Convert a decimal octet into an integer.

        Args:
            octet_str: A string, the number to parse.

        Returns:
            The octet as an integer.

        Raises:
            ValueError: if the octet isn't strictly a decimal from [0..255].

def _parse_octet(self, octet_str):
        """Convert a decimal octet into an integer.

        Args:
            octet_str: A string, the number to parse.

        Returns:
            The octet as an integer.

        Raises:
            ValueError: if the octet isn't strictly a decimal from [0..255].

        """
        if not octet_str:
            raise ValueError("Empty octet not permitted")
        # Whitelist the characters, since int() allows a lot of bizarre stuff.
        if not self._DECIMAL_DIGITS.issuperset(octet_str):
            msg = "Only decimal digits permitted in %r"
            raise ValueError(msg % octet_str)
        # We do the length check second, since the invalid character error
        # is likely to be more informative for the user
        if len(octet_str) > 3:
            msg = "At most 3 characters permitted in %r"
            raise ValueError(msg % octet_str)
        # Convert to integer (we know digits are legal)
        octet_int = int(octet_str, 10)
        # Any octets that look like they *might* be written in octal,
        # and which don't look exactly the same in both octal and
        # decimal are rejected as ambiguous
        if octet_int > 7 and octet_str[0] == '0':
            msg = "Ambiguous (octal/decimal) value in %r not permitted"
            raise ValueError(msg % octet_str)
        if octet_int > 255:
            raise ValueError("Octet %d (> 255) not permitted" % octet_int)
        return octet_int