def get_join_cols(by_entry):
  """ helper function used for joins
  builds left and right join list for join function
  """
  left_cols = []
  right_cols = []
  for col in by_entry:
    if isinstance(col, str):
      left_cols.append(col)
      right_cols.append(col)
    else:
      left_cols.append(col[0])
      right_cols.append(col[1])
  return left_cols, right_cols