Interoperability with existing FORCE_COLOR uses

[epage]

Just found out about this project (see astral-sh/uv#3955) which seems to be trying to document and standardize FORCE_COLORs behavior.

It seems like it would be helpful to specify the behavior to be compatible with major users.

Behaviors I've seen through a quick survey

• FORCE_COLOR is present and non-empty
  • https://force-color.org/
• FORCE_COLOR is present
  • (Python) https://docs.python.org/3.13/using/cmdline.html#controlling-color
  • (Python) https://github.com/Textualize/rich/blob/master/rich/console.py#L952-L956
  • (Python) https://github.com/wntrblm/nox/blob/main/nox/_options.py#L605
  • (Python) https://github.com/pytest-dev/pytest/blob/97ed533f63d5780a05702a711555cb6744247a37/src/_pytest/_io/terminalwriter.py#L26-L37
  • (Python) https://github.com/pypa/build/blob/main/src/build/__main__.py#L46-L53
• FORCE_COLOR is present and bool-ish
  • (Python) https://github.com/tox-dev/tox/blob/main/src/tox/config/cli/parser.py#L352-L361
  • (Python) https://github.com/python/mypy/blob/master/mypy/util.py#L552-L557
  • (JS) https://github.com/chalk/supports-color/blob/main/index.js#L33-L57
    • Note that higher numbers express certain capabilities of the terminal
  • (rust) https://github.com/zkat/supports-color/blob/d5c906d1fcd91cd1bf92d22f3d0cca452de8ca9f/src/lib.rs#L38-L43 (modeled after chalk.supports-color)
    • Note that higher numbers express certain capabilities of the terminal

[shadowspawn]

For reference, two major implementations I have looked at in node ecosystem and linked from force-color.org do not follow the description on force-color.org. The primary use in both cases is levels 0-3 with some other values explicitly supported. I think in practice other environment variables like CLICOLOR_FORCE and NO_COLOR have simpler patterns of usage than FORCE_COLOR for forcing color on/off.

The node implementation mentions being inspired by the supports_color package. Levels 1-3 correspond to 16 colours, 256 colours, and 16M colours.

FORCE_COLOR	result
supports_color
empty	on, level 1
'0'	off
'1', '2', '3'	on
'true'	on, level 1
'false'	off
other non-number	ignored
node
empty	on, level 1
'1', '2', '3'	on
'true'	on, level 1
other	off