[yt-dev] YT field detection

[Matthew Turk]

Hi Doug,

Thanks for going through and looking into this.

On Fri, Aug 29, 2014 at 3:07 PM, Douglas Harvey Rudd <drudd at uchicago.edu> wrote:
> Hi all
>
> I have a set of fixes for Issue #887 (field detection). Most apply without breaking any tests, but to address all of the problems in that issue I need to add a check to YTDataContainer._determine_fields that doesn't allow a field to be added if it doesn't exist in field_list or derived_field_list.

That sounds good to me.

>
> This fixes my bug, but breaks a test based on code in yt/visualization/volume_rendering/camera.py which adds a field "temp_weightfield" (not a tuple!).

Whoops, that should be a tuple!  :)  Does it use ds.add_field?  That
should ensure it gets added to the derived_field_list.

>
> When this field is passed to _determine_fields, _get_field_info first guesses at ("gas","temp_weightfield"), but since that doesn't exist in field_info, it replaces it with ("index","temp_weightfield"), which I don't see as being any better.

I would slightly prefer "index" over "gas" just because "index"
indicates it's an internal field.

>
> There are several places where the syntax _get_field_info(*field) is used, which will break when field is a string, not a tuple.  Should I fix camera.py to use an explicit fluid type?  If so, what?  I've tried to use the fluid type of the input field, but that apparently is also not always a tuple.

I think fixing camera.py is good, and I would go with "index".

>
> e.g.
>         fields = [field]
>         if self.weight is not None:
>             self.weightfield = (field[0], "temp_weightfield")
>             # This is a temporary field, which we will remove at the end.
>             def _make_wf(f, w):
>                 def temp_weightfield(a, b):
>                     tr = b[f].astype("float64") * b[w]
>                     return b.apply_units(tr, a.units)
>                     return tr
>                 return temp_weightfield
>             ds.field_info.add_field(self.weightfield,
>                 function=_make_wf(self.field, self.weight))
>             # Now we have to tell the dataset to add it and to calculate
>             # its dependencies..
>             deps, _ = ds.field_info.check_derived_fields([self.weightfield])
>             ds.field_dependencies.update(deps)
>             fields = [self.weightfield, self.weight]
>
> Douglas Rudd
> Scientific Computing Consultant
> Research Computing Center
> drudd at uchicago.edu
>
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