Python Essentials - Part 42: Logical Operators

Logical Operators

We have already used some logical operators throughout the previous lessons without formally introducing them. In this part we are going to look at them properly and understand what each one does.

Logical operators are symbols or keywords that Python uses to compare values or combine conditions. The result of any logical operation is always a Boolean, True or False. Here are the operators we will cover:

and both conditions must be True
or at least one condition must be True
< less than
> greater than
= assignment
== equal to
=== does not exist in Python, but we will explain why

= vs ==

This is one of the most common sources of confusion for beginners and it is worth taking the time to understand the difference clearly.

A single = is the assignment operator. It is used to store a value in a variable. You are not asking a question, you are giving Python an instruction.

A double == is the equality operator. It is used to compare two values and ask the question "are these the same?" Python will evaluate the expression and return either True or False.

As for ===, this operator exists in languages like JavaScript where it is used for strict equality checking. It does not exist in Python. If you try to use it Python will throw a SyntaxError. In Python == already handles strict comparison, so a third operator is not needed.

Try each of the following in the box below and run it each time to see what Python returns:

name = 'Eric' then print(name)
print(name == 'Eric')
print(name == 'John')
print(name === 'Eric') and note the error

Less Than and Greater Than

The < and > operators compare two numerical values. Just like ==, they return either True or False. You can use them directly inside a print() statement and Python will evaluate the expression and print the result.

Try each of the following expressions in the box below and run it each time to see what Python returns:

print(3 > 10)
print(5 < 9)
print(10 > 10)
print(10 >= 10)
print(7 < 3)

The not Operator

The not operator is the simplest of all the logical operators. It does exactly what it sounds like. It reverses the Boolean value of a condition. If something is True, not makes it False. If something is False, not makes it True.

You have already seen conditions like this:

if is_logged_in:

With not you can flip that condition without changing the variable itself:

if not is_logged_in:

This reads very naturally in plain English. "If the user is not logged in, do something." This makes not a very readable and practical tool for writing conditions that check for the absence of something rather than the presence of it.

Try each of the following in the box below and run it each time to see what Python returns:

print(not True)
print(not False)
print(not 0)
print(not 1)
print(not '')
print(not 'hello')

Time to experiment!

Coding Exercises (VS Code) Instructions:

Create a file named logical_operators.py in your part42 folder.
Complete the tasks and use # comments to explain your answers.

Exercise 1: Comparing Strings

Task: Run the following expressions and observe the results:

print('a' > 'b')
print('a' > 'A')

Goal: Write a comment explaining why 'a' is not greater than 'b' and why 'a' is greater than 'A'. Research the term ASCII value to help you understand why.

Exercise 2: Chained Comparisons

Task: Run the following expressions and observe the results:

print(1 < 2 < 3 < 4)
print(1 < 2 > 3 < 4)
print(1 < 2 > 1 < 4)

Goal: Write a comment explaining what happens at the value 2 in the second expression and why the result changes. Explain how Python evaluates chained comparisons from left to right.

Exercise 3: <=, >=, !=

Task: Run the following expressions and observe the results:

print(10 >= 10)
print(9 <= 10)
print(5 != 5)
print(5 != 6)

Goal: Write a comment next to each expression explaining what the operator does and why Python returned that result. Pay close attention to != and explain in your own words what "not equal to" means in a condition.

Exercise 4: Combining Operators

Task: Create three variables age = 17, has_id = True, is_member = False.
Goal: Write a conditional block that prints "Full access granted." if the age is 18 or above and the user has an id. Print "Members only area." if the user is a member but under 18. Print "Access denied." for everything else. Test by changing each variable and write a comment explaining which operators you used and why.

Exercise 5: Student Check

Task: Create two variables is_student = False and is_enrolled = False.
Goal: Write a conditional block that covers the following three outcomes:

If the user is a student and enrolled: print "You are a student."
If the user is a student but not enrolled: print "Pick a course to begin."
If the user is not a student: print "You cannot learn if you are not a student."

Goal: Test all combinations and write a comment explaining how the not operator is used to check for the absence of enrollment.

Don't Forget to commit and Push!