def iter_fields(self, exclude=None, only=None):
        """This method iterates over all fields that are defined and yields
        ``(key, value)`` tuples.  Per default all fields are returned, but
        it's possible to limit that to some fields by providing the `only`
        parameter or to exclude some using the `exclude` parameter.  Both
        should be sets or tuples of field names.
        """
        for name in self.fields:
            if (exclude is only is None) or \
               (exclude is not None and name not in exclude) or \
               (only is not None and name in only):
                try:
                    yield name, getattr(self, name)
                except AttributeError:
                    pass