fails with ChainReader trajectory about pmda HOT 2 OPEN

The problem is that ParallelAnalysisBase._dask_helper() needs to be pickled by dask. But it is a bound method of ParallelAnalysisBase so that method (or rather, the class that was derived from it, e.g. RMSD) also needs to be pickled. If this class contains anything that cannot be pickled, the whole pickle fails. In particular, there is ParallelAnalysisBase._trajectory, which contains the Reader from the universe (not the universe itself, which is currently no picklable). Almost all readers are picklable, except the ChainReader, because it contains a Generator:

_ChainReader__chained_trajectories_iter': <generator object ChainReader._chained_iterator at 0x7f1c19716468>

Ideally, pickling _dask_helper() would not need to pickle the enclosing class. However, a short term fix should be to make the ChainReader picklable.

from pmda.

ChainReader should work once PR MDAnalysis/mdanalysis#2723 is merged.

from pmda.