code: def _prefix_from_prefix_string(self, prefixlen_str):
          
          try:
              if not _BaseV4._DECIMAL_DIGITS.issuperset(prefixlen_str):
                  raise ValueError
              prefixlen = int(prefixlen_str)
              if not (0 <= prefixlen <= self._max_prefixlen):
                 raise ValueError
          except ValueError:
              raise NetmaskValueError('%s is not a valid prefix length' %
                                      prefixlen_str)
          return prefixlen
docs: Turn a prefix length string into an integer.
  
          Args:
              prefixlen_str: A decimal string containing the prefix length.
  
          Returns:
              The prefix length as an integer.
  
          Raises:
              NetmaskValueError: If the input is malformed or out of range.