Default cache directory should respect XDG Base Directory Specification.

[gerritholl]

Feature Request

Is your feature request related to a problem? Please describe.

As a default cache directory, satpy uses '.':

satpy/satpy/resample.py

Lines 426 to 432 in a18b6e2

	def _create_cache_filename(self, cache_dir=None, prefix='',
	fmt='.zarr', **kwargs):
	"""Create filename for the cached resampling parameters."""
	cache_dir = cache_dir or '.'
	hash_str = self.get_hash(**kwargs)
	return os.path.join(cache_dir, prefix + hash_str + fmt)

This is potentially problematic: the current directory may be unwriteable or otherwise a poor choice to create files.

Describe the solution you'd like

I would like for satpy to respect the XDG Base Directory Specification and use the $XDG_CACHE_HOME directory (probably $XDG_CACHE_HOME/satpy) as a cache directory if none is specified.

Describe any changes to existing user workflow

Users who have $XDG_CACHE_HOME set will find cache files where they previously didn't.

Simply taking $XDG_CACHE_HOME will not introduce additional dependencies. Satpy could consider to use appdirs.user_cache_dir("satpy") using the appdirs package. This solution would add appdirs as a satpy dependency.

Additional context

I can pass cache_dir=appdirs.user_cache_dir("satpy") explicitly to .resample(...)

[djhoese]

Pyspectral is already using appdirs and if we are going to follow any specification it needs to work/apply to all three of the major platforms we support (linux, mac, windows) so I vote for appdirs if we have to.

Side question: Satpy also defaults to putting output files (from Scene.save_dataset/s) in the current directory. Do you see us needing to follow a different standard practice/specification for this as well?

[gerritholl]

I'm not aware of an existing standard for where to put output files. I personally have a small utility

def plotdir(basedir=None, create=False):
    pd = (pathlib.Path(
            (basedir or
             os.environ.get("PLOT_BASEDIR") or
             "/media/nas/x21308/plots_and_maps/")) /
          datetime.datetime.now().strftime("%Y/%m/%d"))
    if create:
        pd.mkdir(parents=True, exist_ok=True)
    return pd

that I pass when I'm creating plots or maps, but that's a bit too specific for my personal taste (in particular the sorting into the current date) to suit as a generic default. For logfiles I deliberately don't use appdirs.user_log_dir() because I don't agree that logfiles should be in the cache directory (I want my logfiles to persist).

[djhoese]

Although appdirs isn't necessarily a standard, it at least tries to use semi-standard directories for things. I agree that the log thing is a little odd (issue on their github?).

For output files, I think the current directory is "good enough" for now and pretty expected by most people.

[djhoese]

I realized this today, Satpy doesn't cache anything by default does it? All of the uses of cache_dir in resampling are triggered by the user passing an actual cache_dir. So the real issue here is that that _create_cache_filename shouldn't have cache_dir be a keyword argument at all. It should be a required positional argument.

@pnuu @mraspaud Am I missing a use case of this function?

[pnuu]

Nothing is cached by default in satpy.resample. And indeed, cache_dir should be a normal argument in _create_cache_filename, and the cache_dir = cache_dir or '.' line removed.