Comparisons and truth values

In the basic ideas you met the bouncer who asks every guest the same question: are you at least 18? Such a condition can always be answered with yes or no, and only once the answer is known can a program take one of two ways. Before the next lesson is about deciding, you learn here how Python asks such questions. You have in fact already seen an answer to one: with lists, in gave you True or False.

True or false: the type bool

Besides numbers and texts, Python knows a third kind of simple value, the truth values. There are exactly two of them, True for true and False for false, and their type is called bool, after the mathematician George Boole:

print(True)
print(type(True))

door_open = False
print(door_open)

True and False are written without quotation marks and start with a capital letter. "True" in quotation marks would be a text, and Python does not know true in lower case. You can keep a truth value in a variable like any other value.

Comparing

Usually you do not write True and False yourself but let a comparison work them out. The bouncer asks about the age:

age = 17

print(age >= 18)
print(age < 18)
print(age == 17)
print(age != 17)

Every comparison is a yes-or-no question, and Python answers it with a truth value. These are the six comparisons:

The result of a comparison is a value like any other, so you can also give it to a variable and use it later:

age = 17
adult = age >= 18
print(adult)

= is not ==

You already know the single equals sign: age = 17 assigns, it gives the name age the value 17. The double equals sign asks whether two values are equal and changes nothing:

age = 17

print(age == 18)
print(age)

The question age == 18 is answered with False, and age is still 17 afterwards. When you read code aloud, it helps to say = as "gets" and == as "is equal to?".

Take care when comparing with input: input() always returns a text, and a text is never equal to a number, even if it holds the same digits.

print("18" == 18)
print(int("18") == 18)

Comparing texts

Two texts are equal if they match character by character, and upper and lower case count too. Type yes here, then Yes and then YES with spaces:

answer = input("Do you have a ticket? ")

print(answer == "yes")
print(answer.strip().lower() == "yes")

For Python, "Yes" and "yes" are different texts. Anyone who wants to check an answer therefore makes it alike first: strip() removes the spaces at the edges and lower() makes all letters small, as you know from working with text. After that a single comparison with "yes" is enough, however someone typed the answer.

and, or and not

A decision often depends on more than one question. The bouncer only lets in those who are at least 18 and on the guest list. Entry to the swimming pool is free for anyone younger than 6 or having a birthday. Python combines such conditions with and and or:

age = 20
on_list = True
print(age >= 18 and on_list)

age = 4
birthday = False
print(age < 6 or birthday)

and only gives True if both sides are true. or gives True as soon as at least one side is true, so also when both are. And not turns a truth value around:

raining = False
print(not raining)
print(not raining and 15 > 10)

When and and or meet in one condition, Python works out the and first, much as multiplication comes before addition. In such cases, put in round brackets, and then it is clear to you and to Python what belongs together.

There is a short way of writing a range. Whether someone is a teenager, that is at least 13 and younger than 20, you can ask with and or by chaining both comparisons, as you know it from mathematics:

age = 15
print(age >= 13 and age < 20)
print(13 <= age < 20)

in: does it occur in it?

You know in from lists: it asks whether a value is in a list, and it also gives a truth value. With not in you ask the opposite, and with texts in checks whether one text occurs in another:

guests = ["Ada", "Alan", "Grace"]

print("Ada" in guests)
print("Linus" in guests)
print("Linus" not in guests)
print("nan" in "banana")

A function that answers yes or no

A function can return a truth value just as it returns a number or a text. Whether a number is even you can find out with % from calculating: then the remainder of dividing by 2 is zero.

def is_even(number):
    return number % 2 == 0


print(is_even(4))
print(is_even(7))

Python first works out number % 2, compares the result with 0 and returns the truth value with return. Such functions are often called is_… or has_…, because their call then reads like a question: is_even(4) means "is 4 even?".

So far the program only prints the answers. How it reacts to an answer and carries out something different depending on the result is what you learn in the next lesson, Decisions with if, elif and else.

Takeaways

• True and False are the two truth values, and their type is called bool.
• The comparisons ==, !=, <, <=, > and >= give a truth value; = assigns, == asks about equality.
• Texts have to match character by character, also in upper and lower case; strip() and lower() make an input alike first.
• and needs two true sides, or at least one, and not turns around; in asks whether something occurs in a list or a text.
• A function such as is_even(number) can return a truth value with return.