[yt-dev] Native field discussion

[Matthew Turk]

Hi Casey and Nathan,

Last night when I read this I felt a bit of dread in my stomach,
worrying that this would make things a lot more complex and difficult.
 But I have to say, now having thought it through and read your
pastes, reflected on the contents of the YTEP ... I think this is
beautiful and elegant.  Very nicely thought out.

I completely agree that -- particularly in 3.0 -- we no longer need to
perform any conversions at IO time, and instead we should be pushing
this to the places where the data will be presented to individual
users.  This will require some thought about where we want this
conversion to take place (i.e., at the display layer?) but it should
be completely workable to do so.  The ensure_cgs decorator is a great
way to manage this.

I'm +1 on this, and can help with the technical details.  As it
stands, all unit conversions happen in a single place in the code, so
we can simply remove the items there and instead just wrap them in the
appropriate "code" units.  We'll need to figure out the specific
semantics of that but it should be okay.  How do we handle the fact
that converting "code_density" to cgs is different for different
arrays, depending on the code?  i.e., if I have two "code_density"
arrays from a code that stores them in comoving code units, how do we
distinguish between those two things during division, say?

-Matt

On Mon, May 6, 2013 at 6:50 PM, Casey W. Stark <caseywstark at gmail.com> wrote:
> Hey Nathan.
>
> Thanks for the reminder about `ensure_cgs`. Definitely helps to see the
> pseudocode too. Users could also write out the array.convert_to(...) in the
> field function for whatever obscure units. I guess this is similar to the
> fields with the units tacked on to the name, but much cleaner.
>
> I think making the code to universal field mapping more explicit will be
> very helpful. I've been confused about where some derived fields come from
> many times!
>
>
> On Mon, May 6, 2013 at 3:39 PM, Nathan Goldbaum <goldbaum at ucolick.org>
> wrote:
>>
>> Hey Casey,
>>
>> That's pretty much what I was thinking, although I wanted to add a little
>> bit of syntactic sugar so the CGS conversion happens automatically:
>>
>> http://paste.yt-project.org/show/3443/
>>
>> The ensure_cgs decorator would simply call convert_to_cgs on the returned
>> YTArray.  The nice thing about this is the flow of data is clear, things
>> come in to the field definition code units and leave in CGS.  This also
>> eliminates the need for the unit conversion functions that are scattered
>> throughout the frontends and universal_fields.py.
>>
>> This has the nice feature that each of the code frontends will need to
>> explicitly define mappings to the CGS fields (like Density) needed by
>> universal fields.
>>
>> -Nathan
>>
>>
>> On Mon, May 6, 2013 at 3:33 PM, Casey W. Stark <caseywstark at gmail.com>
>> wrote:
>>>
>>> Hey yt.
>>>
>>> Nathan and I had a cool idea as an extension of the units work, but I am
>>> hitting a yt internals knowledge wall writing a YTEP. I would like some
>>> input on technical details or general feelings before I move ahead with the
>>> write-up.
>>>
>>> One of the things mentioned in the units YTEP is handling code units.
>>> Basically each frontend static output should register the code units. In Nyx
>>> for instance, we would do something like registering "code_length" as Mpccm,
>>> "code_mass" as Msun, etc. Then any field from the dataset can be loaded in
>>> code units with something like dd["density"].in_units("code_density").
>>>
>>> This got us thinking -- why load native fields in CGS now? Instead, we
>>> can define native fields exactly as they are on disk and users who want code
>>> units will never have to convert by hand again. If frontend developers
>>> define the fields and units on disk, and some mapping to the universal
>>> fields, we shouldn't have to define or convert units anywhere else.
>>>
>>> I made a quick and dirty example of what we're thinking here:
>>> http://paste.yt-project.org/show/3441/
>>>
>>> So, is something like this a good idea? Is it realistic? Would this make
>>> for a simpler field info container? Nathan, is there anything I missed?
>>>
>>> - Casey
>>>
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>>
>>
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