2.2. Rules for Names

The name we give a variable, also called its identifier, is subject to some important rules. Some of these rules are strict - Python will not accept names that do not follow these rules. These rules are part of the syntax of Python. The syntax of any language are the rules for what are valid ways to write things in the language. Other rules for variable names are human conventions. Python will not enforce these rules, but you and other programmers reading your code may get confused.

2.2.1. Naming Syntax

There are restrictions on what you can use as a variable name.

• It must start with a letter (uppercase like A or lowercase like a) or an underscore _

• It can also contain digits, like 1 or 9, just not as the first character

• It can’t have spaces, or special symbols other than _

• It can’t be a Python keyword. Keywords are words that have special meaning in the language

  (see below for examples).

• Case matters. A variable named result is not the same as one named Result.

Python has a few dozen keywords that you can’t use as variable names. Here is a list of the most common ones. If you ever have an error based on one of your variable names and do not know why, compare your name to this list to make sure you are not using a keyword as your name.

and	as	assert	break	class	continue
def	del	elif	else	except	exec
finally	for	from	global	if	import
in	is	lambda	nonlocal	not	or
pass	raise	return	try	while	with
yield	True	False	None

Which of the following is not a legal variable name?

• _a1
• You can use an underscore as the first character in a variable name
• my_name
• You can use an underscore between words in a variable name.
• amountOfStuff
• You can use both uppercase and lowercase letters in a variable name.
• BMP
• You can use only uppercase letters in a variable name.
• 1A
• You can't use a digit as the first letter in a variable name.

Which of the following is not a legal variable name?

• _my_name
• This is legal, but you don't usually start a variable name with an underscore.
• my name
• You can't have a space in a variable name.
• myname
• This may be hard to read, but it is legal.
• myName
• Since you can't have spaces in names, one way to make variable names easier to read is to use camel case (uppercase the first letter of each new word).
• my_name
• Since you can't have spaces in names, one way to make variable names easier to read is to use an underscore between two words.

2.2.2. Naming Convention

Programmers generally choose names for their variables that are meaningful to the human readers of the program — they help the programmer document, or remember, what the variable is used for. To be meaningful, a name must clearly describe what a piece of information is to anyone reading the code, not just the author of the code.

Meaningful names are generally full words like height. Abbreviations are generally not meaningful to anyone other than the original author. If you are reading through code and see a variable h, that will likely not do anything to help you figure out what information it is holding. (The exception to this rule is representing values from a mathematical formula like \(a^b + b^c = c^2\) where a has a well defined menaing.)

Oftentimes, you need multiple words to meaningfully describe a variable. Say you have a program that involves converting a height between inches and centimeters, within that program, the name height might be confusing. Is it referring to a value in inches? centimeters?

In cases like this, since you can’t have spaces in a variable name, you can either join words together by uppercasing the first letter of each new word like heightInInches (called mixed-case or camel-case because the capitals look like the humps of a camel) or use underscores between words like height_in_inches (called snake-case).

Say you have a variable that represents the date someone was hired. Which of these would be the most appropriate name?

• hireDate
• Correct! This or hire_date would both be reasonable choices for a name.
• _hire_date
• hire_date would be reasonable. But you don't usually start a variable name with an underscore.
• hire date
• You can't have a space in a variable name
• date
• date isn't the worst option here, but it might not be clear to other readers of the code what date we are referring to. Is it a person's hire date? their birth day? today's date?
• hd
• An abbreviation would not be appropriate in this case. hd is not meaningful.

Caution

Beginners sometimes confuse “meaningful to the human readers” with “meaningful to the computer”. So they’ll wrongly think that because they’ve called some variable average or pi, it will somehow automagically calculate an average, or automagically associate the variable pi with the value 3.14159.

That is not the case. The computer doesn’t attach semantic meaning to your variable names. The Python interpreter does not care if you call a variable average, a, something, or sue - to it, all of those names are equally meaningful in that they have no real meaning other than naming a value the programmer is working with.

Materials on this page adapted with permission from from:
Foundations of Python Programming (GNU Free Document License)

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