Python String isidentifier() Method (original) (raw)

Last Updated : 04 Jan, 2025

The isidentifier() method in Python is used to check whether a given string qualifies as a valid identifier according to the Python language rules. Identifiers are names used to identify variables, functions, classes, and other objects. A valid identifier must begin with a letter (A-Z or a-z) or an underscore (_), should contain only alphanumeric characters (A-Z, a-z, 0-9) and underscores (_), and should not be a reserved keyword in Python. In this article, we will see how the isidentifier() method works:

Let’s understand with the help of an example:

Python `

s = "variable_name" print(s.isidentifier())

`

**Explanation:

• The string "variable_name" follows all the rules of a valid Python identifier:
  • It contains only letters, digits, and underscores.
  • It does not start with a digit.
  • It is not a Python keyword.
• Hence, the method returns True.

Table of Content

• Syntax of isidentifier() Method
• Examples of isidentifier method()
  • 1. Starting with a number
  • 2. Containing special characters
  • 3. Reserved keywords
  • 4. Empty string

Syntax of isidentifier()

string.isidentifier()

Parameters

• The isidentifier() method does not take any parameters.

Return Type

• Returns True if the string is a valid identifier.
• Returns False otherwise.

Examples of isidentifier method()

1. Starting with a number

An identifier cannot start with a number.

Python `

Starts with a number

s = "1variable"

print(s.isidentifier())

`

**Explanation:

• The string "1variable" begins with the digit 1, which violates the rules for identifiers.
• Consequently, the method returns False.

2. Containing special characters

Special characters, apart from the underscore, are not allowed in identifiers.

Python `

s = "var!name" # Contains a special character (!) print(s.isidentifier())

`

**Explanation:

• The string "var!name" includes the special character !, making it an invalid identifier.
• The method appropriately returns False.

3. Reserved keywords

Python’s reserved keywords cannot be used as identifiers. While they follow the naming rules, their special meaning in the language makes them invalid as identifiers:

Python `

s = "class" # Reserved keyword in Python print(s.isidentifier())

`

**Explanation:

• The string "class" follows all identifier rules, so isidentifier() returns True.
• However, using it as an identifier in code will lead to a SyntaxError because it is a reserved keyword.

4. Empty string

An empty string cannot qualify as an identifier:

Python `

s = "" # Empty string print(s.isidentifier())

`

**Explanation:

• The string is empty, so it cannot meet the rules for identifiers.
• The method returns False accordingly.