Storing Values in Variables On Python

A variable is like a box in the computer’s memory where you can store a single value. If you want to use the result of an evaluated expression later in your program, you can save it inside a variable.

Storing Values in Variables On Python

Assignment Statements

You’ll store values in variables with an assignment statement. An assignment statement consists of a variable name, an equal sign (called the assignment operator), and the value to be stored. If you enter the assignment statement spam = 42, then a variable named spam will have the integer value 42 stored in it.

Think of a variable as a labeled box that a value is placed in, as in the picture below

spam = 42 is like telling the program, “The variable spam now has the integer value 42 in it.”

For example, enter the following into the interactive shell:

A variable is initialized (or created) the first time a value is stored in it u. After that, you can use it in expressions with other variables and values v. When a variable is assigned a new value w, the old value is forgotten, which is why spam evaluated to 42 instead of 40 at the end of the example. This is
called overwriting the variable. Enter the following code into the interactive shell to try overwriting a string:

>>> spam = 'Hello'
>>> spam
'Hello'
>>> spam = 'Goodbye'
>>> spam
'Goodbye'

Just like the box in Figure 1-3, the spam variable in this example stores 'Hello' until you replace it with 'Goodbye'.

When a new value is assigned to a variable,

the old one is forgotten.

Variable Names

The Table has examples of legal variable names. You can name a variable anything as long as it obeys the following three rules:

1. It can be only one word.
2. It can use only letters, numbers, and the underscore (_) character.
3. It can’t begin with a number.

Valid and Invalid Variable Names

Variable names are case-sensitive, meaning that spam, SPAM, Spam, and sPaM are four different variables. It is a Python convention to start your variables with a lowercase letter.

This book uses camelcase for variable names instead of underscores; that is, variables lookLikeThis instead of looking_like_this. Some experienced programmers may point out that the official Python code style, PEP 8, says that underscores should be used. I unapologetically prefer camelcase and point to “A Foolish Consistency Is the Hobgoblin of Little Minds” in PEP 8 itself:

“Consistency with the style guide is important. But most importantly: know when to be inconsistent—sometimes the style guide just doesn’t apply. When in doubt, use your best judgment.” 

A good variable name describes the data it contains. Imagine that you moved to a new house and labeled all of your moving boxes as Stuff. You’d never find anything! The variable names spam, eggs, and bacon are used as generic names for the examples in this book and in much of Python’s documentation (inspired by the Monty Python “Spam” sketch), but in your programs, a descriptive name will help make your code more readable.

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