Python Boolean and Conditional Programming: if.. else

Besides numbers and strings, Python has several other types of data. One of them is the Boolean data type. Booleans are extremely simple: they are either true or false. Booleans, in combination with Boolean operators, make it possible to create conditional programs: programs that decide to do different things, based on certain conditions.

The Boolean data type was named after George Boole, the man that defined an algebraic system of logic in the mid 19th century.

What is a Boolean?

Let’s start with a definition:

Boolean

A boolean is the simplest data type; it’s either True or False.

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.” It’s important to note, that in Python a Boolean value starts with an upper-case letter: True or False. This is in contrast to most other programming languages, where lower-case is the norm.

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

>>> door_is_locked = True
>>> if door_is_locked:
...     print("Mum, open the door!")
...
Mum, open the door!
>>>_

Here’s an interactive version of the same code that you can experiment with:

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 block. This block is executed only when the expression evaluates to False. This way, you can run code for both options. Let’s try this:

>>> door_is_locked = False
>>> if door_is_locked:
...     print("Mum, open the door!")
... else:
...     print("Let's go inside")
...
Let's go inside
>>>_

Thanks to our else-block, we can now print an alternative text if door_is_locked is False. As an exercise, try to modify the interactive code example above to get the same result.

Python 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 directly so far, but more expressions evaluate to either True or False. These expressions often include a so-called operator.

There are multiple types of operators, and for now, we’ll only look at these:

1. Comparison operators: they compare two values to each other
2. Logical operators

Comparison operators

Let’s look at comparison operators first. You can play around with them in the REPL:

>>> 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

This is what all the comparison operators are called:

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

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.

Logical operators

Next up: logical operators. These operators only work on booleans and are used to implement logic. The following table lists and describes them:

Operator	What is does	Examples
and	True if both statements are true	True and False == False False and False == False True and True == True
or	True if one of the statements is true	True or False == True True or True == True False or False == False
not	Negates the statement that follows	not True == False not False == True

Here are some examples in the REPL to help you play around with these:

>>> not True
False
>>> not False
True
>>> True and True
True
>>> True and False
False

Comparing different types in Python

When you try to compare different types, you’ll often get an error. Let’s say you want to compare an integer with a string:

>>> 1 < 'a'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'int' and 'str'
>>>

This is how Python tells you it can’t compare integers to strings. But there are types that can mix and match. I would recommend against it because it makes your code hard to understand, but for sake of demonstration, let’s compare a boolean and an int:

>>> True == 1
True
>>> False == 0
True
>>> True + True
2
>>> False + False
0
>>> False + True
1
>>> True + 3
4
>>>

As can be seen, True has a value of 1, and False has a value of 0. This has to do with the internal representation of booleans in Python: they are a special kind of number in Python.