docs: def is_valid_varname(varname):
      """ Checks syntax and validity of a variable name """
      if not isinstance(varname, six.string_types):
          return False
      match_obj = re.match(varname_regex, varname)
      valid_syntax = match_obj is not None
      valid_name = not keyword.iskeyword(varname)
      isvalid = valid_syntax and valid_name
      return isvalid