Think Python, 3rd Edition

Chapter 2. Variables and Statements

In the previous chapter, we used operators to write expressions that perform arithmetic computations.

In this chapter, you’ll learn about variables and statements, the import statement, and the print function. And I’ll introduce more of the vocabulary we use to talk about programs, including “argument” and “module.”

Variables

A variable is a name that refers to a value. To create a variable, we can write an assignment statement like this:

n = 17
       

An assignment statement has three parts: the name of the variable on the left, the equals operator, =, and an expression on the right. In this example, the expression is an integer. In the following example, the expression is a floating-point number:

pi = 3.141592653589793
       

And in the following example, the expression is a string:

message = 'And now for something completely different'
       

When you run an assignment statement, there is no output. Python creates the variable and gives it a value, but the assignment statement has no visible effect. However, after creating a variable, you can use it as an expression. So we can display the value of message like this:

message
       

'And now for something completely different'
       

You can also use a variable as part of an expression with arithmetic operators:

n + 25
       

42
       

2 * pi
       

6.283185307179586
       

And you can use a variable when you call a function:

round(pi)
       

3
       

len(message)
       

42
       

State Diagrams

A common way to represent variables on paper is to write ...