Section 3.3. Identifiers

3.3. Identifiers

Identifiers are the set of valid strings that are allowed as names in a computer language. From this all-encompassing list, we segregate out those that are keywords, names that form a construct of the language. Such identifiers are reserved words that may not be used for any other purpose, or else a syntax error (SyntaxError exception) will occur.

Python also has an additional set of identifiers known as built-ins, and although they are not reserved words, use of these special names is not recommended. (Also see Section 3.3.3.)

3.3.1. Valid Python Identifiers

The rules for Python identifier strings are like most other high-level programming languages that come from the C world:

• First character must be a letter or underscore ( _ )

• Any additional characters can be alphanumeric or underscore

• Case-sensitive

No identifiers can begin with a number, and no symbols other than the underscore are ever allowed. The easiest way to deal with underscores is to consider them as alphabetic characters. Case-sensitivity means that identifier foo is different from Foo, and both of those are different from FOO.

3.3.2. Keywords

Python's keywords are listed in Table 3.1. Generally, the keywords in any language should remain relatively stable, but should things ever change (as Python is a growing and evolving language), a list of keywords as well as an iskeyword() function are available in the keyword module.

Table 3.1. Python Keywords^[a]

and	as[b]	assert[c]	break
class	continue	def	del
elif	else	except	exec
finally	for	from	global
if	import	in	is
lambda	not	or	pass
print	raise	return	try
while	with[b]	yield[d]	None[e]

^[a] access keyword obsoleted as of Python 1.4.

^[b] New in Python 2.6.

^[c] New in Python 1.5.

^[d] New in Python 2.3.

^[e] Not a keyword but made a constant in Python 2.4.

3.3.3. Built-ins

In addition to keywords, Python has a set of "built-in" names available at any level of Python code that are either set and/or used by the interpreter. Although not keywords, built-ins should be treated as "reserved for the system" and not used for any other purpose. However, some circumstances may call for overriding (aka redefining, replacing) them. Python does not support overloading of identifiers, so only one name "binding" may exist at any given time.

We can also tell advanced readers that built-ins are members of the __builtins__ module, which is automatically imported by the interpreter before your program begins or before you are given the >>> prompt in the interactive interpreter. Treat them like global variables that are available at any level of Python code.

3.3.4. Special Underscore Identifiers

Python designates (even more) special variables with underscores both prefixed and suffixed. We will also discover later that some are quite useful to the programmer while others are unknown or useless. Here is a summary of the special underscore usage in Python:

• _xxx Do not import with 'from module import *'

• __xxx__ System-defined name

• __xxx Request private name mangling in classes

Core Style: Avoid using underscores to begin variable names

Because of the underscore usage for special interpreter and built-in identifiers, we recommend that the programmer avoid beginning variable names with the underscore. Generally, a variable named _xxx is considered "private" and should not be used outside that module or class. It is good practice to use _xxx to denote when a variable is private. Since variables named __xxx__ often mean special things to Python, you should avoid naming normal variables this way.