Booleans

4. Booleans

Summary

Booleans are yet another datatype. They are either True or False and never anything else. That’s it! ## Simple, yes, but used ALL the time In computer science, booleans are used a lot. This has to do with how computers work internally. Many operations inside a computer come down to a simple “true or false.”

In Python, we use booleans in combination with conditional statements to control the flow of a program:

<syntaxhighlight lang="python">>>> door_is_locked = True >>> if door_is_locked: ... print("Mum, open the door!") ... Mum, open the door!</syntaxhighlight> First, we define a variable called door_is_locked and set it to True. Next, you’ll find an if-statement. This is a so-called conditional statement. It is followed by an expression that can evaluate to either True or False. If the expression evaluates to True, the block of code that follows is executed. If it evaluates to False, it is skipped. Go ahead and change door_is_locked to False to see what happens.

An if can be followed by an optional else:

<syntaxhighlight lang="python">>>> door_is_locked = False >>> if door_is_locked: ... print("Mum, open the door!") ... else: ... print("Let's go inside") ... Let's go inside</syntaxhighlight> Thanks to our else-block, we can now print an alternative text if door_is_locked is False. ## Python’s Boolean Operators The ability to use conditions is what makes computers tick; they make your software smart and allow it to change its behavior based on external input. We’ve used True and False directly so far, but more expressions evaluate to either True or False. These expressions often include a so-called boolean operator.

Let’s look at these boolean operators in the REPL:

<syntaxhighlight lang="python">>>> 2 > 1 True >>> 2 < 1 False >>> 2 < 3 < 4 < 5 < 6 True >>> 2 < 3 > 2 True >>> 3 <= 3 True >>> 3 >= 2 True >>> 2 == 2 True >>> 4 != 5 True >>> 'a' == 'a' True >>> 'a' > 'b' False</syntaxhighlight> This is what all the boolean operators are called:

Operator	Meaning
>	greater than
<	smaller than
>=	greater than or equal to
<=	smaller than or equal to
==	is equal
!=	is not equal

Python’s boolean operators

As can be seen in the examples, these operators work on strings too. Strings are compared in the order of the alphabet, with these added rules: - Uppercase letters are ‘smaller’ than lowercase letters, e.g.: ‘M’ < ‘m’ - Digits are smaller than letters: ‘1’ < ‘a’

You’re probably wondering what the logic is behind these rules. Internally, each character has a number in a table. The position in this table determines the order. It’s as simple as that. See Unicode on Wikipedia to learn more about it if you’re interested.