[yt-dev] Issue #804: Querying for index properties before the index is generated fails (yt_analysis/yt)

[Nathan Goldbaum]

New issue 804: Querying for index properties before the index is generated fails
https://bitbucket.org/yt_analysis/yt/issue/804/querying-for-index-properties-before-the

Nathan Goldbaum:

The following IPython session illustrates the issue:


```
#!python

[goldbaum at hyades CenOstriker_feedback]$ yt load DD0002/DD0002
yt : [INFO     ] 2014-03-14 11:20:36,955 Parameters: current_time              = 0.00200008999
yt : [INFO     ] 2014-03-14 11:20:36,955 Parameters: domain_dimensions         = [64 64 64]
yt : [INFO     ] 2014-03-14 11:20:36,955 Parameters: domain_left_edge          = [ 0.  0.  0.]
yt : [INFO     ] 2014-03-14 11:20:36,956 Parameters: domain_right_edge         = [ 1.  1.  1.]
yt : [INFO     ] 2014-03-14 11:20:36,956 Parameters: cosmological_simulation   = 0.0
Python 2.7.6 |Anaconda 1.8.0 (64-bit)| (default, Nov 11 2013, 10:47:18) 
Type "copyright", "credits" or "license" for more information.

IPython 2.0.0-dev -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: ds.derived_field_list
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/home/goldbaum/yt-hg/yt/mods.pyc in <module>()
----> 1 ds.derived_field_list

AttributeError: 'EnzoDataset' object has no attribute 'derived_field_list'

In [2]: ds.index
yt : [INFO     ] 2014-03-14 11:20:56,557 Gathering a field list (this may take a moment.)
yt : [INFO     ] 2014-03-14 11:20:57,316 Loading field plugins.
yt : [INFO     ] 2014-03-14 11:20:57,317 Loaded angular_momentum (6 new fields)
yt : [INFO     ] 2014-03-14 11:20:57,317 Loaded astro (12 new fields)
yt : [INFO     ] 2014-03-14 11:20:57,317 Loaded cosmology (18 new fields)
yt : [INFO     ] 2014-03-14 11:20:57,318 Loaded fluid (53 new fields)
yt : [INFO     ] 2014-03-14 11:20:57,318 Loaded fluid_vector (84 new fields)
yt : [INFO     ] 2014-03-14 11:20:57,319 Loaded geometric (99 new fields)
yt : [INFO     ] 2014-03-14 11:20:57,320 Loaded local (99 new fields)
yt : [INFO     ] 2014-03-14 11:20:57,320 Loaded magnetic_field (105 new fields)
Out[2]: <yt.frontends.enzo.data_structures.EnzoHierarchy at 0x5485190>

In [3]: ds.derived_field_list 
Out[3]: 
[('all', 'ParticleMass'),
 ('all', 'ParticleMassMsun'),
 ** snip **

In [4]: 

```