Configuration model¶

Configuration is the product. This document sketches its shape; exact option names are provisional and will be settled by tests-first API work.

File formats¶

TOML is the primary format; YAML and package.json are supported with the same schema and keys. See ADR 0001 for the rationale.

File	Format
`rainbow.toml`, `.rainbow.toml`	TOML
`rainbow.yaml`, `rainbow.yml`, `.rainbow.yaml`, `.rainbow.yml`	YAML (1.2 core schema)
`pyproject.toml` — `[tool.rainbow]` table	TOML
`package.json` — `"rainbow"` key	JSON

More than one rainbow configuration in the same directory is an error; rainbow-fmt never picks one silently.

Finding the configuration¶

For each formatted file, rainbow-fmt looks in the file's directory, then in each parent directory up to the filesystem root, and uses the nearest configuration. A pyproject.toml without [tool.rainbow] or a package.json without "rainbow" does not count. Configurations in directories further up are not merged in; combine files explicitly with extends.

`extends`¶

# rainbow.toml
extends = ["../team/style.yaml", "local.toml"]   # or a single path

Paths are relative to the file that names them and may be in any supported format (a pyproject.toml or package.json contributes its rainbow section). Extended files may extend others; a cycle is an error. Later files override earlier ones, and the extending file overrides them all:

tables are merged key by key;
[[override]] and [[rule]] entries are concatenated, base entries first (so the extending file's rules win);
every other value, arrays included, is replaced.

Files¶

[files]
exclude = ["tests/fixtures/**", "*.generated.json"]  # skipped when a directory is walked
ignore_unknown = true   # no warning for explicitly named files of unknown type
editorconfig = false    # do not read .editorconfig files (default: true)
verify = false          # skip verifying formatted output (default: true; see cli.md)
cache = false           # neither skip nor record formatted files (default: true)
jobs = 1                # worker processes; 0 (default) is one per CPU (working directory's configuration)

See cli.md.

Values, positions and errors¶

Every format loads into the same data: tables, arrays, strings, integers, floats and booleans. null (YAML, JSON) and dates (TOML, YAML) are rejected, and so are duplicate keys. Every value keeps its position, so errors name the file, line and column (in characters):

rainbow.yaml:2:16: core.line_breaks: expected one of "preserve", "off", "fit"; got boolean false; if you meant the string "off", quote it
rainbow.toml:4:1: rule[0] (json/pair): unknown name 'nope'

TOML is read with the standard library's tomllib; YAML with ruamel.yaml (1.2 core schema); package.json with a small built-in parser that tracks positions (ADR 0001, amendment 1).

Example¶

# rainbow.toml
preset = "rainbow:balanced"     # starting point; optional

[core]
max_width      = 100
indent_style   = "space"        # "space" | "tab" | "preserve"
indent_size    = 4
line_ending    = "lf"           # "lf" | "crlf" | "preserve"
final_newline  = true

[shared]                        # cross-language vocabulary
quotes         = "single"       # "single" | "double" | "preserve" | "fewest-escapes"
trailing_comma = "multiline"    # "never" | "always" | "multiline" | "preserve"
blank_lines    = { max = 2, between_top_level = "preserve" }
align          = { assignments = true, object_values = false }

[language.python]
trailing_comma    = "never"     # overrides [shared] for Python only
bracket_wrap      = "fit"       # "magic_trailing_comma" | "fit" | "preserve"
top_level_blank_lines = 2

[language.javascript]
semicolons        = "as-needed" # "always" | "as-needed" | "preserve"
arrow_parens      = "preserve"
brace_style       = "1tbs"      # "1tbs" | "allman" | "stroustrup" | "preserve"

[language.svelte]
indent_script_and_style = false

[[override]]                    # path-glob overrides, applied in order
files = ["legacy/**/*.js"]
core.indent_size = 2
language.javascript.semicolons = "preserve"

[[rule]]                        # user-level rule override (ADR 0005)
language = "css"
select   = "declaration"
template = "[field('property'), ': ', field('value'), ';']"

The same configuration (abridged) in YAML:

# rainbow.yaml
preset: rainbow:balanced

core:
  max_width: 100
  indent_style: space
  indent_size: 4

shared:
  quotes: single
  trailing_comma: multiline
  blank_lines: { max: 2, between_top_level: preserve }

language:
  python:
    bracket_wrap: fit
  javascript:
    semicolons: as-needed

override:
  - files: ["legacy/**/*.js"]
    core: { indent_size: 2 }
    language: { javascript: { semicolons: preserve } }

Note: YAML is read with the 1.2 core schema, so unquoted off, no, and on are strings, not booleans. A boolean supplied where an enum is expected is rejected with a message suggesting quotes.

Option schema¶

Each option is declared once, with a name, a type (integer with a minimum, a choice of strings, or a boolean), a default and a one-sentence description (rainbow_fmt.options). Core, shared and [files] options are declared centrally; each language pack declares its own (extending.md). Planned: node scope for inline directives, and before/after examples for generated reference docs.

Options available today:

Key	Values	Default
`core.max_width`	integer ≥ 1	`80`
`core.indent_style`	`"space"`, `"tab"`	`"space"`
`core.indent_size`	integer ≥ 0	`4`
`core.tab_width`	integer ≥ 1	`4`
`core.line_ending`	`"lf"`, `"crlf"`, `"preserve"` (the input's first line ending)	`"lf"`
`core.max_blank_lines`	integer ≥ 0: blank lines kept between members, at most (none are added)	`1`
`shared.trailing_comma`	`"never"`, `"multiline"`	`"never"` (`"multiline"` for Python, JavaScript and TypeScript)
`language.json.trailing_comma`	`"never"` (JSON has no trailing commas)	`"never"`
`language.json.object_wrap`	`"preserve"`, `"fit"`, `"always"`	`"preserve"`
`language.json.align_values`	boolean	`false`
`language.css.selector_list`	`"one_per_line"`, `"fit"`	`"one_per_line"`
`language.css.rule_wrap`	`"always"`, `"fit"`, `"preserve"`	`"always"`
`language.css.last_semicolon`	`"always"`, `"never"`, `"preserve"`	`"always"`
`language.python.bracket_wrap`	`"magic_trailing_comma"`, `"fit"`, `"preserve"`	`"magic_trailing_comma"`
`language.python.binary_operator_break`	`"before"`, `"after"`	`"before"`
`language.python.definition_blank_lines`	`"enforce"`, `"cap"`, `"preserve"`	`"enforce"`
`language.python.top_level_blank_lines`	integer ≥ 0	`2`
`language.python.nested_blank_lines`	integer ≥ 0	`1`
`language.python.inline_comment_spaces`	integer ≥ 1	`2`
`language.python.statement_blank_lines`	`"preserve"`, `"separate"` (blank lines around loops and after `if` statements in functions)	`"preserve"`
`language.python.docstrings`	`"preserve"`, `"aligned"` (closing quotes on their own line, continuation lines under the first letter)	`"preserve"`
`language.python.bracket_hug`	boolean: a list of dicts as `[{` … `}, {` … `}]`	`false`
`language.javascript.return_semicolons`	`"as_statements"`, `"unless_last"` (a `return` followed by another statement ends with `;`)	`"as_statements"`
`language.javascript.bracket_hug`	boolean: an array of objects as `[{` … `}, {` … `}]`	`false`
`language.html.whitespace_sensitivity`	`"css"`, `"strict"`, `"ignore"` (`html.md`)	`"css"`
`language.html.bracket_same_line`	boolean	`false`
`language.html.void_elements`	`"no_slash"` (`<br>`), `"slash"` (`<br />`), `"preserve"`	`"no_slash"`
`language.svelte.whitespace_sensitivity`, `language.svelte.bracket_same_line`	as for HTML (`svelte.md`)
`language.yaml.sequence_indent`	`"indent"`, `"none"` (`yaml.md`)	`"indent"`
`language.sql.keyword_case`	`"preserve"`, `"upper"`, `"lower"` (`sql.md`)	`"preserve"`
`language.markdown.fenced_code`	`"format"`, `"preserve"` (`markdown.md`)	`"format"`
`language.markdown.table_width`	integer ≥ 1 (`markdown.md`)	`180`
`files.exclude`	list of patterns (as in `[[override]] files`, relative to the configuration file): files and directories a directory walk skips; a preset's patterns add to the project's; a file named on the command line is always formatted	`[]`
`files.ignore_unknown`	boolean	`false`
`files.editorconfig`	boolean	`true`
`files.verify`	boolean	`true`
`files.cache`	boolean	`true`
`files.jobs`	integer ≥ 0 (0: one per CPU)	`0`

Any core or shared option may also be set in a [language.X] section, for that language only. Language options may appear only in their language's section.

A language may narrow a core or shared option, allowing fewer values. The option then belongs to the language: values in [core] or [shared] (and --set core.…/shared.…) do not apply to it, and a disallowed value in [language.X] is an error. JSON narrows trailing_comma to "never", because trailing commas are not valid JSON; [shared] trailing_comma = "multiline" applies to other languages only.

A language may also change only the default of a core or shared option. Values in [core] and [shared] still apply to it. Python's default for trailing_comma is "multiline" (PEP 8, Black); [shared] trailing_comma = "never" applies to Python too. rainbow-fmt options shows such a default as language.python.trailing_comma.

Everything is validated when a file is formatted: unknown keys, wrong types and malformed [[override]] sections are errors that name the file, line and column. Sections for languages rainbow-fmt does not know (for example [language.html] today) are ignored, so a configuration can be written before a language pack exists.

Resolution order (lowest → highest precedence)¶

Built-in defaults from the schema
Presets — named bundles: rainbow:balanced, rainbow:minimal, rainbow:black, rainbow:prettier and rainbow:styleguide (team presets published as packages are planned)
.editorconfig (mapped to core options)
Project config: any file listed under File formats — nearest directory wins; extends combines files (a target may be in a different format)
[[override]] sections whose files patterns match, in order (later sections win)
Inline directives in the source (rainbow: set, for one member)
CLI flags (--set language.python.bracket_wrap=fit)

A higher level always wins, whatever section it is written in. Within a level, [language.X] beats [shared] beats [core] for the same concept. For example, an override's core.indent_size beats the project's [language.json] indent_size.

Presets¶

preset = "rainbow:balanced"                       # or a list; later presets win

Preset	Sets
`rainbow:balanced`	`core.max_width = 100`, `core.indent_size = 2`, `core.line_ending = "lf"`, `shared.trailing_comma = "never"`, `language.json.object_wrap = "fit"`
`rainbow:minimal`	`core.max_width = 120`, `core.line_ending = "preserve"`, `language.json.object_wrap = "preserve"`
`rainbow:black`	Black's layout where an option reaches it: 88 columns, 4 spaces, `trailing_comma = "multiline"`, the magic trailing comma, blank lines enforced; strings, parentheses and docstrings stay as written, unlike Black
`rainbow:prettier`	Prettier's defaults where an option reaches them: 80 columns, 2 spaces, semicolons, `bracket_spacing`, `object_wrap = "preserve"`, `void_elements = "slash"`; quotes and parentheses stay as written, and a line Prettier would break inside a tag stays long
`rainbow:styleguide`	This repository's `STYLEGUIDE.md` (how it was derived): 4 spaces, 99 columns (120 for HTML and Svelte), `trailing_comma = "multiline"`, blank lines between the thoughts of a function, aligned docstrings, `bracket_hug`, semicolons only after a `return` that is not last, `void_elements = "no_slash"`; Django `migrations` directories excluded

Presets are configuration files shipped with rainbow-fmt (rainbow_fmt/presets/); their contents grow with the option set. No preset applies unless the configuration names one (ADR 0006). preset is an ordinary top-level setting, so it passes through extends; a preset cannot itself name presets, and only the built-in rainbow: names are accepted.

`.editorconfig`¶

For each file, .editorconfig files are read from the file's directory upwards, stopping after one with root = true; sections whose glob matches the file apply, later sections and nearer files winning (EditorConfig semantics, including **, {a,b} and {1..3} in globs).

EditorConfig property	Option
`indent_style`	`core.indent_style`
`indent_size`	`core.indent_size` (`tab` is ignored)
`tab_width`	`core.tab_width`
`max_line_length`	`core.max_width` (`off` is ignored)
`end_of_line`	`core.line_ending` (`cr` is ignored)

Invalid values are ignored, as the EditorConfig specification requires, and unset removes an earlier value. .editorconfig is below the project configuration: a value in rainbow.toml wins. Turn it off with [files] editorconfig = false.

`[[override]]`¶

[[override]]
files = ["legacy/**/*.json", "*.jsonc"]
core.indent_size = 2
language.json.object_wrap = "fit"

files is required. Patterns are matched against the file's path relative to the directory of the configuration file that contains the override: * matches within one directory, ? one character, [abc] one of a set ([!abc] none of it), and ** any number of directories. A pattern without / matches the file name in any directory. An override may contain core, shared and language sections.

`--set`¶

rainbow-fmt format --set core.max_width=100 --set shared.trailing_comma=multiline . sets options for every file, above all configuration files. The value is a TOML value; a bare word is a string. [files] settings cannot be set this way (use -u for ignore_unknown, --no-verify for verify, --no-cache for cache, --jobs for jobs).

Seeing the result¶

rainbow-fmt options FILE prints the options that apply to FILE and where each value came from (cli.md).

Inline directives¶

Comments in the source can switch formatting off, skip one member, or change options for one member. They use each language's comment syntax (#, //, /* … */):

# rainbow: off
MATRIX = [
    1, 0, 0,
    0, 1, 0,
    0, 0, 1,
]
# rainbow: on

x = compute( )  # rainbow: skip-line

# rainbow: set trailing_comma=never
def f(a, b, c): ...

Directive	Where	Effect
`rainbow: off` … `rainbow: on`	own lines	everything between is printed as written; without `on`, to the end of the block, object or list
`rainbow: skip-next`	own line	the next member is printed as written
`rainbow: skip-line`	after code	the member it follows is printed as written (for a Python `if`, `def` …: its header)
`rainbow: set KEY=VALUE …`	own line	the next member is formatted with these option values

A member is a Python, JavaScript or TypeScript statement, a class member, an interface or type literal member, or an item of a bracketed list, a JSON object or array member (or the document's value), a CSS rule, at-rule or declaration. KEY is an option name without its section (trailing_comma, bracket_wrap); VALUE is written as for --set. Options the printer applies to the whole file (max_width, indent_style, indent_size, tab_width, line_ending) cannot be set by a directive. Python and JavaScript statements printed as written move as a whole to their block's indentation; their relative indentation, multi-line strings, template literals and JSX are kept.

Other tools' comments are honoured too: Black's # fmt: off, # fmt: on and # fmt: skip (like skip-line) in Python, and Prettier's prettier-ignore (like skip-next) in JSON, CSS, JavaScript and TypeScript. Where they cannot apply, they are ordinary comments.

A rainbow: comment that is not a valid directive (# rainbow: of; a comment of several words whose first is no directive, such as # rainbow: a formatter, is prose and left alone), is on the wrong kind of line, has nothing to act on (skip-next at the end of a block, on without off) or sits where no member is (inside an expression) is an error: the file is left unchanged and the error names its line and column:

error: app.py:12:5: unknown directive 'rainbow: of', not formatted

`preserve`¶

preserve means: for this decision, reproduce what the source did. It is implemented per option by consulting the trivia and tokens recorded in the CST. Examples:

quotes = "preserve" keeps each string's original quote character.
trailing_comma = "preserve" keeps a trailing comma if and only if one was present.
line_breaks = "preserve" treats every source newline inside a construct as a hard break, and never joins lines.
blank_lines.* = "preserve" keeps the original count (still capped by blank_lines.max, if set).

Provenance and `explain`¶

Every resolved value records its origin: the default, a preset, an .editorconfig line and section, a configuration position (and the override[N] it belongs to), or --set. rainbow-fmt options FILE shows them today; explain (planned) will show them per source position:

$ rainbow-fmt explain src/app.py:42
line 42, col 5  string literal
  quotes = "double"
    set by  rainbow.toml:14  [language.python]
    overrides [shared] quotes = "single" (rainbow.toml:9)
  rule    python/string  (rainbow_fmt.languages.python.format:88)