1:n string attribute concatenation

from qgis.utils import iface
layer = iface.activeLayer()
layer.startEditing()
compare_field_index = 15
concat_field_index = 5
new_field_index = 10
feature_dict = {f.id(): f for f in layer.getFeatures()}
for f in feature_dict.values():
  if f[concat_field_index]:
    new_field_text = f[concat_field_index]
  else:
    new_field_text = ''
  # Another for loop to go over features
  for compare_f in feature_dict.values():
    if (f != compare_f
        and f[compare_field_index] == compare_f[compare_field_index]):
      print '%s matches' % f[compare_field_index]
      if compare_f[concat_field_index]:
        new_field_text += compare_f[concat_field_index]
  print new_field_text
  f[new_field_index] = new_field_text
layer.commitChanges()