On Fri, Dec 19, 2014 at 10:14 AM, José Utreras <jutreras@das.uchile.cl> wrote:
Thanks for your answer John
I have to define sl_right, sl_center :global name 'sl_right' is not defined

I'm not really sure how to do it. I want to do something like these:

vx = data["gas", "velocity_x"][sl_right,sl_center,sl_center]
vx+= data["gas", "velocity_x"][sl_center,sl_right,sl_center]

Hi Jose,

This code looks like came from the definition of the vorticity field.  You're correct that ftype, sl_right, sl_center, and sl_left aren't defined inside the voriticity field definition.  That's because the voriticity field is a closure (a special type of python function that is able to use varibles defined in the enclosing scope around the function definition).  I'd encourage you to take a look at yt/fields/fluid_vector_fields.py to see how this works inside yt:

https://bitbucket.org/yt_analysis/yt/src/5007c9a46622118abab89dc153dbfb6591cc5177/yt/fields/fluid_vector_fields.py?at=yt
 
...

add the x velocity of 6 neighbour cells.


Your field definition will need a ValidateSpatial field validator (like vorticity). ValidateSpatial will make it so data is passed into a field definition along with a user-selectable number of ghost zones.

 
Thanks


2014-12-19 13:31 GMT-03:00 John ZuHone <jzuhone@gmail.com>:
Jose,

Presuming that you have sl_right and sl_center defined in your field already, you should be able to just substitute “gas” for ftype, since the velocity fields are “gas" fields.

Best,

John

> On Dec 19, 2014, at 11:04 AM, José Utreras <jutreras@das.uchile.cl> wrote:
>
> Hi everyone,
>
> I want to write a new field based on the neighbour cells like this:
>
> vx = data[ftype, "velocity_x"][sl_right,sl_center,sl_center]
>
> but I got " global name 'ftype' is not defined". I suppose that I have the same problem with sl_left, sl_right , sl_center.
>
> Thanks,
>
> Jose
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