An identifier is a name given to a variable, function, class or module. Only following are allowed in identifiers:

• Lowercase (a-z) characters
• Uppercase (A-Z) characters
• Digits (0-9)
• Underscore(_)

An identifier can be a combination of lowercase characters, uppercase characters, digits and underscore.

• An identifier can not start with a digit. 1y is invalid identifier whereas y1 is a valid identifier.
• Keywords can not be used as identifier. Keywords are later mentioned in this tutorial.
• Spaces and special symbols like !,@,#,$,%,^,& etc. are not allowed in identifier.
• Identifier can be of any length.
• Identifiers are case sensitive. Sum, SUM and sum all are different identifiers.

Following are some valid identifiers:

sum, first_sum, sum1, sum__1, _sum, sum_1_

Following are some invalid identifiers:

1sum, %sum, 1_sum, 1sum%

Keywords

Keywords are a list of reserved words that have predefined meaning. Keywords can not be used as an identifier in Python. Attempting to use a keyword as an identifier name will cause an error.

List of Keywords in Python

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