Brian preference files don't allow setting perferences registered in externel packages

[denisalevi]

During the import of brian2, the validity of preferences set in preference files is checked. At time of the brian2 import, any external package that defines a new preference (e.g. brian2cuda or brian2genn) will not be imported yet. If the preference files sets a preference registered in the external package, one gets a Preference cateogry ... is unregistered error.

I think it would be nice to support preference files brian2cuda / brian2genn as well. One solution would be to move the preference check from brian2/only.py into the first run call. I'm not sure if this would break things though if one relies on changing a preference between defining brian objects (if that is possible)?

Alternatively, one could implement a way that brian2 is imported without the check and have brian2cuda / brian2genn be imported before brian2 (they would internally still import brian2 but without the preference check). When brian2 is imported in the main script, the check would run, but external preferences would be registered already.

[denisalevi]

Or just hard-code that prefs.devices.cuda_standalone and prefs.devices.genn are excluded from the check and let the packages handle checking those preference in preference files? Probably the easiest solution. And adding any addition package that defines preferences in the future shouldn't be a problem (and will likely occur not so often :P).

[mstimberg]

Hmm, this is a very good point. I think this merits a bit of thought, since it is not the only thing in the preference system that is suboptimal. For example, some preferences are only checked during import (e.g. the logging configuration) and changing them later does not have any effect. I would prefer not adding a workaround on top of our system at this time.

What we could do as a temporary solution in brian2genn and brian2cuda is to allow the user to store their preferences in separate files and add something like this to the end of their __init__.py (when the preferences are already registered):

from brian2 import prefs
user_prefs = os.path.join(os.path.expanduser('~'),
                          '.brian/brian2cuda_preferences')
cur_prefs = 'brian2cuda_preferences'
files = [user_prefs, cur_prefs]
for file in files:
    try:
        prefs.read_preference_file(file)
    except IOError:
        pass