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In psana2 all mpi ranks execute the same code, but not all ranks can create a detector Detector object since there are “hidden” MPI helper-ranks shown in this diagram: MPITaskStructureToSupportScaling.  For those helper-ranks the Detector object will be None.  Those helper-ranks won’t enter psana2 loops over runs/steps/events, so as long as you only use a detector Detector object inside loops your code will run correctly without any special checks.  However, if you use the detector Detector object outside those loops you must check that the Detector object is not None before calling any methods.

Historical background: we went back and forth about how to manage the MPI helper-ranks.  The alternative would have been to use callbacks instead of run/step/event loops to more effectively hide the helper-ranks from user code, but callbacks would have been user-unfriendly in a different way: writing loops is a more natural coding approach for many users.  We felt the loop approach (with more fragile Detector objects that can be None) was the lesser of two evils.