'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.'

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
