What you’ll learn in this lesson

• What a boolean is: True and False
• How comparisons ask yes-or-no questions
• All six comparison operators
• How to store a comparison’s answer in a variable
• Why = and == are different, and why mixing them up is the classic beginner bug

True or False: the boolean

A boolean (named after the mathematician George Boole) is a value
that can only be one of two things: True or False. That is all.
No maybe, no in-between. Computers make millions of yes-or-no
decisions, and booleans are how those decisions are stored.

Note the capital letters: Python writes True and False with a
capital first letter. Lowercase true gives a NameError, because
Python only recognizes the capitalized forms.

print(True)
print(False)

Output:

True
False

On their own, True and False are not very interesting. They become
powerful when a comparison produces one of them — which is next.

Comparisons ask yes-or-no questions

A comparison compares two values and answers True or False.
Here are all six comparison operators:

Try them:

print(10 > 9)
print(10 < 9)
print(10 == 10)
print(10 != 9)
print(7 >= 7)
print(7 <= 6)

Output:

True
False
True
True
True
False

Comparisons work on strings too, in dictionary (alphabetical) order:

print("apple" < "banana")
print("Sam" == "Sam")

Output:

True
True

"apple" < "banana" is True because “apple” comes before “banana”
alphabetically. You will rarely compare strings this way, but it is
good to know the same operators work on both numbers and text.

Storing the answer

A comparison’s answer is just a value, so it can live in a variable
like anything else:

age = 12
old_enough = age >= 10
print(old_enough)

Output:

True

old_enough now holds True. Later, in Lesson 8, you will use
variables like this to make decisions: “if old_enough, do this.”
Storing the answer separately keeps the decision readable — the
variable’s name says what the question meant.

The classic bug: = versus ==

This is the single most common beginner mistake in every programming
language, so learn it now:

• = stores a value: age = 12 puts 12 into age.
• == asks whether two values are equal: age == 12 answers
  True or False.

Using one = where you meant two does not ask a question — it stores
a value, usually by accident. If a comparison ever seems to “always be
true” or your program behaves oddly around a decision, check whether
you wrote = instead of == first.

print(True)
print(False)

Output:
True
False

print(10 > 9)
print(10 < 9)
print(10 == 10)
print(10 != 9)
print(7 >= 7)
print(7 <= 6)

Output:
True
False
True
True
True
False

print("apple" < "banana")
print("Sam" == "Sam")

Output:
True
True

age = 12
old_enough = age >= 10
print(old_enough)

Output:
True

Download the Lesson 8 slides (PowerPoint, with narration)

Download the Lesson 8 slides (PDF)

Hands-on exercise

1. In the shell, work out each of these before running them, then
   check: 5 > 3, 5 < 3, 5 == 5, 5 != 5, 4 >= 4, 4 <= 3.
2. Store your age in a variable. Then store the answer to "am I 10 or
   older?" in a second variable and print it.
3. Compare two names with == — once with the same name twice,
   once with different names — and explain both results.

Press Run to see the output here.

Practice file

Save this as lesson08-practice.py and run it with F5 in IDLE.

# Lesson 8: Booleans and Comparisons
# WHAT TO DO: A boolean is an answer that is either True or False.
# This program stores an age of 12 and should answer "am I 10 or older?"
# But one line is wrong. Fix line 10: change < to >= in the comparison.
# HINT: 12 is 10 or older, so the answer should be True. Only change
# the comparison sign; do not change the age or the number 10.
# Run with F5 in IDLE. You are done when the last line prints True.

age = 12
old_enough = age < 10  # <-- CHANGE THIS LINE: should ask "10 or older"
print(age)
print(old_enough)

Key takeaways

• A boolean is True or False, always capitalized.
• Comparisons (==, !=, >, <, >=, <=) answer yes-or-no questions.
• A comparison's answer can be stored in a variable for later use.
• = stores; == asks. Mixing them up is the classic
  beginner bug.

Next lesson: If Statements.

Quick quiz

1. What values can a boolean hold, and how are they spelled in
   Python?
2. What does 10 != 9 ask, and what is the answer?
3. What is the difference between = and ==?
4. In old_enough = age >= 10, what is stored in
   old_enough, and why?
5. Why does lowercase true give an error?

Answer key: 1. Only True or False, always capitalized.
2. "Is 10 not equal to 9?" — True. 3. = stores a value; == asks
whether two values are equal. 4. A boolean — the answer to the comparison
age >= 10. 5. Python only recognizes the capitalized forms True
and False.