Changing config items at runtime is broken

[bsipocz]

Changing config values at runtime should be possible, yet it's not working: https://docs.astropy.org/en/stable/config/index.html#changing-values-at-runtime

>>> from astroquery.simbad import conf
>>> conf.server='simbad.harvard.edu'
>>> from astroquery.simbad import Simbad
>>> Simbad.SIMBAD_URL
'http://simbad.u-strasbg.fr/simbad/sim-script'

[bsipocz]

cc @saimn and @pllim if you could spot something inherently wrong in our usage, as I recall you have been working on the config system most recently.

[eerovaher]

@eteq has already provided an explanation of the problem together with an outline of a solution in #544. See also https://docs.astropy.org/en/stable/config/index.html#usage-tips

[pllim]

Indeed. As you can see here:

astroquery/astroquery/simbad/core.py

Line 274 in 08e57d7

SIMBAD_URL = 'http://' + conf.server + '/simbad/sim-script'

SIMBAD_URL is a class variable, which means it grabs the config value pretty much at import time (I think) and it does not update when the config changes. If you really want people to be able to change SIMBAD_URL at runtime, it should not be a class variable but a config item itself.

p.s. Or maybe a @property that re-evaluates at runtime.

[bsipocz]

Hmm. I hoped that delaying the import of the class does the trick, but you're absolutely right that it shouldn't.

This brings us back to the same discussion we had recently of what should be on the class and what on the instance level. Maybe even the URLs should be instance, at least when there is an option to choose between the mirrors?

Do you see any downside of having them as a property instead?

OTOH, I would be down for the idea of not allowing changing the mirror at runtime but only doing that at the config level.

[pllim]

Since Python does not have true "private" attributes, personally, I think property vs config is just a matter of preference.

If this Simbad server is used throughout astroquery, I think it makes more sense as config. But if it is just used inside this one class, I am not sure if it matters much. But then again, I am unfamiliar with all the possible use cases of astroquery objects.

[bsipocz]

Again, a very good point. Not much atm, but indeed the simbad module is cross used in other modules so for that use case the config system has to be used and there is no runtime way to change the url.

[eerovaher]

The astropy config system includes features like a built-in context manager for temporarily changing config item values (very useful for testing) and input validation. For these reasons I would prefer to use the astropy config system as much as possible. Not all users can be expected to be familiar with it, so there should also be some other way of specifying values at runtime, whether it be through function arguments or class or instance attributes. But using the config system at runtime should work in all the subpackages. #2292 provides an example of fixing that without breaking the current API.

[ManonMarchand]

Hello,

I wanted to add a method to switch between the two simbad mirrors. How do I get the second one from the configuration of astropy? So far, I have this:

>>> from astroquery.simbad import conf
>>> next(conf.items())
('server', <ConfigItem: name='server' value='simbad.cds.unistra.fr' at 0x7f1b2c26a9d0>)

And I was wondering how I could access the url of the other mirror from that? Am I looking in the wrong place?