old_contents: class Model(object):
      """Abstract base class for all models."""
  
      def __init__(self, **kwargs):
          object.__init__(self)
  
          doc = kwargs.get('doc', None)
          if doc is not None:
              self._id = doc.get('_id', None)
              self._rev = doc.get('_rev', None)
          else:
              self._id = kwargs.get('_id', None)
              self._rev = kwargs.get('_rev', None)
  
      def as_doc_for_store(self):
          rv = {}
          if self._id:
              rv["_id"] = self._id
          if self._rev:
              rv["_rev"] = self._rev
          return rv
  
new_contents: class Model(object):
      """Abstract base class for all models."""
  
      def __init__(self, *args, **kwargs):
          object.__init__(self)
  
          doc = kwargs.get('doc', None)
          if doc is not None:
              self._id = doc.get('_id', None)
              self._rev = doc.get('_rev', None)
          else:
              self._id = kwargs.get('_id', None)
              self._rev = kwargs.get('_rev', None)
  
      def as_doc_for_store(self):
          rv = {}
          if self._id:
              rv['_id'] = self._id
          if self._rev:
              rv['_rev'] = self._rev
          return rv
  
subject: refactor model.py