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
- 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
- ArrayProperty store array.array objects for lean and efficient POD type
  storage
With aetycoon, you'll have all the properties you're ever likely to need.