11.10. "dbhash" — DBM-style interface to the BSD database library
*****************************************************************

2.6 版后已移除: The "dbhash" module has been removed in Python 3.

The "dbhash" module provides a function to open databases using the
BSD "db" library.  This module mirrors the interface of the other
Python database modules that provide access to DBM-style databases.
The "bsddb" module is required  to use "dbhash".

This module provides an exception and a function:

exception dbhash.error

   Exception raised on database errors other than "KeyError".  It is a
   synonym for "bsddb.error".

dbhash.open(path[, flag[, mode]])

   Open a "db" database and return the database object.  The *path*
   argument is the name of the database file.

   The *flag* argument can be:

   +-----------+---------------------------------------------+
   | Value     | Meaning                                     |
   +===========+=============================================+
   | "'r'"     | Open existing database for reading only     |
   |           | (default)                                   |
   +-----------+---------------------------------------------+
   | "'w'"     | Open existing database for reading and      |
   |           | writing                                     |
   +-----------+---------------------------------------------+
   | "'c'"     | Open database for reading and writing,      |
   |           | creating it if it doesn’t exist             |
   +-----------+---------------------------------------------+
   | "'n'"     | Always create a new, empty database, open   |
   |           | for reading and writing                     |
   +-----------+---------------------------------------------+

   For platforms on which the BSD "db" library supports locking, an
   "'l'" can be appended to indicate that locking should be used.

   The optional *mode* parameter is used to indicate the Unix
   permission bits that should be set if a new database must be
   created; this will be masked by the current umask value for the
   process.

参见:

  Module "anydbm"
     Generic interface to "dbm"-style databases.

  Module "bsddb"
     Lower-level interface to the BSD "db" library.

  Module "whichdb"
     Utility module used to determine the type of an existing
     database.


11.10.1. Database Objects
=========================

The database objects returned by "open()" provide the methods  common
to all the DBM-style databases and mapping objects.  The following
methods are available in addition to the standard methods.

dbhash.first()

   It’s possible to loop over every key/value pair 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.

dbhash.last()

   Return the last key/value pair in a database traversal.  This may
   be used to begin a reverse-order traversal; see "previous()".

dbhash.next()

   Returns the key next key/value pair in a database traversal.  The
   following code prints every key in the database "db", without
   having to create a list in memory that contains them all:

      print db.first()
      for i in xrange(1, len(db)):
          print db.next()

dbhash.previous()

   Returns the previous key/value pair in a forward-traversal of the
   database. In conjunction with "last()", this may be used to
   implement a reverse-order traversal.

dbhash.sync()

   This method forces any unwritten data to be written to the disk.
