On Tue, Aug 21, 2018 at 8:12 AM Matthew Turk <matthewturk@gmail.com> wrote:
Hi Corentin,

Great discussion.

On Tue, Aug 21, 2018 at 4:34 AM Corentin CADIOU <corentin.cadiou@iap.fr> wrote:
> Hello,
> As of today, there is no way to configure yt plots per-field. One is stuck with configuration that are yt-wide (e.g. default_colormap). For example, I often find myself typing p.set_cmap('density', 'dusk'). I suggest to add a way to configure the behaviour of yt plotting interface per-field using yt configuration, so that I could set once and for all some defaults for plots. I pushed a first PR on github (https://github.com/yt-project/yt/pull/1931) and opened an issue (https://github.com/yt-project/yt/issues/1888) to discuss the feature.
> Nathan and I discussed a bit about how this should be implemented in the configuration file. Since ini files are quite restricted (e.g. you cannot have subsections), Nathan suggested at one point to use the toml format (https://github.com/toml-lang/toml). However, it is incompatible with the older .ini format. For example, boolean should be in lowercase (true, false) and not Python-like (True, False), strings have to be surrounded by apostrophes, ...

Taking a step back, I suspect that the following are both true:

1) Folks who have been using yt for a lot longer are more likely to
have configuration files scattered about
2) The existing config files don't actually do *that* much

We've also migrated in the past, and could probably implement that
again.  (Kacper did it the first time.) 

This is referring to "yt config migrate" which you should take a look at. We could definitely overload the existing command for this purpose.

I think using the standard library configparser module you could first load the ini-style file, inspect the state of the resulting ConfigParser object, convert the data to a dict, and pass that dict to a toml writer.
So I don't think that this
should be that big of a barrier; we could also consider the idea of
having a field configuration toml file that sits next to the ini file.
Do we need new dependencies if we have toml?

We'd probably want to depend on either this:


which seems to be the most popular library or this:


Which seems to be what pip has vendored and I trust dstufft's opinion on these things...

Another option would be to vender one of those libraries but I don't think that's necessary anymore given the state of the packaging ecosystem and that both of these are pure python.

> Another suggestion is to add each fields' configuration as a new section in the ini file. For example
> [config.field_type.field_name]
> cmap = viridis
> log = False
> This is what is implemented in the PR, but maybe some of you have more clever ideas about this? One fairly strong constraint is that the configuration files should be as backward-compatible as possible. If not, we'd have to write a migration script from the old to the new format.

Or have them be in a separate file, which might even make sense -- and
would be consistent with the idea that we may eventually want to
override these by frontend.

This is starting to get close to my ideas for field_day. I was also hoping to use toml there...

But yes, it would be good to use whatever syntax we come up with here for frontend-specific overrides.

> We should also discuss what can be configured here. There is already the colormap and log scale implemented. One very neat feature would be to be able to configure the default unit, but it may be hard to implement (e.g. how do we configure the default unit for both projection and slices?) and probably confusing for the user.

Definitely default unit for non-integrated quantities!  I think for
integrated quantities like projections what we could do is configure
the default integration unit, and that would combine with the default
unit to produce the final result.

Yup, and we need to be clear to document this use case since it'll come up a lot.

> So here are the questions. Do you think per-field configuration should be part of yt? If so what format should it use? And what are your thoughts on what should and shouldn't be configurable?

1) Yes
2) I don't particularly mind toml, but I'd rather we break hard if
we're going to break from INI-style files.  A second file would make
sense to me.

I agree that this is a worthwhile improvement. I would also like to move to a richer configuration format than ini-style files. ConfigParser is also a pain to deal with.

That said, unless you also want to systematically update everywhere we use the configuration system in yt (might be possible with some sed magic, so don't discount just updating everything) you'd also need to write an emulation layer or figure out how to generate a ConfigParser object from the toml file.

> Corentin
> --
> Corentin Cadiou
> PhD student
> Institut d'Astrophysique de Paris (IAP), desk 142b
> 98 bis boulevard Arago
> 75014 Paris, France
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