Python String isidentifier() Method

The isidentifier() method in Python is used to check whether a given string is a valid Python identifier. An identifier is a name used to identify variables, functions, classes, modules, etc.

Syntax

string.isidentifier()

Parameters:

• No parameters required.

Returns:

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

What is a Valid Identifier?

• Only contains letters (A–Z, a–z), digits (0–9), and underscores (_)
• Cannot start with a digit
• Cannot contain spaces or special characters
• Cannot be a Python keyword (though isidentifier() won't check for keywords)

Example 1: Valid Identifier

name = "my_variable"
print(name.isidentifier())

True

Example 2: Starts with a Digit

name = "1variable"
print(name.isidentifier())

False

Example 3: Contains a Space

name = "my var"
print(name.isidentifier())

False

Example 4: Underscore is Allowed

name = "_temp123"
print(name.isidentifier())

True

Use Cases

• Validating variable names in dynamic code generation
• Building programming tools or editors
• Checking user input before creating variable-like data

Common Mistakes

• Thinking it checks for Python keywords – it doesn't. "class" is a valid identifier technically, but cannot be used as a variable.
• Confusing it with isalnum() – which only checks for letters and digits, not identifier rules.

Interview Tip

Be ready to explain the difference between isidentifier() and isalnum(), and what makes a valid identifier in Python.

Summary

• isidentifier() checks if a string is a valid Python identifier
• Returns True if valid, False if not
• Useful for code generation, validation, and linters

Practice Problem

Write a program that takes input from the user and tells whether it's a valid identifier.

user_input = input("Enter a variable name: ")
if user_input.isidentifier():
    print("Valid identifier")
else:
    print("Invalid identifier")