[yt-users] YTFieldUnitError

[Nathan Goldbaum]

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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