Working with Operators: Logical

In the previous section, you learned how to compare values using operators like == (equal to), != (not equal to), < (less than), and > (greater than). These comparisons always result in a boolean value: either True or False.

Often, you need to check more than one condition at a time. For instance, you might want to know if a user is both logged in and has administrator privileges, or if it's either Saturday or Sunday. This is where logical operators come into play. They allow you to combine boolean values (True or False) to create more complex conditions. Python provides three logical operators: and, or, and not.

The and Operator

The and operator evaluates two boolean expressions. It returns True only if both expressions are True. If either expression (or both) is False, the result is False.

Think of it like needing two forms of identification to enter a secure area. You need ID card A and ID card B. Having only one isn't enough.

Let's look at an example:

age = 25
has_ticket = True

# Is the person old enough AND has a ticket?
can_enter = (age >= 18) and (has_ticket == True)
print(f"Age is 25, has ticket: {can_enter}") # Output: True

age = 16
# Now the first condition (age >= 18) is False
can_enter = (age >= 18) and (has_ticket == True)
print(f"Age is 16, has ticket: {can_enter}") # Output: False

age = 30
has_ticket = False
# Now the second condition (has_ticket == True) is False
can_enter = (age >= 18) and (has_ticket == True)
print(f"Age is 30, no ticket: {can_enter}") # Output: False

The parentheses () around the comparisons (age >= 18) are not strictly required here because and has lower precedence than comparison operators, but they often make the code easier to read by clearly grouping the conditions.

Here's a summary of how and works:

Condition 1	Condition 2	Condition 1 and Condition 2
True	True	True
True	False	False
False	True	False
False	False	False

The or Operator

The or operator also evaluates two boolean expressions. It returns True if at least one of the expressions is True. It only returns False if both expressions are False.

Imagine getting a discount if you are either a student or a senior citizen. Meeting just one of the conditions is sufficient.

Consider this example:

is_weekend = False
is_holiday = True

# Is it the weekend OR a holiday?
day_off = is_weekend or is_holiday
print(f"Weekend: False, Holiday: True -> Day off: {day_off}") # Output: True

is_weekend = True
is_holiday = False
# The first condition is True
day_off = is_weekend or is_holiday
print(f"Weekend: True, Holiday: False -> Day off: {day_off}") # Output: True

is_weekend = False
is_holiday = False
# Both conditions are False
day_off = is_weekend or is_holiday
print(f"Weekend: False, Holiday: False -> Day off: {day_off}") # Output: False

Here's the summary for or:

Condition 1	Condition 2	Condition 1 or Condition 2
True	True	True
True	False	True
False	True	True
False	False	False

The not Operator

The not operator is simpler. It works on a single boolean expression and reverses its value. If the expression is True, not makes it False. If the expression is False, not makes it True.

Think of a light switch. not on means off, and not off means on.

is_logged_in = False
print(f"Is logged in: {is_logged_in}") # Output: False

# Check if the user is NOT logged in
needs_login = not is_logged_in
print(f"Needs login: {needs_login}") # Output: True

is_raining = True
print(f"Is raining: {is_raining}") # Output: True

# Check if it is NOT raining
can_go_outside = not is_raining
print(f"Can go outside: {can_go_outside}") # Output: False

Summary for not:

Condition	not Condition
True	False
False	True

Combining Logical Operators

You can combine multiple logical operators in a single expression. Python evaluates not first, then and, then or. However, just like in arithmetic, you can use parentheses () to control the order of evaluation or simply to make your intentions clearer.

age = 22
is_student = True
has_coupon = False

# Eligible if (age < 25 AND is a student) OR has a coupon
is_eligible_discount = (age < 25 and is_student) or has_coupon
# Evaluates as: (True and True) or False
# -> True or False
# -> True
print(f"Eligible for discount: {is_eligible_discount}") # Output: True

# Change coupon status
has_coupon = True
age = 30
is_student = False

is_eligible_discount = (age < 25 and is_student) or has_coupon
# Evaluates as: (False and False) or True
# -> False or True
# -> True
print(f"Eligible for discount (different criteria): {is_eligible_discount}") # Output: True

Logical operators are fundamental tools for building programs that can make decisions based on multiple criteria. You will see them used extensively when we discuss control flow structures like if statements in the next chapter, allowing your programs to react differently based on whether complex conditions evaluate to True or False.