gistfile1.py

import six

class FooError(Exception):

    message = _(u'An unknown exception occurred.')

    def __str__(self):
        if six.PY3:
            return self.message

        return self.message.encode('utf-8')

    def __unicode__(self):
        return self.message


# elsewhere...

def do_something():
    raise FooError()

try:
    do_something()
except FooError as ex:
    # Returns a UTF-8 string in py2, and a wide string in py3,
    # both of type six.text_type, with no coercion.
    msg = str(ex)

    # Returns unicode type in py2 and str in py3.
    msg = six.text_type(ex)