[yt-dev] Issue #751: PhasePlot is broken in 3.0 (yt_analysis/yt)

[Nathan Goldbaum]

New issue 751: PhasePlot is broken in 3.0
https://bitbucket.org/yt_analysis/yt/issue/751/phaseplot-is-broken-in-30

Nathan Goldbaum:

I cannot create PhasePlots in 3.0.  The same command works fine in the current stable release.

Here is the traceback I'm seeing:


```
#!python traceback

TypeError                                 Traceback (most recent call last)
<ipython-input-7-db8ac28b5d75> in <module>()
----> 1 plot = PhasePlot(dd, "CellMassMsun", "Radius", "Height")

/home/goldbaum/yt-hg/yt/visualization/profile_plotter.pyc in __init__(self, data_source, x_field, y_field, z_fields, weight_field, x_bins, y_bins, accumulation, fractional, profile, fontsize, font_color, figure_size)
    543                weight_field = weight_field,
    544                accumulation=accumulation,
--> 545                fractional=fractional)
    546         self.profile = profile
    547         ImagePlotContainer.__init__(self, data_source, profile.field_data.keys(),

/home/goldbaum/yt-hg/yt/data_objects/profiles.pyc in create_profile(data_source, bin_fields, n, weight_field, fields, accumulation, fractional)
    998     setattr(obj, "fractional", fractional)
    999     if fields is not None:
-> 1000         obj.add_fields(fields)
   1001     for field in fields:
   1002         if fractional:

/home/goldbaum/yt-hg/yt/data_objects/profiles.pyc in add_fields(self, fields)
    758         fields = ensure_list(fields)
    759         temp_storage = ProfileFieldAccumulator(len(fields), self.size)
--> 760         for g in parallel_objects(self.data_source._grids):
    761             self._bin_grid(g, fields, temp_storage)
    762         self._finalize_storage(fields, temp_storage)

/home/goldbaum/yt-hg/yt/utilities/parallel_tools/parallel_analysis_interface.pyc in parallel_objects(objects, njobs, storage, barrier, dynamic)
    431     # If our objects object is slice-aware, like time series data objects are,
    432     # this will prevent intermediate objects from being created.
--> 433     oiter = itertools.islice(enumerate(objects), my_new_id, None, njobs)
    434     for obj_id, obj in oiter:
    435         result_id = obj_id * njobs + my_new_id

TypeError: 'NoneType' object is not iterable
```

Looking at the traceback, the issue seems to be that the profile code expects there to be a `_grids` attribute attached to data sources.