Chapter 2. Variables, Expressions and Statements

One of the most powerful features of a programming language is the ability to manipulate variables. A variable is a name that refers to a value.

Assignment Statements

An assignment statement creates a new variable and gives it a value:

>>> message = 'And now for something completely different'
>>> n = 17
>>> pi = 3.141592653589793

This example makes three assignments. The first assigns a string to a new variable named message; the second gives the integer 17 to n; the third assigns the (approximate) value of π to pi.

A common way to represent variables on paper is to write the name with an arrow pointing to its value. This kind of figure is called a state diagram because it shows what state each of the variables is in (think of it as the variable’s state of mind). Figure 2-1 shows the result of the previous example.

Figure 2-1. State diagram.

Variable Names

Programmers generally choose names for their variables that are meaningful—they document what the variable is used for.

Variable names can be as long as you like. They can contain both letters and numbers, but they can’t begin with a number. It is legal to use uppercase letters, but it is conventional to use only lowercase for variables names.

The underscore character, _, can appear in a name. It is often used in names with multiple words, such as your_name or airspeed_of_unladen_swallow ...

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