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# 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.