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.

Cheers,
Kacper

> On Tue, Nov 24, 2015 at 7:32 AM, Matthew Turk <matthewturk@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@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@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@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@lists.spacepope.org
>>>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> yt-users mailing list
>>>> yt-users@lists.spacepope.org
>>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>>
>>>
>>>
>>>
>>> --
>>> Cameron Hummels
>>> NSF Postdoctoral Fellow
>>> Department of Astronomy
>>> California Institute of Technology
>>> http://chummels.org
>>>
>>> _______________________________________________
>>> yt-users mailing list
>>> yt-users@lists.spacepope.org
>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>
>> _______________________________________________
>> yt-users mailing list
>> yt-users@lists.spacepope.org
>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>
>
>
>
>
>
> _______________________________________________
> yt-users mailing list
> yt-users@lists.spacepope.org
> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>

_______________________________________________
yt-users mailing list
yt-users@lists.spacepope.org
http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org