[yt-users] YTFieldUnitError

Tue Nov 24 10:33:57 PST 2015

On Tue, Nov 24, 2015 at 9:10 AM, Kacper Kowalik <xarthisius.kk at gmail.com>
wrote:
>
>
> I'd be strongly -1 on "force_units". In that scenario you invoke several
> costly calls to sympy just to disregard the result. IMO you should drop
> units at the very beginning and create YTArray during return in the
> derived field's definition.
>

By this you mean something like:

def _temperature(field, data):
    tr = array_of_wrong_units
    data.apply_units(tr, 'K')
    return tr

ds.add_field('temperature', function=_temperature, units='K')

?

Or is the syntax wrong here?  I'm not really sure how to use this.

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