What Is snake_case? A Practical Guide for Developers

snake_case is a naming style that writes multi-word identifiers entirely in lowercase, with underscores between words:

user_id
first_name
get_user_by_id
max_retry_count
is_active

The name comes from the visual: the underscores make text look like it’s slithering along the ground like a snake.

What makes a valid snake_case identifier?

• All letters are lowercase
• Words are separated by single underscores
• No spaces, hyphens, or capital letters
• Numbers are allowed anywhere: user_v2, http_200_response
• Leading underscores are a convention for “private” in Python: _internal_func
• Double leading underscores invoke name mangling in Python classes: __private

These are not snake_case:

First_Name    ← capital letter
user-id       ← hyphen, not underscore (that's kebab-case)
userID        ← camelCase, no underscore
User_ID       ← mixed case

Where snake_case is the standard

Python — PEP 8, the official Python style guide, mandates snake_case for all function names, method names, and variables:

def get_user_by_id(user_id: int) -> User:
    ...

max_connections = 10
is_authenticated = False

Classes use PascalCase; constants use UPPER_SNAKE_CASE.

Ruby — Ruby follows the same convention: methods and variables are snake_case, classes are PascalCase:

def send_welcome_email(user_id)
  ...
end

active_users = User.where(active: true)

Rust — Rust enforces snake_case at the compiler level. A variable named myVar triggers a warning; my_var is correct:

fn get_user_by_id(user_id: u64) -> User {
    ...
}

let max_retry_count = 3;

SQL and database columns — snake_case is the dominant convention for column and table names across PostgreSQL, MySQL, and SQLite:

CREATE TABLE user_accounts (
    user_id       SERIAL PRIMARY KEY,
    first_name    TEXT NOT NULL,
    last_name     TEXT NOT NULL,
    created_at    TIMESTAMPTZ DEFAULT NOW()
);

Some ORMs (Django, ActiveRecord) generate snake_case column names automatically from PascalCase model names.

Environment variables — UPPER_SNAKE_CASE (a variant) is the standard for environment variable names:

DATABASE_URL=postgres://localhost/mydb
SECRET_KEY=abc123
MAX_CONNECTIONS=20

Where snake_case is not the standard

• JavaScript/TypeScript — camelCase for variables, PascalCase for classes
• Java — camelCase for variables and methods, PascalCase for classes
• Go — camelCase for unexported names, PascalCase for exported
• CSS — kebab-case for class names and custom properties
• URL paths — kebab-case (/user-settings) is preferred over snake_case

snake_case vs camelCase vs kebab-case

Style	Example	Standard use
snake_case	user_id	Python, Ruby, Rust, SQL
camelCase	userId	JavaScript, TypeScript, Java
PascalCase	UserId	Classes in most languages
UPPER_SNAKE	USER_ID	Constants, environment variables
kebab-case	user-id	CSS, HTML, URLs

The distinction matters at API boundaries. A Python backend that exposes user_id and a JavaScript frontend that expects userId need explicit conversion — either in the serializer or the client. Libraries like humps (Python) or change-case (JavaScript) handle this.

Why Python chose snake_case

Guido van Rossum’s original rationale (captured in PEP 8 and various Python mailing list posts) was that snake_case reads more like natural English, particularly for longer identifiers. get_user_by_id parses as four distinct words; getUserById requires training your eye to find the word boundaries.

It’s partly a historical accident: Python emerged in a Unix/C context where underscored identifiers were common. JavaScript emerged from a Java/ECMAScript context where camelCase was already the norm.

UPPER_SNAKE_CASE for constants

When you want a constant in Python, Ruby, or Rust, the convention is snake_case with all letters uppercased:

MAX_CONNECTIONS = 100
DEFAULT_TIMEOUT = 30
API_BASE_URL = "https://api.example.com"

In Python, UPPER_SNAKE doesn’t technically prevent mutation (unlike const in JavaScript), but the convention signals intent to other developers.

Convert between styles

Working across a Python backend and a JavaScript frontend? The caseconvert.io tool converts identifiers between snake_case, camelCase, PascalCase, and kebab-case — paste a block of identifiers and get the converted output instantly.