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2.8. Variable Names and Keywords

Variable names can be arbitrarily long. They can contain both letters and digits, but they have to begin with a letter or an underscore. Although it is legal to use uppercase letters, by convention we don’t. If you do, remember that case matters. Bruce and bruce are different variables.

Caution

Variable names can never contain spaces.

The underscore character ( _) can also appear in a name. It is often used in names with multiple words, such as my_name or price_of_tea_in_china. There are some situations in which names beginning with an underscore have special meaning, so a safe rule for beginners is to start all names with a letter.

If you give a variable an illegal name, you get a syntax error. In the example below, each of the variable names is illegal.

76trombones = "big parade"
more$ = 1000000
class = "Computer Science 101"

76trombones is illegal because it does not begin with a letter. more$ is illegal because it contains an illegal character, the dollar sign. But what’s wrong with class?

It turns out that class is one of the Python keywords. Keywords define the language’s syntax rules and structure, and they cannot be used as variable names. Python has thirty-something keywords (and every now and again improvements to Python introduce or eliminate one or two). This list is current as of Python version 3.5:

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

You might want to keep this list handy. If the interpreter complains about one of your variable names and you don’t know why, see if it is on this list.

Check your understanding

True or False: the following is a legal variable name in Python: A_good_grade_is_A+

• True
• - The + character is not allowed in variable names.
• False
• - The + character is not allowed in variable names (everything else in this name is fine).

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