[yt-users] gradient field in yt3

[Yuan Li]

Hi all:

I am having trouble creating a gradient field in yt3. What I want is the
gradient of logP, where logP is defined as:

@derived_field(name="logP",units="")
def _logP(field,data):
    return numpy.log(numpy.array(data["gas","pressure"]))

Then I add the gradient field:

pf.add_gradient_fields(("gas","logP"))

which seems to work and I can find it in pf.derived_field_list.

But if I try to use it, I get this error message:
yt.units.unit_object.UnitParseError: Cannot parse for unit data from '()'.
Please supply an expression of only Unit, Symbol, Pow, and Mulobjects.

Is this because logP does not have a unit?

Also, I had another unrelated question about the gradient fields:
Can I add the gradient fields globally instead of to a specific dataset?


Thanks!
Yuan
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