gistfile1.txt

import uproot
from pathlib import Path
import numpy as np

cwd = Path('.')

out_branches = ['bjets_n']

for input_fname in cwd.glob('*.root*'):
  output_fname = Path(f'out_{input_fname.parts[-1]}')
  with uproot.recreate(output_fname) as out_file:
    in_file = uproot.open(input_fname)
    print(f"Opening {input_fname} and writing to {output_fname}")
    for tree_name, tree in in_file.items():
      print(f"  ... processing {tree_name}")
      branches = {b:tree[b].interpretation.todtype for b in out_branches}
      out_treename = tree_name.split(b';')[0]
      out_file[out_treename] = uproot.newtree(branches)
      for events in tree.iterate(branches=out_branches, namedecode='utf-8'):
        out_file[out_treename].extend(events)