equal To Examples

# Age range check
age = 18
if age <= 18:
    print("You are 18 or younger")

# Budget check
budget = 1000
expense = 1000
if expense <= budget:
    print("Within budget")

# Grade boundaries
score = 90
if score >= 90 and score <= 100:
    print("Grade A")

Greater Than or Equal To (>=)

The >= operator checks if the left operand is greater than or equal to the right operand.

Basic Greater Than or Equal To

# Numeric comparison
print(5 >= 5)    # Output: True
print(5 >= 3)    # Output: True
print(3 >= 5)    # Output: False

Greater Than or Equal To Examples

# Minimum age requirement
age = 21
if age >= 21:
    print("You can enter")

# Passing grade
score = 60
if score >= 60:
    print("You passed!")

# Stock level check
stock = 50
min_stock = 50
if stock >= min_stock:
    print("Stock level is adequate")

Chaining Comparisons

Python allows you to chain multiple comparisons together, which is more readable than using and:

# Chained comparison (more readable)
x = 5
print(3 < x < 10)  # Output: True (equivalent to 3 < x and x < 10)

# Multiple chained comparisons
print(1 < 2 < 3 < 4)  # Output: True
print(1 < 2 < 3 > 4)  # Output: False

# Practical example
age = 25
if 18 <= age <= 65:
    print("Working age")

# Temperature range
temp = 72
if 70 <= temp <= 75:
    print("Comfortable temperature")

Note: Chained comparisons are evaluated left to right, and each comparison must be true for the whole expression to be true.

Comparison with Different Types

Numeric Types

# Integers and floats
print(5 == 5.0)      # True
print(5 < 5.1)       # True
print(5 <= 5.0)      # True

Strings

# String comparison is case-sensitive
print("Hello" == "hello")  # False
print("Hello" < "hello")   # True (uppercase < lowercase in ASCII)

# Comparing strings of different lengths
print("abc" < "abcd")      # True

Mixed Types

# Numbers and strings cannot be directly compared
# print("5" < 4)  # TypeError: '<' not supported between 'str' and 'int'

# Need to convert first
print(int("5") < 4)  # False

None Comparison

# None comparisons
value = None
print(value == None)   # True (but use 'is' instead - covered later)
print(value != None)   # False

Logical Operators

Logical operators are used to combine or modify boolean expressions. Python has three logical operators: and, or, and not.

Logical Operators Overview

Logical AND (and)

The and operator returns True only if both operands are True.

AND Truth Table

Basic AND

# Both True
print(True and True)   # Output: True

# One False
print(True and False)  # Output: False
print(False and True)  # Output: False

# Both False
print(False and False) # Output: False

AND with Comparisons

# Combining conditions
age = 25
has_license = True

if age >= 18 and has_license:
    print("Can drive")

# Multiple conditions
score = 85
attendance = 0.95
if score >= 80 and attendance >= 0.90:
    print("Eligible for scholarship")

Short-Circuit Evaluation

The and operator uses short-circuit evaluation: if the left operand is False, the right operand is not evaluated.

# Short-circuit example
x = 0
# This won't cause a division by zero error
if x != 0 and 10 / x > 1:
    print("Safe division")

# Practical use
user = None
if user is not None and user.is_active():
    print("User is active")

AND Examples

# Password validation
password = "Secret123"
if len(password) >= 8 and password.isalnum():
    print("Valid password")

# Range check
number = 15
if number > 10 and number < 20:
    print("Number is between 10 and 20")

# Multiple AND conditions
username = "admin"
password = "secret"
is_active = True
if username == "admin" and password == "secret" and is_active:
    print("Login successful")

Logical OR (or)

The or operator returns True if at least one operand is True.

OR Truth Table

Basic OR

# At least one True
print(True or False)   # Output: True
print(False or True)   # Output: True
print(True or True)    # Output: True

# Both False
print(False or False)  # Output: False

OR with Comparisons

# Alternative conditions
day = "Saturday"
if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")

# Multiple OR conditions
grade = "A"
if grade == "A" or grade == "B" or grade == "C":
    print("Passing grade")

Short-Circuit Evaluation

The or operator also uses short-circuit evaluation: if the left operand is True, the right operand is not evaluated.

# Short-circuit example
default_value = "guest"
user_input = "admin"
result = user_input or default_value
print(result)  # Output: "admin" (if user_input is truthy, use it)

# Practical use
config_value = None
value = config_value or "default"
print(value)  # Output: "default"

OR Examples

# Discount eligibility
age = 65
is_student = True
if age >= 65 or is_student:
    print("Eligible for discount")

# Error handling
value = None
if value is None or value == "":
    print("Value is missing")

# Multiple conditions
status = "pending"
if status == "active" or status == "pending" or status == "processing":
    print("Status is valid")

Logical NOT (not)

The not operator returns the opposite boolean value.

NOT Truth Table

Basic NOT

# Negation
print(not True)   # Output: False
print(not False)  # Output: True

# With comparisons
print(not (5 > 3))  # Output: False
print(not (5 < 3))  # Output: True

NOT Examples

# Inverting conditions
is_weekend = False
if not is_weekend:
    print("It's a weekday")

# Checking for absence
items = []
if not items:
    print("List is empty")

# Negating complex conditions
age = 20
has_license = False
if not (age >= 18 and has_license):
    print("Cannot drive")

Common NOT Patterns

# Checking if value is not None
value = None
if value is not None:
    print("Value exists")

# Checking if string is not empty
text = ""
if not text:
    print("String is empty")

# Inverting boolean variables
is_active = True
if not is_active:
    print("Account is inactive")

Combining Logical Operators

You can combine multiple logical operators to create complex conditions.

Operator Precedence

When combining operators, Python follows this precedence (highest to lowest):

1. Comparison operators (<, >, <=, >=, ==, !=)
2. not
3. and
4. or

Examples of Combined Operators

# Complex condition
age = 25
has_license = True
has_insurance = False

if age >= 18 and has_license and not has_insurance:
    print("Can drive but needs insurance")

# Using parentheses for clarity
if (age >= 18 and has_license) or (age >= 16 and has_permit):
    print("Can drive")

# Multiple conditions
score = 85
attendance = 0.95
is_enrolled = True

if score >= 80 and attendance >= 0.90 and is_enrolled:
    print("Eligible for honors")

Precedence Examples

# Without parentheses (uses precedence)
result = 5 > 3 and 2 < 4 or 1 > 2
# Evaluates as: (5 > 3 and 2 < 4) or (1 > 2)
# = (True and True) or False
# = True or False
# = True

# With parentheses (explicit grouping)
result = (5 > 3 and 2 < 4) or (1 > 2)
# Same result, but clearer

# Different grouping
result = 5 > 3 and (2 < 4 or 1 > 2)
# = True and (True or False)
# = True and True
# = True

Best Practices

# Use parentheses for clarity, even when not strictly necessary
# Good
if (age >= 18 and has_license) or (age >= 16 and has_permit):
    pass

# Also good (precedence makes it work, but less clear)
if age >= 18 and has_license or age >= 16 and has_permit:
    pass

# Better: break into variables
can_drive_with_license = age >= 18 and has_license
can_drive_with_permit = age >= 16 and has_permit
if can_drive_with_license or can_drive_with_permit:
    pass

Common Patterns and Use Cases

1. Range Checking

# Check if value is in range
value = 15
if 10 <= value <= 20:
    print("Value is in range")

# Using and
if value >= 10 and value <= 20:
    print("Value is in range")

2. Multiple Conditions

# All conditions must be true
username = "admin"
password = "secret"
is_active = True

if username == "admin" and password == "secret" and is_active:
    print("Access granted")

# At least one condition must be true
status = "pending"
if status == "active" or status == "pending" or status == "processing":
    print("Valid status")

3. Negation Patterns

# Check if not in list
item = "apple"
if item not in ["banana", "orange", "grape"]:
    print("Item not found")

# Check if not equal
value = 5
if value != 0:
    print("Value is not zero")

4. Input Validation

# Validate user input
user_input = "25"
if user_input.isdigit() and int(user_input) > 0:
    age = int(user_input)
    print(f"Valid age: {age}")
else:
    print("Invalid input")

5. Conditional Assignment

# Set default value
value = None
result = value or "default"
print(result)  # Output: "default"

# Conditional value
age = 25
status = "adult" if age >= 18 else "minor"
print(status)  # Output: "adult"

Common Pitfalls and Mistakes

1. Using = Instead of ==

# Wrong
# if x = 5:  # SyntaxError!

# Correct
if x == 5:
    pass

2. Chaining == Incorrectly

# This doesn't work as you might expect
x = 5
y = 5
z = 5
# print(x == y == z)  # This works! Returns True

# But this is different
# print(x == y == 5)  # This also works! Returns True

3. Comparing Floats

# Floating-point precision issues
result = 0.1 + 0.2
print(result == 0.3)  # False! (due to floating-point precision)

# Better approach
print(abs(result - 0.3) < 0.0001)  # True

4. String Comparison Case Sensitivity

# Case-sensitive comparison
print("Hello" == "hello")  # False

# Case-insensitive comparison
print("Hello".lower() == "hello".lower())  # True

5. Confusing and/or Precedence

# This might not work as expected
age = 25
has_license = True
has_insurance = False

# Wrong interpretation
# if age >= 18 and has_license or has_insurance:  # This is: (age >= 18 and has_license) or has_insurance

# Clear with parentheses
if (age >= 18 and has_license) or has_insurance:
    pass

Practice Exercise

Exercise: Comparison and Logical Operators Practice

Objective: Create a Python program that demonstrates the use of comparison and logical operators in practical scenarios.

Instructions:

1. Create a file called comparison_practice.py

2. Write a program that:

   • Performs various comparisons
   • Uses logical operators to combine conditions
   • Validates user input
   • Makes decisions based on multiple conditions
   • Demonstrates operator precedence

3. Your program should include:

   • Age verification system
   • Grade calculation with multiple conditions
   • Password validation
   • Temperature range checking
   • Discount eligibility

Example Solution:

"""
Comparison and Logical Operators Practice
This program demonstrates the use of comparison and logical operators
in various practical scenarios.
"""

print("=" * 60)
print("COMPARISON AND LOGICAL OPERATORS PRACTICE")
print("=" * 60)
print()

# 1. Basic Comparisons
print("1. BASIC COMPARISONS")
print("-" * 60)
a = 10
b = 5
c = 10

print(f"a = {a}, b = {b}, c = {c}")
print(f"a == b: {a == b}")      # False
print(f"a == c: {a == c}")      # True
print(f"a != b: {a != b}")      # True
print(f"a < b: {a < b}")        # False
print(f"a > b: {a > b}")        # True
print(f"a <= c: {a <= c}")      # True
print(f"a >= b: {a >= b}")      # True
print()

# 2. Chained Comparisons
print("2. CHAINED COMPARISONS")
print("-" * 60)
value = 15
print(f"value = {value}")
print(f"10 <= value <= 20: {10 <= value <= 20}")  # True
print(f"5 < value < 10: {5 < value < 10}")        # False
print()

# 3. Age Verification System
print("3. AGE VERIFICATION SYSTEM")
print("-" * 60)
ages = [16, 18, 21, 25, 65]

for age in ages:
    can_vote = age >= 18
    can_drink = age >= 21
    senior_discount = age >= 65

    print(f"Age {age}:")
    print(f"  Can vote: {can_vote}")
    print(f"  Can drink: {can_drink}")
    print(f"  Senior discount: {senior_discount}")
    print()

# 4. Grade Calculation with Multiple Conditions
print("4. GRADE CALCULATION")
print("-" * 60)
scores = [
    {"name": "Alice", "score": 95, "attendance": 0.98},
    {"name": "Bob", "score": 75, "attendance": 0.85},
    {"name": "Charlie", "score": 88, "attendance": 0.92},
    {"name": "Diana", "score": 92, "attendance": 0.75},
]

for student in scores:
    name = student["name"]
    score = student["score"]
    attendance = student["attendance"]

    # Determine grade
    if score >= 90:
        grade = "A"
    elif score >= 80:
        grade = "B"
    elif score >= 70:
        grade = "C"
    else:
        grade = "F"

    # Check eligibility for honors
    eligible_for_honors = score >= 85 and attendance >= 0.90
    passed = score >= 60 and attendance >= 0.75

    print(f"{name}:")
    print(f"  Score: {score}, Attendance: {attendance:.0%}")
    print(f"  Grade: {grade}")
    print(f"  Passed: {passed}")
    print(f"  Eligible for honors: {eligible_for_honors}")
    print()

# 5. Password Validation
print("5. PASSWORD VALIDATION")
print("-" * 60)
passwords = ["short", "NoNumbers", "nouppercase123", "ValidPass123", "Another123!"]

for password in passwords:
    length_ok = len(password) >= 8
    has_upper = any(c.isupper() for c in password)
    has_lower = any(c.islower() for c in password)
    has_digit = any(c.isdigit() for c in password)

    is_valid = length_ok and has_upper and has_lower and has_digit

    print(f"Password: '{password}'")
    print(f"  Length >= 8: {length_ok}")
    print(f"  Has uppercase: {has_upper}")
    print(f"  Has lowercase: {has_lower}")
    print(f"  Has digit: {has_digit}")
    print(f"  Valid: {is_valid}")
    print()

# 6. Temperature Range Checking
print("6. TEMPERATURE RANGE CHECKING")
print("-" * 60)
temperatures = [32, 50, 68, 75, 85, 95, 100]

for temp in temperatures:
    freezing = temp <= 32
    cold = 32 < temp < 50
    moderate = 50 <= temp < 70
    warm = 70 <= temp < 85
    hot = temp >= 85

    print(f"Temperature: {temp}°F")
    if freezing:
        print("  Status: Freezing")
    elif cold:
        print("  Status: Cold")
    elif moderate:
        print("  Status: Moderate")
    elif warm:
        print("  Status: Warm")
    elif hot:
        print("  Status: Hot")
    print()

# 7. Discount Eligibility
print("7. DISCOUNT ELIGIBILITY")
print("-" * 60)
customers = [
    {"age": 25, "is_student": True, "is_member": False},
    {"age": 65, "is_student": False, "is_member": True},
    {"age": 30, "is_student": False, "is_member": False},
    {"age": 20, "is_student": True, "is_member": True},
]

for customer in customers:
    age = customer["age"]
    is_student = customer["is_student"]
    is_member = customer["is_member"]

    student_discount = is_student
    senior_discount = age >= 65
    member_discount = is_member

    eligible_for_discount = student_discount or senior_discount or member_discount

    print(f"Customer: Age {age}, Student: {is_student}, Member: {is_member}")
    print(f"  Student discount: {student_discount}")
    print(f"  Senior discount: {senior_discount}")
    print(f"  Member discount: {member_discount}")
    print(f"  Eligible for any discount: {eligible_for_discount}")
    print()

# 8. Logical Operator Combinations
print("8. LOGICAL OPERATOR COMBINATIONS")
print("-" * 60)
x = 10
y = 5
z = 15

print(f"x = {x}, y = {y}, z = {z}")
print(f"x > y and x < z: {x > y and x < z}")           # True
print(f"x > y or x > z: {x > y or x > z}")             # True
print(f"not (x > y): {not (x > y)}")                   # False
print(f"x > y and not (x > z): {x > y and not (x > z)}")  # True
print()

# 9. String Comparisons
print("9. STRING COMPARISONS")
print("-" * 60)
str1 = "apple"
str2 = "banana"
str3 = "Apple"

print(f"'{str1}' < '{str2}': {str1 < str2}")           # True
print(f"'{str1}' == '{str3}': {str1 == str3}")         # False
print(f"'{str1}.lower()' == '{str3}.lower()': {str1.lower() == str3.lower()}")  # True
print()

print("=" * 60)
print("PRACTICE COMPLETE!")
print("=" * 60)

Expected Output (truncated for brevity):

============================================================
COMPARISON AND LOGICAL OPERATORS PRACTICE
============================================================

1. BASIC COMPARISONS
------------------------------------------------------------
a = 10, b = 5, c = 10
a == b: False
a == c: True
a != b: True
a < b: False
a > b: True
a <= c: True
a >= b: True

[... rest of output ...]

Challenge (Optional):

• Add user input validation
• Create a more complex eligibility system
• Implement a voting eligibility checker
• Build a loan approval system based on multiple criteria
• Create a weather advisory system

Key Takeaways

1. Comparison operators (==, !=, <, >, <=, >=) return boolean values
2. == vs =: Use == for comparison, = for assignment
3. Chained comparisons (3 < x < 10) are more readable than using and
4. Logical AND (and) returns True only if both operands are True
5. Logical OR (or) returns True if at least one operand is True
6. Logical NOT (not) returns the opposite boolean value
7. Short-circuit evaluation: and and or don't evaluate the right operand if not needed
8. Operator precedence: Comparisons > not > and > or
9. Use parentheses for clarity when combining multiple logical operators
10. Be careful with float comparisons due to precision issues

Quiz: Comparison and Logical Operators

Test your understanding with these questions:

1. What is the result of 5 == 5.0?

   • A) True
   • B) False
   • C) Error
   • D) None

2. What is the result of True and False?

   • A) True
   • B) False
   • C) Error
   • D) None

3. What is the result of True or False?

   • A) True
   • B) False
   • C) Error
   • D) None

4. What is the result of not True?

   • A) True
   • B) False
   • C) Error
   • D) None

5. What is the result of 5 < 3 or 2 > 1?

   • A) True
   • B) False
   • C) Error
   • D) None

6. What is the result of 3 < x < 10 when x = 5?

   • A) True
   • B) False
   • C) Error
   • D) None

7. What is the result of "apple" < "banana"?

   • A) True
   • B) False
   • C) Error
   • D) None

8. What is the result of 5 != 5?

   • A) True
   • B) False
   • C) Error
   • D) None

9. What is the result of not (5 > 3 and 2 < 4)?

   • A) True
   • B) False
   • C) Error
   • D) None

10. Which operator has the highest precedence?

    • A) and
    • B) or
    • C) not
    • D) ==

Answers:

1. A) True (Python converts int to float for comparison)
2. B) False (AND requires both to be True)
3. A) True (OR requires at least one to be True)
4. B) False (NOT inverts the value)
5. A) True (5 < 3 is False, but 2 > 1 is True, so OR returns True)
6. A) True (5 is between 3 and 10)
7. A) True (lexicographic comparison)
8. B) False (5 equals 5, so not equal is False)
9. B) False (5 > 3 and 2 < 4 is True, so not True is False)
10. D) == (comparison operators have higher precedence than logical operators)

Next Steps

Excellent work! You've mastered comparison and logical operators. You now understand:

• All comparison operators and their usage
• How to combine conditions with logical operators
• Operator precedence and how to use parentheses
• Common patterns and best practices
• How to avoid common pitfalls

What's Next?

• Lesson 2.3: Assignment Operators
• Lesson 5.1: Conditional Statements (if, elif, else)
• Practice building programs that make decisions
• Experiment with complex logical expressions

Additional Resources

• Python Comparisons: docs.python.org/3/reference/expressions.html#comparisons
• Boolean Operations: docs.python.org/3/reference/expressions.html#boolean-operations
• Operator Precedence: docs.python.org/3/reference/expressions.html#operator-precedence
• Truth Value Testing: docs.python.org/3/library/stdtypes.html#truth-value-testing

Lesson completed! You're ready to move on to the next lesson.