def _update_docstring(old_str, append_str):
  """Update old_str by inserting append_str just before the "Args:" section."""
  old_str = old_str or ""
  old_str_lines = old_str.split("\n")

  # Step 0: Prepend spaces to all lines of append_str. This is
  # necessary for correct markdown generation.
  append_str = "\n".join("    %s" % line for line in append_str.split("\n"))

  # Step 1: Find mention of "Args":
  has_args_ix = [
      ix for ix, line in enumerate(old_str_lines)
      if line.strip().lower() == "args:"]
  if has_args_ix:
    final_args_ix = has_args_ix[-1]
    return ("\n".join(old_str_lines[:final_args_ix])
            + "\n\n" + append_str + "\n\n"
            + "\n".join(old_str_lines[final_args_ix:]))
  else:
    return old_str + "\n\n" + append_str
Update old_str by inserting append_str just before the "Args:" section.