[Yt-dev] Field definitions, derived fields, whats-in-a-file and the "deliberate_fields" branch

david collins antpuncher at gmail.com
Wed Nov 9 14:34:07 PST 2011 

> To be perfectly honest, you were in my list of top one people I thought would
> either love or hate this proposed change.

It wouldn't be so much "hate" as much as "gripe because my
non-standard code that worked with the 1987 vintage of yt doesn't work
with the changes you made in 1995." :D  I'm actually really impressed
that it all went so smoothly, that's a pretty major change with many
opportunities for bear traps to step in.

> Ah!  I think this is what TranslateFunc is designed for.  I think the way I
> envision it working is to choose a canonical name, maybe Bx, and then do:
> add_enzo_field("Bx", function=NullFunc)
> add_field("Bx", function=TranslateFunction("MagneticField_C_1"))

Is this now functioning?   I don't quite get the logic behind the
function calls here, why is Bx itself an "add_enzo_field" but the
translate is an add_field?

Thanks a ton,
d.

> Thank you.  :)  I have updated the pull request with the new changeset where
> con_args is fixed.
> Best,
> Matt
>
>>
>> Thanks a ton!
>> d.
>>
>> Trace for the first bug:
>>  File "go", line 20, in <module>
>>    from yt.mods import *
>>  File "/Users/dccollins/local/src/yt_turk/yt/mods.py", line 44, in
>> <module>
>>    from yt.data_objects.api import \
>>  File "/Users/dccollins/local/src/yt_turk/yt/data_objects/api.py",
>> line 31, in <module>
>>    from grid_patch import \
>>  File "/Users/dccollins/local/src/yt_turk/yt/data_objects/grid_patch.py",
>> line 35, in <module>
>>    from yt.data_objects.data_containers import YTFieldData
>>  File
>> "/Users/dccollins/local/src/yt_turk/yt/data_objects/data_containers.py",
>> line 3407
>>    _con_args = {"regions"}
>>                          ^
>> SyntaxError: invalid syntax
>>
>>
>>
>> On Wed, Nov 9, 2011 at 1:25 PM, j s oishi <jsoishi at gmail.com> wrote:
>> > Hi Matt,
>> >
>> > This sounds like a much needed overhaul. However, I'm not quite clear
>> > on exactly what this will entail, or how it will work once
>> > implemented. Could you or Casey provide an example of a new field or
>> > two, demonstrating how these dictionaries, fallbacks, and Null
>> > functions work? I think this is likely a very simple thing, but I'm
>> > having trouble visualizing it.
>> >
>> > thanks,
>> >
>> > j
>> >
>> > On Wed, Nov 9, 2011 at 9:05 AM, Matthew Turk <matthewturk at gmail.com>
>> > wrote:
>> >> Hi all,
>> >>
>> >> Over the last couple months, Casey and I have been working -- on and
>> >> off! -- on a new branch of the code called "deliberate_fields."  This
>> >> branch will change, in a substantial but easy-to-update way, how
>> >> fields are handled in yt.
>> >>
>> >> I recognize this email is long.  But if you use non-standard fields, a
>> >> bunch of derived fields, unit modifications, any of that, it may
>> >> affect you.  So I *please* ask that you read it and, if you like,
>> >> contribute back to the discussion.
>> >>
>> >> This is one of the items I really want to have done for a hypothetical
>> >> 2.3 release.
>> >>
>> >> = Background =
>> >>
>> >> The way fields work currently was designed a bit haphazardly.  They
>> >> use FieldInfoContainers, objects which share state and which contain
>> >> unions of the known derived fields and the known IO-based fields.  One
>> >> of the problems with this is that the only thing that separates a
>> >> derived field from a known field is that function that generates the
>> >> field: the IO-based fields all use a lambda which returns None, and
>> >> the non-IO based fields return actual fields.  This is pretty
>> >> sub-optimal, and it actually lands us in trouble when (for instance)
>> >> we have fields wandering around named things like "Thermal_Energy" and
>> >> "ThermalEnergy"; the mechanism by which one is selected and the other
>> >> not is problematic, and to get around infinite recursion, hacks have
>> >> had to be applied.
>> >>
>> >> As it stands, to find a field, the shared-state "field info" on a
>> >> parameter file is queried; this then will try to check universal
>> >> fields.  But because of how the fields are stored, the field info
>> >> cascade can also operate in reverse.  The big problem is that the
>> >> field selection mechanism doesn't seem to have a bus factor >= 1.0.
>> >> And, it has a number of hacks to make it work with conflicting field
>> >> definitiosn and the like.
>> >>
>> >> Unfortunately, layering these hacks on top of each other makes it much
>> >> harder for other codes to be supported; translations are not reliable,
>> >> and sometimes cause too many levels of recursion to be added.
>> >> Something simpler is necessary.
>> >>
>> >> = What this does =
>> >>
>> >> Essentially, this creates multi-level, explicit fallbacks.  The field
>> >> info container, which was a bloated, weird shared state object, is now
>> >> simply a dictionary subclass with a "fallback" option.  When you
>> >> create them, you can either create it in isolation (with no fallback)
>> >> or with a fallback.  When you query it, if it does not have a field,
>> >> it checks its fallback.  There are, additionally, two new functions
>> >> for IO: the translation function and the null function.  The first is
>> >> to translate, for instance, "density" to "Density" and the second is
>> >> to indicate that a field is expected to be found in an output from the
>> >> simulation code.
>> >>
>> >> There are now affiliated with each simulation code two field info
>> >> objects: the "known" fields, which may appear in files, and the
>> >> non-known (i.e., code-specific derived) fields.  These live as the
>> >> attributes _fieldinfo_fallback and _fieldinfo_known on the
>> >> StaticOutput sublcass corresponding to a simulation code.  When the
>> >> Hierarchy (not static output) is instantiated, the first step is to
>> >> create a new field_info object.  This has, as a fallback, the
>> >> _fieldinfo_fallback, which itself has as a fallback the
>> >> universally-known derived fields.  The hierarchy then queries the
>> >> output file for which fields are available.  This process then looks
>> >> for a corresponding field in fieldinfo_known, and if it finds it, it
>> >> adds it to the field_info object, *overriding* any possible derived
>> >> fields.  (In this manner, for instance, yt will not recalculate a
>> >> "CoolingTime" field if one exists in the output.)
>> >>
>> >> = What it aims to do in the future =
>> >>
>> >> This will be utilized in three main ways:
>> >>
>> >> 1) Making it more clear which fields belong to which code, and which
>> >> come from disk and which are derived
>> >> 2) Help move IO into fields, to optimize for geometries and data
>> >> containers
>> >> 3) Make units more clear and specific
>> >> 4) This is all designed around better supporting the GDF.
>> >>
>> >> = Where from here? =
>> >>
>> >> It would be hugely beneficial if you could test this and report back.
>> >> I have created a pull request:
>> >>
>> >>
>> >> https://bitbucket.org/yt_analysis/yt/pull-request/27/field-overhaul-to-utilize-explicit
>> >>
>> >> This is by no means a settled matter; I think we need to have testing
>> >> on this, buy-in from developers and users, and to make sure that old
>> >> code doesn't beak.  The test cases all pass for me for Enzo.
>> >>
>> >> Before this can be merged, I would hope we can get some testing from:
>> >>
>> >>  * Enzo
>> >>  * Nyx
>> >>  * FLASH
>> >>  * Orion
>> >>
>> >> and any other codes that can hear me.
>> >>
>> >> Thanks very much for your time; please let me know if you have any
>> >> questions, concerns, jokes, comments, improvements, CDs of your band,
>> >> suggestions, and so on.  For this major of a change I'd like to keep
>> >> discussion on list, so the record of this is a bit more prominent.
>> >>
>> >> Best,
>> >>
>> >> Matt
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>>
>>
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