Hi Matt, I just pushed a change to my fork that changes the way the data_size property of our ARTIOChunk (same as old DomainSubset) is exposed. This catches cases when .data_size is read before a fill is called. You reordered calls somewhere in the main code that was causing this to fail for SlicePlot, but now I am running into it for ProjectionPlot. from yt.mods import * pf = load("tests/artdat/sizmbhloz-clref04SNth-rs9_a0.9011.art") ProjectionPlot(pf, "x", "Density", width = (100,'kpc') ).save() leads to yt : [ERROR ] 2013-03-14 10:39:05,002 ARTIOChunk.data_size called before fill I've tracked it back to this line in YTSelectionContainer._chunked_read old_size, self.size = self.size, chunk.data_size Here is the traceback: File "./testprojection.py", line 4, in <module> ProjectionPlot(pf, "x", "Density", width = (100,'kpc') ).save() File "/Users/drudd/Programs/yt-x86_64/src/yt-hg/yt/visualization/plot_window.py", line 1336, in __init__ center=center, field_parameters = field_parameters) File "/Users/drudd/Programs/yt-x86_64/src/yt-hg/yt/data_objects/construction_data_containers.py", line 243, in __init__ self.get_data(field) File "/Users/drudd/Programs/yt-x86_64/src/yt-hg/yt/data_objects/construction_data_containers.py", line 283, in get_data for chunk in self.data_source.chunks(None, "io"): File "/Users/drudd/Programs/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py", line 448, in chunks with self._chunked_read(chunk): File "/Users/drudd/Programs/yt-x86_64/lib/python2.7/contextlib.py", line 17, in __enter__ return self.gen.next() File "/Users/drudd/Programs/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py", line 525, in _chunked_read import traceback; traceback.print_stack() It looks like the problem is the attempt to save state in _chunked_read while _chunk iterates to call .get_data. Since size is just a wrapper around the chunk.data_size, would it be better to have something like this: _size = None @property def size(self) : if self._size is None : if not self._current_chunk is None : self._size = self._current_chunk.data_size return self._size and def _chunked_read(self, chunk): # There are several items that need to be swapped out # field_data, size, shape old_field_data, self.field_data = self.field_data, YTFieldData() old_chunk, self._current_chunk = self._current_chunk, chunk self._size = None old_locked, self._locked = self._locked, False yield self.field_data = old_field_data self._current_chunk = old_chunk self._size = None self._locked = old_locked That may just push the problem further down the call stack, but at least means this code won't ask for .get_size until needed. Douglas Rudd Scientific Computing Consultant Research Computing Center drudd@uchicago.edu