[yt-dev] yt-3.0 derived fields

[Douglas Harvey Rudd]

Okay, I had noticed this thread but hadn't fully digested that I had hit the very same issue.

I'm +1 on Chris' change, for the exact same reason (ParticleMassMsun for star particle types).

Matt: I didn't quite follow your message.  I would assume most derived fields should be neutral 
to the type of particle, since they're typically unit conversions.  Having two particle types with the 
same field name that are not compatible should be forbidden, otherwise ("all", ) would be nonsensical.

Why is discriminating particles and fluids desirable here?  By the particle_ naming convention
for particle fields there shouldn't be any overlap.  I feel that I must have missed something.

In a similar vein, I've changed our GeometryHandler._detect_fields function to generate all valid 
(particle_type, field) pairs, so that ("nbody","creation_time") is not a valid field, for example, while 
("all","particle_mass"), ("nbody","particle_mass"), and ("stars","particle_mass") are all valid.  This 
has broken the known field support, however, since we currently only add naked field types, 
without particle types.  It would be a pain to need to add all valid pairs in fields.py as well.

Douglas Rudd
Scientific Computing Consultant
Research Computing Center
drudd at uchicago.edu



On Mar 10, 2013, at 7:21 PM, Matthew Turk <matthewturk at gmail.com> wrote:

> I'd rather that not work, to tell the truth.  I think Chris's
> suggestion, where you define a derived field that is neutral to the
> *type* of particle, is powerful, but I don't see an advantage to it
> during the call to getting the data, particularly since that loses a
> bit of ability to discriminate between particles/fluids.
> 
> On Sun, Mar 10, 2013 at 8:14 PM, Nathan Goldbaum <nathan12343 at gmail.com> wrote:
>> Just to clarify, dd[…, "ParticleMassMsun"] would work as well, right?
>> 
>> On Mar 10, 2013, at 5:05 PM, Christopher Moody <chrisemoody at gmail.com>
>> wrote:
>> 
>> Hi Sam,
>> Yeah; I think it should change only the derived field. You want the derived
>> field to pass along whatever field type you asked for, or use the implicit
>> one if you didn't ask for the type explicitly. If you tried to implement
>> this in the original call, you'd do something like
>> dd[(Ellipsis,"ParticleMassMsun")].   I can't see use cases where this would
>> be better than dd[("all","ParticleMassMsun")] -- so yeah, no changes to the
>> original call.
>> 
>> chris
>> 
>> 
>> On Sun, Mar 10, 2013 at 4:30 PM, Sam Skillman <samskillman at gmail.com> wrote:
>>> 
>>> Hi Chris,
>>> 
>>> Just to be clear, this change would occur in the derived field definition,
>>> not the call to get the star ParticleMassMsun itself, correct?
>>> 
>>> If so, I think something like this, or with a yt-specific term, would be
>>> great.
>>> 
>>> Thanks,
>>> Sam
>>> 
>>> 
>>> On Sun, Mar 10, 2013 at 5:20 PM, Matthew Turk <matthewturk at gmail.com>
>>> wrote:
>>>> 
>>>> Hi Chris,
>>>> 
>>>> On Sat, Mar 9, 2013 at 4:00 PM, Christopher Moody <chrisemoody at gmail.com>
>>>> wrote:
>>>>> Hi everyone,
>>>>> Fields in yt-3.0 are now defined like (field_type,field_name) but for
>>>>> backwards-compatibility still maintains a field_name ->
>>>>> ("all",field_name)
>>>>> mapping. Consider a few use cases:
>>>>> 
>>>>> keys = ["ParticleMassMsun",
>>>>>        ("all","ParticleMassMsun"),
>>>>>        ("stars","particle_mass"),
>>>>>        ("stars","ParticleMassMsun") ]#broken
>>>>> 
>>>>> I think we all have natural expectations for what should happen, but at
>>>>> the
>>>>> moment the last key in the series is broken. particle_mass is a native
>>>>> field, ParticleMassMsun is a derived field:
>>>>> 
>>>>> def _ParticleMassMsun(field, data):
>>>>>    return data["particle_mass"]/mass_sun_cgs
>>>>> add_field("ParticleMassMsun", function=_ParticleMassMsun,
>>>>> particle_type=True)
>>>>> 
>>>>> Note that in this case data["particle_mass"] maps to
>>>>> data[("all","particle_mass")] even when I originally asked for
>>>>> dd[("stars","ParticleMassMsun")], which is why things break.
>>>>> 
>>>>> So I propose a syntax where in the derived field definition we can do
>>>>> something like data[(Ellipsis,"ParticleMassMsun")] or alternatively
>>>>> data[(field.field_type,"ParticleMassMsun")]. This ensures that derived
>>>>> fields will work intuitively and be as specific to a field type as they
>>>>> want
>>>>> to be or just pass through whatever field type they may want.
>>>>> 
>>>> 
>>>> I like this, but maybe we should use a yt-specific object instead of
>>>> Ellipsis.  This would be very nice and is something I have thought /
>>>> worried about and been unable to come up with an elegant solution for.
>>>> Seems like you found one!  :)
>>>> 
>>>> This week I need to address this for my own work on a simulation with
>>>> multiple particle types; I'll take a pass at implementing something
>>>> like this.  We're also a bit overdue for a YTEP that details access to
>>>> multiple fluids and multiples particles.  Once some things get off my
>>>> plate on Tuesday I will make that a priority.  Then we can use that as
>>>> a place to sort of hash this out and how it'll work.
>>>> 
>>>> -Matt
>>>> 
>>>>> Thanks!
>>>>> chris
>>>>> 
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