Inspecting the stack trace with pdb reveals that fname is 'particle_mass'.

On Tue, Apr 1, 2014 at 4:56 PM, Matthew Turk <matthewturk@gmail.com> wrote:

Hi Nathan,

Weird.  I think this is probably related to at some point guessing the
field type; you'll note that it comes from _determine_fields, which
will call _get_field_info with "unknown".  If it can't figure out what
it is (it defaults to the last type found) it throws the error.  Can
you check what the fname is?

-Matt

On Tue, Apr 1, 2014 at 7:55 PM, Nathan Goldbaum
<issues-reply@bitbucket.org> wrote:
> New issue 824: Field detection issue when recreating data objects
> https://bitbucket.org/yt_analysis/yt/issue/824/field-detection-issue-when-recreating-data
>
> Nathan Goldbaum:
>
> I ran into this when using a notebook.  I found that when I re-ran a cell that creates a data object and then calculates a derived quantity, I get a field detection error the *second* time I run the cell.
>
> The following script exposes the buggy behavior: http://paste.yt-project.org/show/4462/
>
> When run using the current experimental bookmark I get the following output and traceback:
>
>
> ```
> goldbaum at ROUS in ~
> ○ python test.py
> [8.15217983473e+49 g, 0.833333333333 code_mass]
> Traceback (most recent call last):
>   File "test.py", line 21, in <module>
>     print ad.quantities.total_mass()
>   File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/derived_quantities.py", line 194, in __call__
>     rv = super(TotalMass, self).__call__(fields)
>   File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/derived_quantities.py", line 163, in __call__
>     rv = super(TotalQuantity, self).__call__(fields)
>   File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/derived_quantities.py", line 59, in __call__
>     sto.result = self.process_chunk(ds, *args, **kwargs)
>   File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/derived_quantities.py", line 169, in process_chunk
>     for field in fields]
>   File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/data_containers.py", line 205, in __getitem__
>     f = self._determine_fields([key])[0]
>   File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/data_containers.py", line 464, in _determine_fields
>     raise YTFieldTypeNotFound(ftype)
> yt.utilities.exceptions.YTFieldTypeNotFound: Could not find field 'all'.
> ```
>
>
>
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