Datastore list of lists

Question

Hey,

I need to make a list property that will contain lists, something like: db.ListProperty(list(str))

I know list(str) is not a supported value type so as I imagined I received a "ValueError" exception. Thought maybe there is a creative idea out there of how to overcome this :)

Thanks!

Answer 1

You could use pickle to serialize your list and store it in a BlobProperty field.

Answer 2

Expanding on Adam's suggestion, you can push the pickling into its own Property class. Here is an example that handles validation, and converting the provided list into and out of a Blob type. The provided list can contain any data type or combination of datatypes, since it just stores a standard python list.

import pickle

class GenericListProperty(db.Property):
  data_type = db.Blob

  def validate(self, value):
    if type(value) is not list:
      raise db.BadValueError('Property %s must be a list, not %s.' % (self.name, type(value), value))
    return value

  def get_value_for_datastore(self, model_instance):
    return db.Blob(pickle.dumps(getattr(model_instance,self.name)))

  def make_value_from_datastore(self, value):
    return pickle.loads(value)

You use it as you would any other property.

class ModelWithAGenericList(db.Model):
  mylist = GenericListProperty()

class MainHandler(webapp.RequestHandler):
  def get(self):
    db.delete(ModelWithAGenericList.all())

    m = ModelWithAGenericList(mylist = [[1,2,3],[4,5,6],6])
    m.put()

    m = ModelWithAGenericList.all().fetch(1)[0]
    self.response.out.write(str(m.mylist))
    # Outputs: [[1, 2, 3], [4, 5, 6], 6]