Type Unit

object --+
         |
        Unit

Unit(**kwarg properties). A generic, persistent object.

Units are the building-block of Dejavu. They are purposefully lightweight,
relying on Sandboxes to cache them, which in turn rely on Storage Managers
to load and save them.

They maintain their own "schema" via UnitProperty objects, so that the
Storage Managers don't need to know every detail about every Unit.
Storage Managers for simple databases, for example, will simply create
a single flat table for each unit type. If you write a custom Storage
Manager, you can do as you like; the only place you might run into a
problem is if you write a custom Storage Manager for custom Unit types,
because the knowledge between the two is indeterminate. For example,
if we provide a standard StorageManagerForLotusNotes, and you create
custom Units which interface with it, you should probably subclass and
extend our StorageManagerForLotusNotes with some custom storage logic.

sandbox: The sandbox in which the Unit "lives". Also serves as a flag
    indicating whether this Unit has finished the initial creation
    process.
    
    Sandboxes receive Units during recall() and memorize();
    these processes should set the sandbox attribute.

dirty: indicates whether elements in the _properties dictionary
    have been modified. This flag is used by Sandboxes to optimize
    forget(): they do not ask Storage Managers to save data for Units
    which have not been modified. Because SM's may cache Units, no code
    should set this flag other than UnitProperty.__set__ and SM's.