How to Check If a String Is a Valid Identifier in Python

When performing metaprogramming, processing user input for dynamic variable creation, or validating configuration files, you often need to check if a string can be used as a valid variable, function, or class name in Python. This is known as checking if a string is a valid identifier.

This guide explains the rules of Python identifiers and how to validate them programmatically using the built-in isidentifier() method and the keyword module.

Understanding Python Identifier Rules

Before writing code, it is important to know what constitutes a valid identifier in Python 3:

1. Start Character: Must start with a letter (a-z, A-Z) or an underscore (_). It cannot start with a number.
2. Body Characters: Can contain letters, numbers, and underscores.
3. No Special Characters: Symbols like @, $, -, or spaces are not allowed.
4. Case Sensitivity: myVar and myvar are different identifiers.
5. Reserved Keywords: Words like if, class, or return are reserved and cannot be used as names, even if they fit the character rules.

Method 1: Using str.isidentifier()

Python strings have a built-in method .isidentifier() that returns True if the string adheres to syntax rules (points 1-3 above).

var1 = "my_variable"
var2 = "_private"
var3 = "var123"

print(f"'{var1}' is valid? {var1.isidentifier()}")
print(f"'{var2}' is valid? {var2.isidentifier()}")
print(f"'{var3}' is valid? {var3.isidentifier()}")

Output:

'my_variable' is valid? True
'_private' is valid? True
'var123' is valid? True

Handling Invalid Strings

Use this method to detect syntax errors like starting with numbers or including spaces.

invalid_start = "1st_variable"
invalid_char = "my-variable"
invalid_space = "my variable"

print(f"'{invalid_start}' is valid? {invalid_start.isidentifier()}")
print(f"'{invalid_char}' is valid? {invalid_char.isidentifier()}")
print(f"'{invalid_space}' is valid? {invalid_space.isidentifier()}")

Output:

'1st_variable' is valid? False
'my-variable' is valid? False
'my variable' is valid? False

Method 2: Checking for Reserved Keywords

Even if a string passes isidentifier(), it might still cause a SyntaxError if it is a reserved keyword (e.g., None, True, def). To check this, you must use the keyword module.

import keyword

reserved_word = "class"
print(f"Is 'class' syntactically valid? {reserved_word.isidentifier()}")

is_reserved = keyword.iskeyword(reserved_word)
print(f"Is 'class' a reserved keyword? {is_reserved}")

Output:

Is 'class' syntactically valid? True
Is 'class' a reserved keyword? True

Robust Solution: Combining Both Checks

To ensure a string is truly safe to use as a variable name, you must verify that it is syntactically correct AND not a reserved keyword.

import keyword

def is_valid_variable_name(name):
    return name.isidentifier() and not keyword.iskeyword(name)

test_names = ["my_var", "123var", "class", "my-var", "yield"]

print(f"{'Name':<10} | {'Valid?':<6}")
print("-" * 20)

for name in test_names:
    valid = is_valid_variable_name(name)
    print(f"{name:<10} | {valid}")

Output:

Name       | Valid?
--------------------
my_var     | True
123var     | False
class      | False
my-var     | False
yield      | False

Conclusion

To check if a string is a valid identifier in Python:

1. Use str.isidentifier() to check for character validity (no starting numbers, no special symbols).
2. Use keyword.iskeyword() to ensure the string isn't reserved.
3. Combine both for a robust validation function suitable for metaprogramming or sanitizing inputs.