0UT1S's

_charset is the character set parameter added to the Content-Type
        header.  This defaults to "us-ascii".  Note that as a side-effect, the
        Content-Transfer-Encoding header will also be set.
        """

# If no _charset was specified, check to see if there are non-ascii
        # characters present. If not, use 'us-ascii', otherwise use utf-8.
        # XXX: This can be removed once #7304 is fixed.
        if _charset is None:
            try:
                _text.encode('us-ascii')
                _charset = 'us-ascii'
            except UnicodeEncodeError:
                _charset = 'utf-8'

MIMENonMultipart.__init__(self, 'text', _subtype, policy=policy,
                                  **{'charset': str(_charset)})

self.set_payload(_text, _charset)