aetycoon provides a library of useful App Engine datastore property classes.

The property classes included here cover use cases that are too specialized to
be included in the SDK, or simply weren't included, but are nevertheless
generally useful. They include:

- DerivedProperty, which allows you to automatically generate values
  - LowerCaseProperty, which stores the lower-cased value of another property
  - LengthProperty, which stores the length of another property
- DerivedDateProperty is a date which is derived from a datetime property on the
  same model.
- ChoiceProperty efficiently handles properties which may only be assigned a
  value from a limited set of choices
- CompressedBlobProperty and CompressedTextProperty store data/text in a
  compressed form
- PacificDateTimeProperty is a DateTimeProperty whose value is always returned
  in Pacific time.

With aetycoon, you'll have all the properties you're ever likely to need.