Add tests to BasicDatabase and merge implementations test/session_common.py: add new tests: * test set with bad __timestamps * test get on missing item with no default * test clear() method * test new lifetime() method everything below here was needed to get the tests to work across all db's. roundup/backends/sessions_dbm.py: make set() validate __timestamp as float. If invalid and item is new, set it to time.time(). If invalid and item exists keep original timestamp. add lifetime(key_lifetime) method. Given key_lifetime in seconds, generate a __timestamp that will be deleted after that many seconds. roundup/backends/sessions_rdbms.py: make set() behave the same as session_dbm. add lifetime method as above. roundup/backends/sessions_sqlite.py: import session_rdbms::BasicDatabase and override __init__. rather than cloning all the code. Kept a few logging and sql methods. roundup/test/memorydb.py: make get() on a missing key return KeyError make set() conform to dbm/rdbms implementations.