Chapter 9. Variables and Readability

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In this chapter, you’ll see how sloppy use of variables makes a program harder to understand.

Specifically, there are three problems to contend with:

  1. The more variables there are, the harder it is to keep track of them all.

  2. The bigger a variable’s scope, the longer you have to keep track of it.

  3. The more often a variable changes, the harder it is to keep track of its current value.

The next three sections discuss how to deal with these issues.

Eliminating Variables

In Chapter 8, Breaking Down Giant Expressions, we showed how introducing “explaining” or “summary” variables can make code more readable. These variables were helpful because they broke down giant expressions and acted as a form of documentation.

In this section, we’re interested in eliminating variables that don’t improve readability. When a variable like this is removed, the new code is more concise and just as easy to understand.

In the following section are a few examples of how these unnecessary variables show up.

Useless Temporary Variables

In the following snippet of Python code, consider the now variable:

now = datetime.datetime.now()
root_message.last_view_time = now

Is now a variable worth keeping? No, and here are the reasons:

  • It isn’t breaking down a complex expression.

  • It doesn’t add clarification—the expression datetime.datetime.now() is clear enough.

  • It’s used only once, so it doesn’t ...

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