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3.3. Logical Operations

In section 3.1. Basic Conditionals, we learned about if and else constructs that are used to perform operation on some condition.

In this section, we'll learn about logical operations that are used to manipulate the conditions in if statements.

The boolean type

In the section 2.1. Variables and Data Types, we learned about the "boolean data type" which is referred to as bool in Python.

For a recap, bool data type is used to represent a boolean value which in turn, means either True or False. Result of logical operations is a boolean value.

Basic logical operations

Let us understand this with an example, if we have a variable age representing the age of a user, we can check whether the user can drive or not by checking whether the age is above 18 or not.

age = 12
print('Can user drive?', age > 18)

Can user drive? False

If we change the value of age to say, 23, we get:

age = 23
print('Can user drive?', age > 18)

Can user drive? True

Here, age > 18 is a logical expression. This expression uses the > logical operator to check whether the age is greater than 18.

A logical expression is formed using logical operators and can either evaluate to True or False. The basic logical operators that can be used are:

  • == (Equals to)
  • != (Not Equals to)
  • > (Greater than)
  • < (Less than)
  • >= (Greater than or equal to)
  • <= (Less than or equal to)
age = int(input('Enter an age:'))

print('Is 18?', age == 18)
print('Not 18?', age != 18)
print('Above 18?', age > 18)
print('Above 18 or Is 18?', age >= 18)
print('Below 18?', age < 18)
print('Below 18 or Is 18?', age <= 18)

Enter an age: 18
Is 18? True
Not 18? False
Above 18? False
Above 18 or Is 18? True
Below 18? False
Below 18 or Is 18? True

Enter an age: 12
Is 18? False
Not 18? True
Above 18? False
Above 18 or Is 18? False
Below 18? True
Below 18 or Is 18? True

Enter an age: 23
Is 18? False
Not 18? True
Above 18? True
Above 18 or Is 18? True
Below 18? False
Below 18 or Is 18? True

We can use logical expressions in an if statement (as seen in section 3.1) to perform an action based on whether expression evaluates to True or False.

age = int(input('Enter an age:'))

if age >= 18:
    print('You can drive.')
else:
    print('You cannot drive.')

Enter an age: 23
You can drive.

Enter an age: 18
You can drive.

Enter an age: 12
You cannot drive.

Here, age >= 18 is the logical expression and the print statement is only executed when this expression evaluates to True.

AND, OR and NOT operators

In some cases, you have multiple expressions that you have to check for.

AND operator

and is a logical operator that checks whether all conditions are True. If any one or more of the conditions are not satisfied, the result is False.

As an example, in order for user to be able to drive, following conditions must be satisfied:

  • the user must be 18 years or above
  • the user must have above 6.5 on driving test

We can represent this like so:

age = int(input('Enter your age: '))
driving_test_score = float(input('Enter your driving test score: '))

if age >= 18 and driving_test_score > 6.5:
    print('You can drive.')
else:
    print('You cannot drive.')

Enter your age: 18
Driving test score: 10
You can drive.

Here, driving_test_score >= 6.5 expression results in True and age >= 18 results in True. The overall expression results in True because both expressions are True.

Enter your age: 12
Driving test score: 10
You cannot drive.

Here, driving_test_score >= 6.5 expression results in True but age >= 18 results in False. The overall expression results in False.

Enter your age: 23
Driving test score: 6
You cannot drive.

Here, driving_test_score >= 6.5 expression results in False but age >= 18 results in True. The overall expression results in False.

Tip

A logical expression produces a boolean value which can also be assigned to a variable. This is particularly useful when we have large expressions which could become confusing.

age = 23
driving_test_score = 6.5

over_18 = age >= 18
passed_test = driving_test_score > 7

print('Over 18?', over_18)
print('Passed test?', passed_test)
print('Can drive?', over_18 and passed_test)

Over 18? True
Passed test? False
Can drive? False

OR operator

The or operator evaluates to True if any one or more of the given expressions are True.

For example, lets say a customer is only eligible for discount if he or she spends at least 100$ or if the customer has shopped at store more than 10 times.

amount_spent = int(input('Enter amount spent shopping: '))
old_customer = input('Are you an old customer? ')

if amount_spent > 150 or old_customer == 'yes':
    print('Customer gets a discount.')
else:
    print('Discount not applied.')

Enter amount spent shopping: 250
Are you an old customer? yes
Customer gets a discount.

Here, amount_spent > 150 is True and old_customer == 'yes' is also True so overall expression is True.

Enter amount spent shopping: 50
Are you an old customer? yes
Customer gets a discount.

Here, amount_spent > 150 is False but old_customer == 'yes' is True so overall expression is True.

Enter amount spent shopping: 40
Are you an old customer? no
Discount not applied.

Here, amount_spent > 150 is False and old_customer == 'yes' is also False so overall expression is False.

Simply put, if you test the following logical expressions, you'd get the mentioned result:

  • True and True evaluates to True
  • True and False evaluates to False
  • False and False evaluates to False
  • True or True evaluates to True
  • True or False evaluates to True
  • False or False evaluates to False

Info

  • For and, if all expressions are True, result is True. If any expression is False, result is False.

  • For or, if any expression is True, result is True. If all expressions are False, result is also False.

NOT operator

We also have a not operator which simply inverts the given boolean value. That is, not True evaluates to False. not False evaluates to True.

For example, the expression in code below is True only when X is not greater than Y (either X < Y or X = Y):

X = int(input('X: '))
Y = int(input('Y: '))
print('X is not greater than Y:', not (X > Y))

X: 20
Y: 30
X is not greater than Y: True

X: 30
Y: 20
X is not greater than Y: False

This expression is just an equivalent of X <= Y.