code: def _prefix_from_prefix_string(cls, prefixlen_str):
          
          # int allows a leading +/- as well as surrounding whitespace,
          # so we ensure that isn't the case
          if not _BaseV4._DECIMAL_DIGITS.issuperset(prefixlen_str):
              cls._report_invalid_netmask(prefixlen_str)
          try:
              prefixlen = int(prefixlen_str)
          except ValueError:
              cls._report_invalid_netmask(prefixlen_str)
          if not (0 <= prefixlen <= cls._max_prefixlen):
              cls._report_invalid_netmask(prefixlen_str)
          return prefixlen
docs: Return prefix length from a numeric string
  
          Args:
              prefixlen_str: The string to be converted
  
          Returns:
              An integer, the prefix length.
  
          Raises:
              NetmaskValueError: If the input is not a valid netmask