def _cast(self, out, peek=None):

        # Empty output is done here
        if not out:
            if 'Content-Length' not in response:
                response['Content-Length'] = 0
            return []
        # Join lists of byte or unicode strings. Mixed lists are NOT supported
        if isinstance(out, (tuple, list))\
        and isinstance(out[0], (bytes, unicode)):
            out = out[0][0:0].join(out) # b'abc'[0:0] -> b''
        # Encode unicode strings
        if isinstance(out, unicode):
            out = out.encode(response.charset)
        # Byte Strings are just returned
        if isinstance(out, bytes):
            if 'Content-Length' not in response:
                response['Content-Length'] = len(out)
            return [out]
        # HTTPError or HTTPException (recursive, because they may wrap anything)
        # TODO: Handle these explicitly in handle() or make them iterable.
        if isinstance(out, HTTPError):
            out.apply(response)
            out = self.error_handler.get(out.status_code, self.default_error_handler)(out)
            return self._cast(out)
        if isinstance(out, HTTPResponse):
            out.apply(response)
            return self._cast(out.output)

        # File-like objects.
        if hasattr(out, 'read'):
            if 'wsgi.file_wrapper' in request.environ:
                return request.environ['wsgi.file_wrapper'](out)
            elif hasattr(out, 'close') or not hasattr(out, '__iter__'):
                return WSGIFileWrapper(out)

        # Handle Iterables. We peek into them to detect their inner type.
        try:
            out = iter(out)
            first = next(out)
            while not first:
                first = next(out)
        except StopIteration:
            return self._cast('')
        except HTTPResponse:
            first = _e()
        except (KeyboardInterrupt, SystemExit, MemoryError):
            raise
        except Exception:
            if not self.catchall: raise
            first = HTTPError(500, 'Unhandled exception', _e(), format_exc())

        # These are the inner types allowed in iterator or generator objects.
        if isinstance(first, HTTPResponse):
            return self._cast(first)
        if isinstance(first, bytes):
            return itertools.chain([first], out)
        if isinstance(first, unicode):
            return imap(lambda x: x.encode(response.charset),
                                  itertools.chain([first], out))
        return self._cast(HTTPError(500, 'Unsupported response type: %s'\
                                         % type(first)))
Try to convert the parameter into something WSGI compatible and set
dataset/ETHPy150Open adampresley/bottlepy-bootstrap/app/bottle.py/Bottle._cast