code: def dirty(self):
          '''The set of instances in this :class:`Session` which have
  been modified.'''
          return frozenset(chain(*tuple((sm.dirty for sm
                                         in itervalues(self._models))))f dirty(self):
          '''The set of instances in this :class:`Session` which have
  been modified.'''
          return frozenset(chain(*tuple((sm.dirty for sm
                                         in itervalues(self._models)))))
docs: The set of instances in this :class:`Session` which have
  been modified.