[yt-dev] Issue #913: Field generation issue is breaking fit_spectrum.py (yt_analysis/yt)

[Nathan Goldbaum]

New issue 913: Field generation issue is breaking fit_spectrum.py
https://bitbucket.org/yt_analysis/yt/issue/913/field-generation-issue-is-breaking

Nathan Goldbaum:

>From the latest docs build (http://tests.yt-project.org/job/yt-docs-3.0/333/):


```
#!python traceback

Traceback (most recent call last):
  File "helper_scripts/run_recipes.py", line 75, in <module>
    RES.get()
  File "/home/xarth/yt-x86_64/lib/python2.7/multiprocessing/pool.py", line 558, in get
    raise self._value
Exception: Traceback (most recent call last):
  File "helper_scripts/run_recipes.py", line 40, in run_receipe
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/doc/source/cookbook/simple_slice_with_multiple_fields.py", line 8, in <module>
    width = (800.0, 'kpc')).save()
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/visualization/plot_window.py", line 1842, in SlicePlot
    return AxisAlignedSlicePlot(ds, normal, fields, *args, **kwargs)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/visualization/plot_window.py", line 1014, in __init__
    slc.get_data(fields)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/data_objects/data_containers.py", line 602, in get_data
    self.index._identify_base_chunk(self)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/data_objects/data_containers.py", line 129, in index
    self._index = self.ds.index
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/data_objects/static_output.py", line 279, in index
    self.create_field_info()
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/data_objects/static_output.py", line 334, in create_field_info
    self.field_info.load_all_plugins()
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/fields/field_info_container.py", line 228, in load_all_plugins
    self.find_dependencies(loaded)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/fields/field_info_container.py", line 241, in find_dependencies
    deps, unavailable = self.check_derived_fields(loaded)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/fields/field_info_container.py", line 310, in check_derived_fields
    fd = fi.get_dependencies(ds = self.ds)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/fields/derived_field.py", line 158, in get_dependencies
    e[self.name]
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/fields/field_detector.py", line 113, in __missing__
    vv = finfo(self)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/fields/derived_field.py", line 184, in __call__
    dd = self._function(self, data)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/doc/source/cookbook/fit_spectrum.py", line 12, in _OVI_number_density
    return data['H_number_density']*2.0
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/fields/field_detector.py", line 93, in __missing__
    finfo = self.ds._get_field_info(*field)
  File "/var/lib/jenkins/jobs/yt-docs-3.0/workspace/yt/data_objects/static_output.py", line 482, in _get_field_info
    raise YTFieldNotFound((ftype, fname), self)
YTFieldNotFound: Could not find field '('all', 'H_number_density')' in sloshing_nomag2_hdf5_plt_cnt_0150.
 in module: simple_slice_with_multiple_fields
```