Python Booleans

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• Slides

In Python, Boolean values are stored in the bool data type. Just like in pseudocode, variables of the bool data type can only store one of two values, True or False.

To create a Boolean variable in Python, we can simply assign those values to a variable in an assignment statement:

a = True
b = False
print(a)
print(type(b))

When we execute that code, we’ll see the following output:

True
<class 'bool'>

In Python, we use the keywords True and False to represent Boolean values. Notice that they are capitalized, unlike the true and false values we saw in pseudocode. In Python, it is very important to make sure these values are capitalized, otherwise the program will not work properly. Also, since these are keywords and not strings, we don’t need to put them in quotation marks.

Converting Between Data Types

Python includes the special bool() function, which can be used to convert any data type into a Boolean value. The bool() function follows these rules to determine what to return:

1. If the input is the value False, the value 0, the value None, or anything with 0 length, including the empty string, it will return False.
2. Otherwise, for all other values it will return True.

Let’s look at a couple of quick examples. First, let’s try to convert the strings "True" and "False" to their Boolean equivalents:

print(bool("True"))
print(bool("False"))

When this code is executed, we’ll see this output:

True
True

This seems a bit strange, since the string "False" ended up creating the Boolean value True. However, if we look at the rules above, since the string "False" has a length greater than 1, and it is not any of the special values listed above, it should always result in the Boolean value True.

In this example, we can check a couple of special values using the bool() function:

print(bool(0))
print(bool(1))
print(bool(""))

In this case, we’ll see the following output:

False
True
False

Here, we see that the value 0, as well as the empty string "", both result in a value of False. However, the value 1 is True, since it is a non-zero value.

In practice, we won’t use the bool() function directly very often. Instead, if we want to determine if a user inputs a True or False value, we can just use one of the Boolean comparators that we’ll see later in this lab.