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RAMSES: issue with particle fields #1554

Closed cphyc closed 6 years ago

cphyc commented 7 years ago

Bug report

Bug summary

When RAMSES does a run with stars but none have been created yet, the number of star in the output is 0. This leads to a bug as the frontend doesn't automatically read the particle_age and particle_metallicity fields as it should. This means that the field would be only present for outputs that have formed stars, even though they are present in the files for all outputs.

This is due to line https://github.com/yt-project/yt/blob/736c39cbcf43b48358525df65db989b6426324a4/yt/frontends/ramses/data_structures.py#L170, where the check is wrong as long as no star have been created.

This lead to a bug if the extra_particle_fields is used. The explanation is the following. Imagine we have a dataset with 11 records (3 positions, 3 velocities, 1 mass, 1 id, and 1 for the refinement level + 1 particle age, 1 particle metallicity). As long as no star are formed, yt will only load the first 9 records, so that the extra_particle_fields dict will be applied to the records >9. Once a star is formed, yt will load the first 11 records, so that the extra_particle_fields will be applied to the records >11.

There are 2 solutions to the problem:

  1. Remove the automatic detection if the extra_particle_fields argument is passed
  2. Improve the automatic detection of the fields to load them as particle_field_1, particle_field_2, … and give the user the possibility to name to give them a name. However, this option would have to assume that all the optional fields have a single type (e.g. real), because the Fortran data layout doesn't give any information about the type of the records.
ngoldbaum commented 7 years ago

I guess option 1 is least dangerous? What do you think?

cphyc commented 7 years ago

The other way round is to automatically detect the first 9 records that are always here, and then assume all the remaning one to be of kind real, throwing a warning to the user like:

yt has detected N extra fields assuming real kind. Consider using the extra_particle_fields option if you have unexpected behavior.