str.isidentifier()

Added in v3.8 · Updated March 13, 2026 · String Methods

python string methods

The .isidentifier() method checks whether a string is a valid Python identifier. It returns True if the string qualifies as a valid identifier according to Python’s naming rules, and False otherwise.

A valid Python identifier must:

Start with a letter (a-z, A-Z) or underscore (_)
Contain only letters, digits (0-9), and underscores
Not be a Python keyword

Syntax

str.isidentifier()

Parameters

None. This method takes no parameters.

Returns

bool: True if the string is a valid identifier, False otherwise.

Examples

Basic usage

>>> "hello".isidentifier()
True
>>> "hello_world".isidentifier()
True
>>> "_private".isidentifier()
True
>>> "class".isidentifier()
True  # valid identifier, even though it's a keyword

Invalid identifiers

>>> "123abc".isidentifier()
False  # starts with a digit
>>> "hello world".isidentifier()
False  # contains a space
>>> "hello-world".isidentifier()
False  # contains a hyphen
>>> "".isidentifier()
False  # empty string

Using with other string methods

# Filter valid identifiers from a list
names = ["valid_name", "123invalid", "another-valid", "okay"]
valid = [name for name in names if name.isidentifier()]
print(valid)  # ['valid_name', 'okay']

# Chain with other validation
def is_valid_variable(name):
    return name.isidentifier() and not name.startswith("_")

>>> is_valid_variable("my_var")
True
>>> is_valid_variable("_private")
False

Practical use cases

This method is useful for:

Validating user-provided variable names
Parsing code or config files
Checking if a string can be used as a Python identifier
Input validation in code generation tools
Building linters or code analysis tools

It is easy to confuse .isidentifier() with similar string methods. Here’s how they differ:

.isalpha() returns True if all characters are alphabetic, but allows single characters like “a” which aren’t valid identifiers by themselves.
.isalnum() returns True if all characters are alphanumeric, which includes digits anywhere in the string.
.isdigit() returns True only if all characters are digits.

>>> "hello".isalpha()
True
>>> "hello".isidentifier()
True
>>> "hello123".isalnum()
True
>>> "hello123".isidentifier()
False  # contains digits

Note that .isidentifier() returns True for Python keywords like if, class, and return. If you need to exclude keywords, use the keyword module:

Python 3 also supports Unicode identifiers beyond ASCII. This means you can use non-English letters in your variable names:

>>> "café".isidentifier()
True
>>> "变量".isidentifier()
True
>>> "müller".isidentifier()
True

This is useful when writing code for international teams or localized applications.

import keyword

def is_valid_not_keyword(s):
    return s.isidentifier() and not keyword.iskeyword(s)

>>> is_valid_not_keyword("class")
False
>>> is_valid_not_keyword("myclass")
True