def _encodeValues(self, values):
        """
        Encode a L{list} of header values to a L{list} of UTF-8 encoded
        bytestrings if required.

        @param values: A list of HTTP header values.
        @type values: L{list} of L{bytes} or L{unicode} (mixed types allowed)

        @return: C{values}, with each item encoded if required
        @rtype: L{list} of L{bytes}
        """
        newValues = []

        for value in values:
            newValues.append(self._encodeValue(value))
        return newValues

        Encode a L{list} of header values to a L{list} of UTF-8 encoded
        bytestrings if required.

        @param values: A list of HTTP header values.
        @type values: L{list} of L{bytes} or L{unicode} (mixed types allowed)

        @return: C{values}, with each item encoded if required
        @rtype: L{list} of L{bytes}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/http_headers.py