Allow override of configuration options via the CLI

[zanieb]

Currently, Ruff includes many configuration options that are only available via a configuration file. These options should be available via the CLI as well so users can either:

• Provide configuration options without defining a configuration file
• Override configuration options per invocation of Ruff without modifying files

There have been previous requests for this, but I could not find them. If you know where they're at please share so we can have more historical context here.

Proposals

There are a few options for exposing this capability.

Environment variables

Add environment variable overrides for all settings. Environment variables would take precedence over all persistent configuration options but not CLI flags.

A repeatable CLI option with key value pairs

A CLI option for overriding settings e.g. --setting <NAME>=<VALUE>

Alternative names include --settings, --override-setting, --override-config, --set-config, --configure, --override, --option, --options

Ruff already uses --config <CONFIG> to provide a path to a configuration file.
Cargo uses --config for both overriding options and providing the path to a configuration file.

A CLI option with JSON support

A CLI option for overriding settings in bulk e.g. --settings {...}

This has been requested for programmatic use of Ruff previously e.g. you can use jq to pass a configuration file in bulk. However, this is not a very user-friendly approach for most use cases.

Dedicated CLI options

A dedicated CLI option generated for each setting e.g. --set-<name> VALUE.

We would probably omit enumerating these in the help menus and document the pattern instead. We could omit the set- prefix but then we need to worry about collisions between configuration options and existing CLI flags. Additionally, this option does not work well with the <section>.<name> syntax used in our configuration.

[zanieb]

For what it's worth, I think that --config NAME=VALUE matching Cargo's design is reasonable.

[T-256]

Loved these proposals, here are examples and cons of each:

1. Environment variables

RUFF_FORMAT_LINE_LENGTH=120 ruff format .

cons:

• All words capitalized and - replaced by _
• Problem with <section>.<name> settings
• Long, less user friendly because they may be varied by different shells/OSs.

2. A repeatable CLI option with key value pairs (e.g. --config)

ruff format . --config line-length=120

cons:

• it's long and it may not preferable where e.g. --line-length already available to choose.

3. A CLI option with JSON support (e.g. --config)

ruff format . --config {"line-length":120}
ruff format . --config eyJsaW5lLWxlbmd0aCI6MTIwfQ==

cons:

• it's not clear it should merge with already loaded settings or reset it.
• it's not clear what should happen when using it multiple times.

4. Dedicated CLI options (without any prefixes)

ruff format . --line-length=120

cons:

• Collisions with existing CLI flags
• Problem with <section>.<name> settings

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I think 1, 2 and 3 can be supported at same time.
4 can be preferable over 2 when its conditions are available (no . in settings name).