'Decodes an argument from the request.
The argument has been percent-decoded and is now a byte string.
By default, this method decodes the argument as utf-8 and returns
a unicode string, but this may be overridden in subclasses.
This method is used as a filter for both `get_argument()` and for
values extracted from the url and passed to `get()`/`post()`/etc.
The name of the argument is provided if known, but may be None
(e.g. for unnamed groups in the url regex).'

def decode_argument(self, value, name=None):

    try:
        return _unicode(value)
    except UnicodeDecodeError:
        raise HTTPError(400, ('Invalid    unicode    in    %s:    %r' % ((name or 'url'), value[:40])))
