CopyTree with selection

[dastudillo]

Try adding gROOT->cd(); (its PyROOT equivalent, rather), or creating a new TFile, between getting the original tree and doing CopyTree. See this (even the title of this post is the same as yours, so you would have found it searching the Forum):

CopyTree (with a selection)

Well, if the problem persists, one thing you could do, is loop the first tree, read all the entries, and in the same loop only fill the second tree, if the entry fits the cut. Longer (not that much) but efficient.