7.10.1 Database Objects 
The database objects returned by open() provide the methods 
common to all the DBM-style databases.  The following methods are
available in addition to the standard methods.
- first()
 - 
  It's possible to loop over every key in the database using this method 
  and the next() method.  The traversal is ordered by
  the databases internal hash values, and won't be sorted by the key
  values.  This method returns the starting key.
 
- last()
 - 
  Return the last key in a database traversal.  This may be used to
  begin a reverse-order traversal; see previous().
 
- next(key)
 - 
  Returns the key that follows key in the traversal.  The
  following code prints every key in the database 
db, without
  having to create a list in memory that contains them all:
k = db.first()
while k != None:
    print k
    k = db.next(k)
 
- previous(key)
 - 
  Return the key that comes before key in a forward-traversal of 
  the database.  In conjunction with last(), this may be used 
  to implement a reverse-order traversal.
 
- sync()
 - 
  This method forces any unwritten data to be written to the disk.
 
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