On Oct 27, 2015, at 11:05 AM, Matthew Turk <> wrote:

Hi Britton,

Yes -- in fact, I am going to try adding it to your PR.  Is there a
notebook or script that demonstrates how to make the setup fail as-is?

On Tue, Oct 27, 2015 at 10:04 AM, Britton Smith <> wrote:
Hi Matt,

If we could do that, it would be the cleanest and most effective solution.
When you have time, can you lay out how this can be done?


On Tue, Oct 27, 2015 at 2:56 PM, Matthew Turk <> wrote:

Hi Britton,

I've thought about this a bit, and I think this can be addressed.  In
the ytdata frontend, we should be able to make it recognize the index
fields; if we can't, we are overdue for having coordinate handlers
check things out.  There should be logic that prevents derived fields
overriding on-disk fields, so if there is an on-disk field that
presents "index" it should work.

On Tue, Oct 27, 2015 at 9:35 AM, Britton Smith <>
Hi all,

I have spent a while looking at the issue involving the derived
and I have come up with a solution for some of them, but not all of
them.  I
have pushed a couple changes to the PR that add a cell_volume and
field specific to YTDataContainerDatasets that makes use of their "dx",
"dy", and "dz" fields.  As of now, the following derived quantities
- min location
- max location
- extrema
- bulk velocity
- total mass
- weighted average quantity
- weighted variance
- total quantity

The ones that don't work are:
- spin parameter
- center of mass
- angular momentum vector

The final three all don't work for the same reason, which is that they
require access to the "index" position fields.  Unfortunately, these
be overridden for reasons that have to do with the _determine_fields
function.  I have made a point during this development process to alter
little as possible outside of the ytdata frontend.  However, at this
there is no more room to maneuver.  The only solutions I can come up
are ugly and would make it very difficult to smoothly swap in a proper
hierarchy when we get around to creating one and doing this properly.

The last thing I can think of would be to swap in special derived
functions for this frontend only that use position fields appropriate
the frontend.  The functions themselves would be simple to implement,
because the derived quantity collection is added onto data container
and not dataset objects, I have no idea how one would do the swap.  For
similar reason, I don't know how to simply not allow these problem cases
be called, or to fail in a clean way for them.

If anyone has any ideas, I would love to hear them.


On Mon, Oct 26, 2015 at 10:12 PM, John ZuHone <> wrote:

No worries—the main issues now are the tests (which don’t run under
2) and Python 3.

Tomorrow I’ll point you at the head that I’ve been making changes on to
address these.

On Oct 26, 2015, at 6:09 PM, Britton Smith <>

Hi John,

At the PR hangout, we discussed the possibility of eventually creating
method to extract subsets of AMR hierarchies that would include only
patches that are at least partially contained within the data
With them, we would also include the mask arrays to be applied to each
patch such that you would only get the cells within the container.
this was deemed something that should not hold up this PR, but that
eventually subbed in to fill in any gaps in functionality created by
treating the grid data like particles.  We decided that patching the
existing machinery with a frontend-specific cell_volume field was the
thing as it would get this on front of users faster, especially since
contains a lot of other things that are fully working.  I may have
some important parts of the discussion, but my memory faded a lot
than I thought it would.

I probably won't have much time to work on this until Friday, but I'll
to play around with the derived quantities when I can.


On Mon, Oct 26, 2015 at 9:45 PM, John ZuHone <> wrote:

Hi all,

Matt asked someone to send a quick note about the current issues
regarding the ytdata frontend PR:

I’m sending it because I’m working on it currently, but I was not able
attend today’s PR hangout so anyone else who was more to add or to
should chime in.

Currently, there are 3 issues, from most important to least important:

1. Tests don’t work
2. Doesn’t support Python 3
3. Derived quantities don’t work

I’m currently working on 1 and 2, and there have been some ideas about
The main issue with the latter is that for non-gridded data containers
sphere, disk, etc.) created from grid datasets, we treat the new
datasets as
“particle data”. For that reason there is an issue with calculating
cell_volume properly. Without that, we cannot use the total_mass()
Given that we have the cell deltas in the dataset, we could have a
derived field for the cell_volume.

There are other issues with some of the other derived quantities
weighted_average_quantity returning the wrong units), but I haven’t
able to diagnose them yet.

I’m working on some changes that will need to be merged into Britton’s
fork eventually to address at least 1 and 2.



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