Description

This ticket does a few things:

• Attempts to synthesize various open tickets related to configuration system woes
• Proposes adding support for ASTROPY_CONFIG_DIR and ASTROPY_CACHE_DIR environment variables, allowing total customization of these paths by users if desired
• De-emphasizes use of XDG_ environment variables for configuring where Astropy-related files go, as it is a somewhat system-biased solution and could cause problems for other apps when used by unwary users (while still maintaining support for the XDG directory specification and documenting it in more "intermediate" config documentation)
• Ditto implementation of Consider using appdirs for config #8245 to vendor the appdirs module rather than maintaining our own config path code (or at least a more minimal amount).

I believe this proposal would, in one stroke, resolve:

• XDG_CONFIG_HOME failing silently caused confusion #6511
• Why doesn't astropy use ~/.config/astropy by default (on Unix-like systems)? #6803
• Use user-specified custom cache directory #10244

and maybe some others. Also related is #10090 which I don't think is in conflict with this proposal--it would still just suppress loading any config file. Originally I thought of incorporating #8245 into this, but upon further reflection I think it would be counter-productive.

Detailed proposal

Much of this proposal is already in line with the status quo, but I wanted to write out the logic clearly:

1. By default all Astropy-related directories remain under the path ~/.astropy/. This includes ~/.astropy/config/ and ~/.astropy/cache; this is the current status quo, and is applicable to all platforms, where its platform-specific expansion depends on how os.path.expanduser works on that platform (see also MNT: Change config home path logic #10705)

2. Astropy will honor the XDG base directory specification if and only if XDG_CONFIG_HOME and/or XDG_CACHE_HOME are explicitly set in the environment. Otherwise it falls back the the default behavior in 1. If either one of the XDG_*_HOME variables is set we commit to the XDG specification. For example if XDG_CONFIG_HOME is set, then the astropy config dir is $XDG_CONFIG_HOME/astropy, and the astropy cache dir is ~/.cache/astropy and not ~/.astropy/cache.

   • Use of XDG_* environment variables will be de-emphasized in the docs, though this logic will be made clear in an admonition box for Linux/BSD users.

3. If ASTROPY_CONFIG_DIR is set, it overrides both 1. and 2. For example setting ASTROPY_CONFIG_DIR=/etc/astropy fixes that path and overrides any other processing. Same for ASTROPY_CACHE_DIR. These variables can both be used independently.

   • These will be documented as the recommended way to change astropy's default directories.

4. Suppress config loading with ASTROPY_SUPPRESS_CONFIG #10090 is maintained within this scheme as well. If ASTROPY_SUPPRESS_CONFIG is set then all config file loading is suppressed, overriding all of 1., 2., and 3.