When a header parameter is encoded in RFC 2231 format, Message.get_param() may return a 3-tuple containing the character set, language, and value. collapse_rfc2231_value() turns this into a string. Optional errors is passed to the errors argument of str’s encode() method; it defaults to 'replace'. Optional fallback_charset specifies the character set to use if the one in the RFC 2231 header is not known by Python; it defaults to 'us-ascii'.