from pyLCIO import IOIMPL, UTIL, EVENT
lcioReader = IOIMPL.LCFactory.getInstance().createLCReader()
lcioReader.open("something.slcio")

event = lcioReader.readNextEvent()

mcps = event.getCollection("MCParticlesSkimmed")

d = {}

for mcp in mcps:
    d[mcp] = 0 #assigns only

for i, mcp in enumerate(mcps):
    print i
    d[mcp] += 1 #attempts to access and assign and throws KeyError