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Python Identifiers and Reserved Words

Identifiers

In Python, a name used in a program is called an identifier. It can be the name of a:

  • Class
  • Function
  • Module
  • Variable

Example:

a = 10

Rules for Defining Identifiers in Python

  1. Allowed Characters Identifiers can only contain:
  • Alphabet symbols (a–z, A–Z)
  • Digits (0–9)
  • Underscore symbol _

❌ Using any other symbol like $ will result in a syntax error.

cash = 10   # Valid
ca$h = 20   # Invalid
  1. Should Not Start with a Digit
123total = 50   # Invalid
total123 = 50   # Valid
  1. Case Sensitivity Identifiers in Python are case-sensitive.
total = 10
TOTAL = 999

print(total)  # Output: 10
print(TOTAL)  # Output: 999
  1. Cannot Use Reserved Words Reserved words in Python cannot be used as identifiers.
def = 10   # Invalid

  1. No Length Limit There is no maximum length for an identifier, but it's best to avoid excessively long names.

  2. Dollar Symbol Not Allowed $ is not permitted in Python identifiers.

  3. Identifiers Starting with Underscore

  • _identifier: Indicates private.
  • __identifier: Indicates strongly private.

  • __identifier__: Indicates a language-defined special name (also known as magic methods).

    Example:

    __add__  # Magic method used for operator overloading

Valid and Invalid Identifiers

Identifier Validity
123total ❌ Invalid
total123 ✅ Valid
java2share ✅ Valid
ca$h ❌ Invalid
_abc_abc_ ✅ Valid
def ❌ Invalid (reserved word)
if ❌ Invalid (reserved word)

Reserved Words

In Python, certain words are reserved to represent specific meanings or functionalities. These are known as reserved words or keywords.

There are 33 reserved words in Python (as of version 3.x):

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

Notes:

  1. All reserved words contain only alphabet characters.
  2. All are in lowercase, except:
  • True
  • False
  • None
a = true   # Invalid
a = True   # Valid

Checking Reserved Words in Python

You can view the list of reserved words using the keyword module:

import keyword
print(keyword.kwlist)

Output:

['False', 'None', 'True', 'and', 'as', 'assert', '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']