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This script lives in /reg/g/psdmin /sdf/group/lcls/ds/ana/tutorials/examplePythonpsana1_examples/randomAccess.py and demonstrates how to jump to a specific event using the three "timestamp" numbers that uniquely identify an LCLS event: seconds/nanoseconds/fiducials (fiducials increment at 360Hz and repeat every ~20 minutes, the other two numbers are a standard unix seconds/nanoseconds timestamp). In LCLS-II there will be only one number that will identify an event.
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from psana import * ds = DataSource('exp=xpptut15:run=54:smd') # LCLSLCLS1 uses 3 numbers to define an event. In LCLS2 this will beis one number. seconds = [] nanoseconds = [] fiducials = [] # get some times of events (these could come from a saved "small data" file, for example) for nevent,evt in enumerate(ds.events()): evtId = evt.get(EventId) seconds.append(evtId.time()[0]) nanoseconds.append(evtId.time()[1]) fiducials.append(evtId.fiducials()) if nevent==2: break # now that we have the times, jump to the events in reverse order ds = DataSource('exp=xpptut15:run=54:idx') runmyrun = next(ds.runs().next() for sec,nsec,fid in zip(reversed(seconds),reversed(nanoseconds),reversed(fiducials)): et = EventTime(int((sec<<32)|nsec),fid) evt = runmyrun.event(et) print (evt.get(EventId).fiducials()) |
This example shows how one uses "idx" (or "index") mode to iterate over events randomly and learn the total number of events in a run:
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from psana import *
ds = DataSource('exp=xpptut15:run=54:idx')
run = next(ds.runs())
times = run.times()
print(f'Found {len(times)} events in run')
for nevt, t in enumerate(times):
evt = run.event(t)
if nevt==2:
break |
Note that for technical reasons the number of events in a run can be different in idx and the more-common smd mode.