[yt-users] YTFieldUnitError

[Cameron Hummels]

Interesting.  I was totally unaware of this functionality, as it doesn't
appear to be documented.  I'd still be a fan of the force_units kwarg,
though.  But this may help for Elizabeth's current quandary.

On Tue, Nov 24, 2015 at 7:32 AM, Matthew Turk <matthewturk at gmail.com> wrote:

> If absolutely necessary, you can "force" units with:
>
> data.apply_units(array_to_override, unit_string)
>
> On Tue, Nov 24, 2015 at 9:31 AM, Cameron Hummels <chummels at gmail.com>
> wrote:
> > I would be a fan of making a "force_units" kwarg to enable one to create
> a
> > new field with different units than those specified.  This could resolve
> > some of the problems with native frontend fields being stuck in the wrong
> > unit, like I was running into last week.  As long as force_unit is off by
> > default, it would just enable us to give full control to the user.
> >
> > On Tue, Nov 24, 2015 at 4:25 AM, Nathan Goldbaum <nathan12343 at gmail.com>
> > wrote:
> >>
> >> Hi Elizabeth,
> >>
> >> This is discussed in the docs here:
> >>
> >>
> >>
> http://yt-project.org/doc/developing/creating_derived_fields.html#defining-a-new-field
> >>
> >> You can do one of two things. First, the way I'd handle this is to
> ensure
> >> that your field definition is returning data that have units of Kelvin.
> >>
> >> For example, something like:
> >>
> >>     from yt.utilities.physical_constants import kb, mh
> >>
> >>     mu = 0.6
> >>
> >>     @derived_field(name=‘Temp’, units=“K”)
> >>     def _Temp(field, data):
> >>         return data['thermal_energy'] / kb * mu * mh
> >>
> >> You could also make mu a field parameter rather than a constant, or if
> you
> >> have a simulation where mu varies with position, make a mean molecular
> >> weight derived field.
> >>
> >> You can also ensure that your field definition returning a dimensionless
> >> value by stripping the units:
> >>
> >>     @derived_field(name=‘Temp’, units=“K”)
> >>     def _Temp(field, data):
> >>         ret = data['thermal_energy'])
> >>         return ret.ndarray_view()
> >>
> >> We don't currently have a way to *force* the units to be whatever you
> >> specify in add_field. That said, I don't think it would be terribly
> hard to
> >> implement.
> >>
> >> We already have units='auto', which forces the units to be the same as
> the
> >> return value of your field function. We might add a keyword
> >> 'force_units=True' to add_field which does what you're looking for. I'd
> be
> >> happy to review a pull request adding this, or feel free to open an
> issue
> >> about it to add to our project backlog.
> >>
> >> Also, worth pointing out that this was designed this way not to anger
> >> users, but to protect them from making mistakes in their field units,
> which
> >> we found to be a common cause of bugs in yt-2. It does mean you need to
> "buy
> >> into" the unit system in your field definition, but the bonus is that
> you
> >> can be much more sure that you're not making a mistake.
> >>
> >> Hope that helps,
> >>
> >> Nathan
> >>
> >> On Tue, Nov 24, 2015 at 12:41 AM, Elizabeth Tasker
> >> <tasker at astro1.sci.hokudai.ac.jp> wrote:
> >>>
> >>> Hi everyone,
> >>>
> >>> While using yt-3, I created a derived field:
> >>>
> >>> @derived_field(name=‘Temp’, units=“K”)
> >>> def _Temp …..
> >>>
> >>> It crashes with a YTFieldUnitError because it believes the units should
> >>> be cm**2/s**2. I understand why yt thinks this — I’m using
> thermal_energy in
> >>> the definition for Temp. However, the units are truly Kelvin and I
> think I
> >>> should be able to specify whatever I like in “units”!
> >>>
> >>> Is there a way to stop yt crashing when it disagrees with your life
> >>> choices?
> >>>
> >>> Elizabeth
> >>> _______________________________________________
> >>> yt-users mailing list
> >>> yt-users at lists.spacepope.org
> >>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
> >>
> >>
> >>
> >> _______________________________________________
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> >>
> >
> >
> >
> > --
> > Cameron Hummels
> > NSF Postdoctoral Fellow
> > Department of Astronomy
> > California Institute of Technology
> > http://chummels.org
> >
> > _______________________________________________
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> >
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>



-- 
Cameron Hummels
NSF Postdoctoral Fellow
Department of Astronomy
California Institute of Technology
http://chummels.org
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