Choosing good names

What you will be able to do

After this lesson you can

• say whether a name is allowed in Python, and why,
• choose a name that makes a program easy to read,
• avoid the names that Python already uses, and
• write a comment that helps the next reader.

Two kinds of rule

There are two kinds of rule about names.

1. Rules that Python enforces. If you break one, the program does not run.
2. Habits that programmers share. If you break one, the program runs, but people find it hard to read. And the person who finds it hard to read is often you, one month later.

The rules of Python

A name

• can have letters, digits and the underscore _,
• cannot start with a digit, so 2nd_place is not allowed and place_2 is,
• cannot have a space or a hyphen, so my age and my-age are not allowed (Python reads the hyphen as a minus sign), and my_age is,
• cannot be a keyword, one of the words that Python keeps for itself, such as if, for, while, class or True,
• is case sensitive: score and Score are two different names.

Here is the full list of the keywords.

Small and capital letters

Two names that differ only by a capital letter are easy to mix up. Python allows them, but good programs avoid them.

Names that Python already uses

Python has many functions that are always ready: print, str, int, float, len, sum, max, min, list, type. They are not keywords, so Python allows you to use their names for your own variables. But then your variable hides the function, and it stops working. Watch this.

The rule: never use the name of a function that Python has. If you want a name for a word, choose word or text. For a total, choose total and not sum. For the biggest number, choose highest and not max.

Habits of good names

A name should say what the variable holds.

Comments

A line that starts with # is a comment. Python ignores it. It is a note from you to a person who reads the program. Use a comment to explain why, or what a step does. Do not repeat the code in words.

• A weak comment: x = x + 1 # add 1 to x
• A helpful comment: visits += 1 # one more visitor has arrived

The programs in this course start each step with a comment. Read them as you read a recipe.

Common mistakes

• Starting a name with a digit, as in 1st_prize. Write first_prize.
• Using a hyphen or a space, as in my-age or my age. Write my_age.
• Using a keyword, as in class = "7B" or for = 3. Write class_name, count.
• Taking the name of a Python function, as in str, sum, max, list. Choose another word.
• Changing the capitals by accident, as in Total and total.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the problem Which names are valid? Say yes or no for each name before you read the solution.
2. Build. Open the lab Fix the names. The program does not run at first. Change the names until the checker shows PASS for all four.
3. Stretch. Take any program from the earlier lessons. Rename its variables to be worse (a, b, c), and ask a friend to say what the program does. Then change them back, and ask again.

Remember these

• A name has letters, digits and underscores. It does not start with a digit, has no space or hyphen, and is not a keyword.
• score and Score are different names.
• Never use str, print, sum and other function names for your variables.
• Choose names that say what the variable holds, in small letters with underscores.
• A comment starts with # and explains the program to a person.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which of these are allowed? total_1, 1_total, my total, my_total, for.

2. Why is sum = 0 a risky line?

3. Is Age = 12 followed by print(age) correct?

4. Write a better name for x = 98.6, if it is a body temperature.