005. Identifiers

Aryan’s RAM monitor reads process names from a config file that users edit. A user types a field name like 2nd_process or mem-usage — and the monitor crashes when Python tries to use it as a variable.

He needs a validator: before accepting any user-supplied field name, check that it’s a legal Python identifier AND not a reserved keyword.

Rules for valid identifiers:

Must start with a letter (a–z, A–Z) or underscore (_)
Can contain letters, digits (0–9), underscores — nothing else
Cannot be a Python keyword

The broken code only does the syntax check but forgets to exclude keywords — so for and while pass as valid, which would crash the monitor later.

💡 Fun fact: Python 3 extended identifier rules to allow Unicode letters — so café, 名前, and Ärger are all valid variable names. PEP 3131 (2007) made this possible, meaning Python identifiers can represent names from almost any human language.

⚠️ Watch out: str.isidentifier() returns True for keywords like for and class — it only checks character rules, not reservation status. You must combine it with not keyword.iskeyword() for a complete validity check.

🤔 Think about it: `_` (a single underscore) is a perfectly valid Python identifier used by convention for “throwaway” variables and in the REPL for the last result. What other single-character identifiers carry special meaning by convention in Python?

Learning objectives

Know the rules for valid Python identifiers
Use str.isidentifier() for syntax checking
Combine identifier check with keyword exclusion

Key concepts

identifiers
naming rules
keyword module

Try it

Concept detail

A Python identifier is any name used for variables, functions, classes, modules, etc. Rules: must start with a letter (a-z, A-Z) or underscore (_), followed by any combination of letters, digits (0-9), or underscores. Case-sensitive: myVar != myvar.

str.isidentifier() checks only the character-set rules. It returns True for keywords like ‘for’ and ‘class’ because those are syntactically valid name shapes — the parser rules them out separately.

For a complete validation you need BOTH checks: name.isidentifier() and not keyword.iskeyword(name)

The broken code passes the character-set check but skips keyword exclusion, so ‘for’, ‘while’, ‘class’ all appear valid. In the RAM monitor context, accepting ‘for’ as a config field name would cause a SyntaxError the moment that name was used in generated code.

Leading underscores carry convention meaning: _name (internal), __name (name-mangled in classes), name (dunder/magic). These are valid identifiers, not keywords.

Solution

import keyword

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

Tests

def test_valid_simple():
    assert is_valid_identifier("my_var") == True

def test_valid_with_underscore_prefix():
    assert is_valid_identifier("_private") == True

def test_invalid_starts_with_digit():
    assert is_valid_identifier("1abc") == False

def test_invalid_has_space():
    assert is_valid_identifier("my var") == False

def test_invalid_keyword():
    assert is_valid_identifier("for") == False

def test_invalid_keyword_while():
    assert is_valid_identifier("while") == False

def test_invalid_has_dash():
    assert is_valid_identifier("my-var") == False