[yt-users] Field existence check

[Matthew Turk]

Hi Dave,

Quick followup.  I realized we do this already for Gas_Energy and
Total_Energy, as you can see in yt/lagos/EnzoFields.py on line 71.
Note that this might get sticky if you are deriving from StaticOutput
or EnzoStaticOutput with a new field container and want these to be
communicated up and down the hierarchy of possibilities.  I think that
is probably an extreme corner case, though.

So, in short, it should work well, and if it doesn't, it is a bug and  
it should be fixed.

-Matt

On Wed, May 27, 2009 at 3:39 PM, Matthew Turk <matthewturk at gmail.com>  
wrote:
> Hi Dave,
>
> It should be the same call to add field. However, the more I think
> about it, the more I think it will default to using the data field
> over the derived field, and so you should be safe adding it as a
> derived field in *all* cases.  I have conducted a few experiments and
> this seems to be true over here.
>
> You should check this by adding a field that supplies bad values, or
> raises an exception.  However, from my reading of the
> AMRGridPatch.get_data function, I believe it will first try the data
> reader and then fall back to generating it on the fly.
> hierarchy.field_list *only* contains fields it finds in the data
> files, and that is what the grid type checks against.
> AMRData.get_data also checks against this list and will generate the
> field if it is not there.
>
> Can you give it a shot, let us know if this is not the case?
>
> -Matt
>
>
>
> On May 27, 2009, at 3:14 PM, David Collins <dcollins at physics.ucsd.edu>
> wrote:
>
>>> pf = load("my_data")
>>> if not "DavesField" in pf.h.field_list:
>>>    add_field(...)
>>
>>
>> add_field is the same one I've been using from lagos?
>>
>> I have some datasets that have this field, some that don't, all being
>> used at the same time.  Can I sometimes add the field, sometimes not?
>>
>> Thanks a bunch,
>>
>> d.
>>
>>>
>>> Hope that helps!
>>>
>>> -Matt
>>>
>>> On Wed, May 27, 2009 at 2:45 PM, david collins
>>> <antpuncher at gmail.com> wrote:
>>>> Hi--
>>>>
>>>> I have a derived quantity that I only write to disk in some
>>>> simulations.  Is there a way to check for the existence of a field
>>>> in
>>>> the data set, so that if it doesn't exist I can derive it?
>>>>
>>>> Thanks,
>>>> d.
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