[yt-dev] Adding new derived fields

[Matthew Turk]

On Jan 23, 2014 5:36 PM, "Nathan Goldbaum" <nathan12343 at gmail.com> wrote:
>
> 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?
>

Yes.

>
> 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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