Naming and Using Variables in Python

When you’re using variables in Python, you need to adhere to a few rules and guidelines. Breaking some of these rules will cause errors; other guidelines just help you write code that’s easier to read and understand. Be sure to keep the following variable rules in mind:

Variable names can contain only letters, numbers, and underscores. They can start with a letter or an underscore, but not with a number. For instance, you can call a variable message_1 but not 1_message.

Spaces are not allowed in variable names, but underscores can be used to separate words in variable names. For example, greeting_message works, but greeting message will cause errors.

Avoid using Python keywords and function names as variable names; that is, do not use words that Python has reserved for a particular programmatic purpose, such as the word print. (See “Python Keywords and Built-in Functions” on page 489.)

Variable names should be short but descriptive. For example, name is better than n, student_name is better than s_n, and name_length is better than length_of_persons_name.

Be careful when using the lowercase letter l and the uppercase letter O because they could be confused with the numbers 1 and 0.

It can take some practice to learn how to create good variable names, especially as your programs become more interesting and complicated. As you write more programs and start to read through other people’s code, you’ll get better at coming up with meaningful names.

The Python variables you’re using at this point should be lowercase. You won’t get errors if you use uppercase letters, but it’s a good idea to avoid using them for now.

Avoiding Name Errors When Using Variables

Every programmer makes mistakes, and most make mistakes every day. Although good programmers might create errors, they also know how to respond to those errors efficiently. Let’s look at an error you’re likely to make early on and learn how to fix it.

We’ll write some code that generates an error on purpose. Enter the following code, including the misspelled word mesage shown in bold:

message = "Hello Python Crash Course reader!"

When an error occurs in your program, the Python interpreter does its best to help you figure out where the problem is. The interpreter provides a traceback when a program cannot run successfully. A traceback is a record of where the interpreter ran into trouble when trying to execute your code. Here’s an example of the traceback that Python provides after you’ve accidentally misspelled a variable’s name:

Traceback (most recent call last):
File "", line 2, in <module>
NameError: name 'mesage' is not defined

The output at u reports that an error occurs in line 2 of the file The interpreter shows this line to help us spot the error quickly v and tells us what kind of error it found w. In this case it found a name error and reports that the variable being printed, mesage, has not been defined. Python can’t identify the variable name provided. A name error usually means we either forgot to set a variable’s value before using it, or we made a spelling mistake when entering the variable’s name.

Of course, in this example we omitted the letter s in the variable name message in the second line. The Python interpreter doesn’t spellcheck your code, but it does ensure that variable names are spelled consistently. For example, watch what happens when we spell message incorrectly in another place in the code as well:

mesage = "Hello Python Crash Course reader!"

In this case, the program runs successfully!

Hello Python Crash Course reader!

Computers are strict, but they disregard good and bad spelling. As a result, you don’t need to consider English spelling and grammar rules when you’re trying to create variable names and writing code.

Many programming errors are simple, single-character typos in one line of a program. If you’re spending a long time
searching for one of these errors, know that you’re in good company. Many experienced and talented programmers spend hours hunting down these kinds of tiny errors. Try to laugh about it and move on, knowing it will happen frequently throughout your programming life.

The best way to understand new programming concepts is to try using them in your programs. If you get stuck while working on an exercise in this book, try doing something else for a while. If you’re still stuck, review the relevant part of that chapter. If you still need help, see the suggestions in Appendix C.

In the next article, we will examine the usage of strings.

This article is an excerpt from A Hands-On, Project-Based Introduction to Programming by Eric Matthes

Reproduced with permission from No Starch Press

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