1-3: Booleans

Our next data type is simple, but crucial. The bool type represents boolean values: either True or False.

Note

Boolean values are named for George Boole, a 19th-century English logician who pioneered logic through binary values.

True and False

The two variants of type bool are True and False. The capitals matter; those specific words are reserved for the bool variants. You can see this in action yourself by running the following expressions.

type(True)
# => bool
type(true)
# => NameError: name 'true' is not defined

Expressions and Comparisons

Most of the time, we don’t explicitly write out bool variants. Instead, we derive these values from expressions that evaluate to either True or False. An expression is any syntactically complete statement in Python that evaluates to a single value. 5 is a valid Python expression, but it does not evaluate to True or False. We need to test for truth or falsity.

Comparisons are a common way to perform such tests (but not the only way, as we’ll see later). Comparisons take two values, apply a test, and evaluate to True or False. You’re familiar with this even if you’ve never written a line of code. Here are the comparison operators in Python

• ==: Equivalence (i.e. a is equivalent to b)
• !=: Not equivalent
• >: Greater than
• <: Less than
• >=: Greater than or equal to
• <=: Less than or equal to

In practice this looks like:

7 > 5
# => True
2 <= 3
# => False
2 == 2.0
# => True
2 != 2
# => False

Negation

You see that != is the negation of the equivalence comparison. In some languages, ! is used to negate any boolean expression. In Python, we use the not keyword. Go ahead—put not in front of any of the comparisons you tried above. not flips the value.

Boolean Operators

Comparisons are useful, but we often need to evaluate more than one boolean expression at a time. Or we need to combine them for a different effect. That’s where the and and or operators come in.

These allow us to combine boolean expressions. For example, True or False evaluates to True. True and False will be False.

Just like in mathematical expressions, parentheses matter and affect order of operations. Try these two:

True and not False or True
# => True
True and not (False or True)
# => False

None

There is another data type closely related to bool known as NoneType, with a single value of None.

type(None)
# => NoneType

This is Python’s way of saying “Nothing’s here.” Other languages use null or nil to express this concept. I mention it here because None is “falsy,” meaning that it kinda evaluates as false. But only kinda.

None == False
# => False

None is not equivalent to False. But given an or statement between None and another value:

None or 10
# => 10

You’ll get the non-None value.

A cool way of seeing this is with a chained or expression. or expressions will evaluate to the first truthy value they encounter. So:

False or None or 6
# => 6

The first two values are ignored, but the int 6 is truthy, and therefore the result of the or expression.

Put simply, None will evaluate like False in comparisons.