[yt-dev] Issue #1165: angular_momentum_vector derived quantity fails for an ORION dataset (yt_analysis/yt)

[Nathan Goldbaum]

New issue 1165: angular_momentum_vector derived quantity fails for an ORION dataset
https://bitbucket.org/yt_analysis/yt/issues/1165/angular_momentum_vector-derived-quantity

Nathan Goldbaum:

This is a weird one.

With the following dataset (spread over three files):

http://use.yt/upload/5e496e7c
http://use.yt/upload/99731ae6
http://use.yt/upload/ac7409e1

and the following test script:


```
#!python

import yt

file = "data.0100.3d.hdf5"
ds = yt.load(file)
sp = ds.sphere("center", (0.1, 'pc'))

L = sp.quantities.angular_momentum_vector()

```

you will get the following error:


```
#!python

Traceback (most recent call last):
  File "test.py", line 7, in <module>
    L = sp.quantities.angular_momentum_vector()
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/derived_quantities.py", line 72, in __call__
    values[i].append(storage[key][i])
IndexError: list index out of range

```

When I step through in a debugger, it seems that the `parallel_objects` inside of the `DerivedQuantity` base class is not properly combining data from the two chunks that get processed. More specifically, the `parallel_passthrough` decorator on the `par_combine_objects` method is only returning data from the first chunk, not combining data from both chunks.

This is totally weird because it's very well tested, often-exercised code. I'm not terribly familiar with the internals of the parallel analysis interface and would appreciate another set of eyes here.

@brittonsmith @MatthewTurk maybe?