Engineering Craft › Developer Tooling

Formatter

A tool that rewrites code to a consistent style automatically.

Also known as: code formatter, auto formatter, Prettier, Black, gofmt

A formatter rewrites your code into one consistent layout (indentation, spacing, line breaks, quote style) automatically. You stop thinking about style and stop arguing about it.

# before
def  add( a,b ):
    return {'sum':a+b,
      'inputs':[a,b]}

# after Black / ruff format
def add(a, b):
    return {"sum": a + b, "inputs": [a, b]}

Common ones: Prettier (JavaScript, TypeScript, CSS, Markdown), Black and Ruff (Python), gofmt (Go), rustfmt (Rust).

Why use one

• No style debates in code reviews. The tool decides.
• Smaller diffs: only real changes show up, not whitespace noise.
• Consistency across the team and over time (style consistency).

Making it automatic

• Turn on format on save in your editor.
• Run it in a pre-commit hook (pre-commit hooks), so nothing unformatted is committed.
• Check it in CI (prettier --check ., black --check .), so the build fails if someone forgot.

npx prettier --write .
black .

Notes

• A formatter changes layout. It does not find bugs. That’s the job of a linter. They work together, and their rules should not fight each other.
• Keep configuration minimal. Use the default settings unless the team has reasons not to.
• When you first add a formatter to an existing project, do it in its own commit so it doesn’t bury real changes (and so git blame can skip it).
• Settings can be shared via .editorconfig or the tool’s config file.