A Python identifier—the name used for a variable, function, class, or other object—must start with a letter or underscore, and may then contain letters, digits, or underscores. Python also permits many Unicode letters. Reserved keywords such as class cannot be used as ordinary names. These are syntax rules; conventions such as snake_case are style guidance, not parser requirements.
What names does Python accept?
Python’s lexical rules define a name as a starting character followed by zero or more continuation characters. ASCII letters and the underscore can start a name; digits can appear after the first character. Eligible non-ASCII characters are allowed as well. The Python 3.14.7 Language Reference says: “Names must contain at least one character, but have no upper length limit.”
| Example | Valid identifier? | Why |
|---|---|---|
count2 |
Yes | Starts with a letter; digits may follow. |
_cache |
Yes | An underscore may start a name. |
2count |
No | A digit cannot be the first character. |
item and Item |
Both valid and distinct | Python names are case-sensitive. |
There is no language-imposed maximum identifier length, though extremely long names can make code harder to read.
Why can’t a keyword be an ordinary name?
Keywords are reserved by Python’s grammar. Names such as class, for, and return have language-defined roles and cannot be used as ordinary identifiers. The reserved set can change between Python versions. If code needs to check names against the interpreter it will run on, use the standard library’s keyword module, for example keyword.iskeyword("class").
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Soft keywords are different: they have special meaning only in particular grammar contexts, so the same spelling can still be an identifier elsewhere. In the reference’s pattern-matching contexts, match and case are soft keywords, and _ acts as a wildcard in a case pattern. Consult the Language Reference’s soft-keyword section when a name’s meaning depends on its syntax context.
Can Python identifiers contain Unicode?
Yes. Python supports many Unicode identifier characters, but not every symbol or pictograph. The Language Reference gives ř_1, 蛇, and साँप as valid examples, and r〰2, €, and 🐍 as invalid ones. Eligibility follows Unicode-derived character rules, rather than a rule that all non-ASCII symbols are accepted.
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Python normalizes identifiers to NFKC during parsing. Consequently, two spellings that normalize to the same form can resolve to the same name; a visually unusual spelling does not necessarily create a distinct identifier. See PEP 3131 for the design and the Language Reference for the current lexical rules.
Unicode clarity and review
Different script characters can look alike while remaining different identifiers. PEP 672 discusses these Unicode-related risks. In multilingual code, use a consistent script, make names clear to the team, and review unfamiliar identifiers carefully. This is a readability and code-review concern, not a reason to assume every Unicode name is unsafe.
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What naming conventions should you follow?
PEP 8 describes style conventions, not restrictions on what the parser accepts. Its common patterns are:
| What you are naming | Common PEP 8 form | Example |
|---|---|---|
| Functions and variables | Lowercase words joined by underscores (snake_case) |
item_count |
| Classes | CapWords | OrderItem |
| Constants | Uppercase words joined by underscores | MAX_RETRIES |
| Modules | Generally short and lowercase; underscores may help readability | data_tools |
For public APIs, PEP 8 says: “Names that are visible to the user as public parts of the API should follow conventions that reflect usage rather than implementation.” Choose names that make sense to callers, not just names that expose how the code currently works.
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When a keyword conflicts with a useful name
If an argument name would clash with a keyword, PEP 8 recommends adding a trailing underscore rather than abbreviating or distorting the word. For example, use class_ if that is the clearest argument name. PEP 8 also cautions against using lowercase l, uppercase O, or uppercase I alone as variable names because some fonts make them hard to distinguish from digits.
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How to choose between two possible names
Check these questions in order:
- Does the grammar accept it? It must start with an allowed starting character, and its remaining characters must be allowed continuations.
- Is it a reserved keyword? If so, it cannot be an ordinary identifier.
- Does it follow the convention for its role? Use PEP 8’s variable, function, class, or constant pattern where appropriate.
- Will it be clear at the call site? Prefer a name that communicates how a public function or argument is used.
- Could normalization or visual similarity cause confusion? Avoid relying on similar-looking Unicode characters to distinguish names.
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