[yt-dev] Adding new derived fields

[Nathan Goldbaum]

Yes, I think it would be impossible or at least very hard to support that
in general.

In the current system, the user can specify the field units in the
add_enzo_field call (for example).  Right now a user can do that in a
plugin file or the top of a script.  Would something similar be possible in
the new system?


On Thu, Jan 23, 2014 at 2:23 PM, Matthew Turk <matthewturk at gmail.com> wrote:

> On Thu, Jan 23, 2014 at 5:13 PM, Nathan Goldbaum <nathan12343 at gmail.com>
> wrote:
> > I think I have a use case that this design might make more difficult.
> >
> > Let's say I'm adding new fields to an existing code for some reason (like
> > radiation, chemistry, or MHD).  I'd like to make it so users can read in
> my
> > fields with yt and then make derived fields of their own using my fields.
>
> Yes, that's an ideal use case we need to support.
>
> >
> > Under your proposal, would I need to modify the yt source to be able to
> read
> > the fields in properly and assign them units?  If so, that seems a bit
> > awkward to me since there would need to be upstream changes to yt that
> only
> > make sense relative to someone's private fork of a hydro code.
>
> Hmm, reading in units for a field that yt does not know about, that is
> actually an outstanding problem with units that predates my changes to
> the field system.  I don't know what to do about it.
>
> >
> >
> > On Thu, Jan 23, 2014 at 1:34 PM, Matthew Turk <matthewturk at gmail.com>
> wrote:
> >>
> >> Hi all,
> >>
> >> The method for adding derived fields has somewhat changed in the
> >> unitrefactor of 3.0, and we have the opportunity to make it a bit
> >> easier to add new fields -- especially after a parameter file has been
> >> created, which previously has been a pain.  Below is a brief summary.
> >>
> >> The current way we do this is with:
> >>
> >> add_field(...)
> >>
> >> or
> >>
> >> @derived_field
> >> def some_function(...
> >>
> >> The field system is now plugin based, which means we load plugins for
> >> different things and fields are dynamically added.  One big advantage
> >> is that logic for things like strides and whatnot doesn't need to
> >> happen inside the field functions, but it also provides some
> >> compartmentalization of things.
> >>
> >> I haven't implemented @derived_field and add_field yet in this system,
> >> but my plan is to do so by creating a "custom" plugin, to which these
> >> will add field definitions, which will always get loaded.  But if you
> >> do:
> >>
> >> pf = ...
> >> pf.h.whatever...
> >>
> >> @derived_field
> >> def func(...)...
> >>
> >> currently, and in the proposed implementation, this will not let them
> >> be detected.  But I want to make it so that we *can* do this:
> >>
> >> pf.add_field( ... ) and @pf.derived_field, which will both dynamically
> >> add fields to existing parameter files.  What this means:
> >>
> >>  * This will not change behavior of @derived_field and add_field, in
> >> that they only apply to pfs instantiated *after* the call happens.
> >>  * There will be a way to dynamically add fields, with field
> >> dependencies and all, by using pf attributes.
> >>  * Adding fields *specific to an output type* will no longer be
> >> possible outside of field plugins.  This means there will be the
> >> ability to add pf-specific and universal fields dynamically in a
> >> script, but all fields that are specific to an output type will need
> >> to be added in the fields.py file for that frontend.  So this means
> >> that anything you might have used add_enzo_field or add_orion_field on
> >> *in your scripts* (not in anything inside yt/frontends/*/fields.py)
> >> will no longer work.
> >>
> >> I'm going to be implementing the uncontroversial part of this, which
> >> is the dynamic field adding to a "custom" plugin, and unless I hear
> >> otherwise I'll continue on this track with the dynamic pf field
> >> addition.
> >>
> >> -Matt
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> >
> >
> >
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