dbm —- Interfaces to Unix "databases"

源代码:Lib/dbm/init.py

dbm is a generic interface to variants of the DBM database —-dbm.gnu or dbm.ndbm. If none of these modules is installed, theslow-but-simple implementation in module dbm.dumb will be used. Thereis a third party interface tothe Oracle Berkeley DB.

  • exception dbm.error

  • A tuple containing the exceptions that can be raised by each of the supportedmodules, with a unique exception also named dbm.error as the firstitem —- the latter is used when dbm.error is raised.

  • dbm.whichdb(filename)

  • This function attempts to guess which of the several simple database modulesavailable —- dbm.gnu, dbm.ndbm or dbm.dumb —- shouldbe used to open a given file.

Returns one of the following values: None if the file can't be openedbecause it's unreadable or doesn't exist; the empty string ('') if thefile's format can't be guessed; or a string containing the required modulename, such as 'dbm.ndbm' or 'dbm.gnu'.

  • dbm.open(file, flag='r', mode=0o666)
  • Open the database file file and return a corresponding object.

If the database file already exists, the whichdb() function is used todetermine its type and the appropriate module is used; if it does not exist,the first module listed above that can be imported is used.

可选的 flag 参数可以是：

值

意义

'r'

以只读方式打开现有数据库（默认）

'w'

以读写方式打开现有数据库

'c'

以读写方式打开数据库，如果不存在则创建它

'n'

始终创建一个新的空数据库，以读写方式打开

The optional mode argument is the Unix mode of the file, used only when thedatabase has to be created. It defaults to octal 0o666 (and will bemodified by the prevailing umask).

The object returned by open() supports the same basic functionality asdictionaries; keys and their corresponding values can be stored, retrieved, anddeleted, and the in operator and the keys() method areavailable, as well as get() and setdefault().

在 3.2 版更改: get() and setdefault() are now available in all database modules.

在 3.8 版更改: Deleting a key from a read-only database raises database module specific errorinstead of KeyError.

Key and values are always stored as bytes. This means that whenstrings are used they are implicitly converted to the default encoding beforebeing stored.

These objects also support being used in a with statement, whichwill automatically close them when done.

在 3.4 版更改: Added native support for the context management protocol to the objectsreturned by open().

The following example records some hostnames and a corresponding title, andthen prints out the contents of the database:

  1. import dbm
  2.  
  3. # Open database, creating it if necessary.
  4. with dbm.open('cache', 'c') as db:
  5.  
  6.     # Record some values
  7.     db[b'hello'] = b'there'
  8.     db['www.python.org'] = 'Python Website'
  9.     db['www.cnn.com'] = 'Cable News Network'
  10.  
  11.     # Note that the keys are considered bytes now.
  12.     assert db[b'www.python.org'] == b'Python Website'
  13.     # Notice how the value is now in bytes.
  14.     assert db['www.cnn.com'] == b'Cable News Network'
  15.  
  16.     # Often-used methods of the dict interface work too.
  17.     print(db.get('python.org', b'not present'))
  18.  
  19.     # Storing a non-string key or value will raise an exception (most
  20.     # likely a TypeError).
  21.     db['www.yahoo.com'] = 4
  22.  
  23. # db is automatically closed when leaving the with statement.

参见

  • 模块 shelve
  • Persistence module which stores non-string data.

The individual submodules are described in the following sections.

dbm.gnu —- GNU's reinterpretation of dbm

源代码:Lib/dbm/gnu.py

This module is quite similar to the dbm module, but uses the GNU librarygdbm instead to provide some additional functionality. Please note that thefile formats created by dbm.gnu and dbm.ndbm are incompatible.

The dbm.gnu module provides an interface to the GNU DBM library.dbm.gnu.gdbm objects behave like mappings (dictionaries), except that keys andvalues are always converted to bytes before storing. Printing a gdbmobject doesn't print thekeys and values, and the items() and values() methods are notsupported.

  • exception dbm.gnu.error

  • Raised on dbm.gnu-specific errors, such as I/O errors. KeyError israised for general mapping errors like specifying an incorrect key.

  • dbm.gnu.open(filename[, flag[, mode]])

  • Open a gdbm database and return a gdbm object. The _filename_argument is the name of the database file.

可选的 flag 参数可以是：

值

意义

'r'

以只读方式打开现有数据库（默认）

'w'

以读写方式打开现有数据库

'c'

以读写方式打开数据库，如果不存在则创建它

'n'

始终创建一个新的空数据库，以读写方式打开

The following additional characters may be appended to the flag to controlhow the database is opened:

值

意义

'f'

以快速模式打开数据库。写入数据库将不会同步。

's'

同步模式。这将导致数据库的更改立即写入文件。

'u'

不要锁定数据库。

Not all flags are valid for all versions of gdbm. The module constantopen_flags is a string of supported flag characters. The exceptionerror is raised if an invalid flag is specified.

The optional mode argument is the Unix mode of the file, used only when thedatabase has to be created. It defaults to octal 0o666.

In addition to the dictionary-like methods, gdbm objects have thefollowing methods:

  • gdbm.firstkey()

  • It's possible to loop over every key in the database using this method and thenextkey() method. The traversal is ordered by gdbm's internalhash values, and won't be sorted by the key values. This method returnsthe starting key.

  • gdbm.nextkey(key)

  • Returns the key that follows key in the traversal. The following code printsevery key in the database db, without having to create a list in memory thatcontains them all:
  1. k = db.firstkey()
  2. while k != None:
  3.     print(k)
  4.     k = db.nextkey(k)
  • gdbm.reorganize()

  • If you have carried out a lot of deletions and would like to shrink the spaceused by the gdbm file, this routine will reorganize the database. gdbmobjects will not shorten the length of a database file except by using thisreorganization; otherwise, deleted file space will be kept and reused as new(key, value) pairs are added.

  • gdbm.sync()

  • When the database has been opened in fast mode, this method forces anyunwritten data to be written to the disk.

  • gdbm.close()

  • Close the gdbm database.

dbm.ndbm —- Interface based on ndbm

源代码:Lib/dbm/ndbm.py

The dbm.ndbm module provides an interface to the Unix "(n)dbm" library.Dbm objects behave like mappings (dictionaries), except that keys and values arealways stored as bytes. Printing a dbm object doesn't print the keys andvalues, and the items() and values() methods are not supported.

This module can be used with the "classic" ndbm interface or the GNU GDBMcompatibility interface. On Unix, the configure script will attemptto locate the appropriate header file to simplify building this module.

  • exception dbm.ndbm.error

  • Raised on dbm.ndbm-specific errors, such as I/O errors. KeyError is raisedfor general mapping errors like specifying an incorrect key.

  • dbm.ndbm.library

  • Name of the ndbm implementation library used.

  • dbm.ndbm.open(filename[, flag[, mode]])

  • Open a dbm database and return a ndbm object. The filename argument is thename of the database file (without the .dir or .pag extensions).

The optional flag argument must be one of these values:

值

意义

'r'

以只读方式打开现有数据库（默认）

'w'

以读写方式打开现有数据库

'c'

以读写方式打开数据库，如果不存在则创建它

'n'

始终创建一个新的空数据库，以读写方式打开

The optional mode argument is the Unix mode of the file, used only when thedatabase has to be created. It defaults to octal 0o666 (and will bemodified by the prevailing umask).

In addition to the dictionary-like methods, ndbm objectsprovide the following method:

  • ndbm.close()
  • Close the ndbm database.

dbm.dumb —- Portable DBM implementation

源代码:Lib/dbm/dumb.py

注解

The dbm.dumb module is intended as a last resort fallback for thedbm module when a more robust module is not available. The dbm.dumbmodule is not written for speed and is not nearly as heavily used as the otherdatabase modules.

The dbm.dumb module provides a persistent dictionary-like interface whichis written entirely in Python. Unlike other modules such as dbm.gnu noexternal library is required. As with other persistent mappings, the keys andvalues are always stored as bytes.

该模块定义以下内容：

  • exception dbm.dumb.error

  • Raised on dbm.dumb-specific errors, such as I/O errors. KeyError israised for general mapping errors like specifying an incorrect key.

  • dbm.dumb.open(filename[, flag[, mode]])

  • Open a dumbdbm database and return a dumbdbm object. The filename argument isthe basename of the database file (without any specific extensions). When adumbdbm database is created, files with .dat and .dir extensionsare created.

可选的 flag 参数可以是：

值

意义

'r'

以只读方式打开现有数据库（默认）

'w'

以读写方式打开现有数据库

'c'

以读写方式打开数据库，如果不存在则创建它

'n'

始终创建一个新的空数据库，以读写方式打开

The optional mode argument is the Unix mode of the file, used only when thedatabase has to be created. It defaults to octal 0o666 (and will be modifiedby the prevailing umask).

警告

It is possible to crash the Python interpreter when loading a databasewith a sufficiently large/complex entry due to stack depth limitations inPython's AST compiler.

在 3.5 版更改: open() always creates a new database when the flag has the value'n'.

在 3.8 版更改: A database opened with flags 'r' is now read-only. Opening withflags 'r' and 'w' no longer creates a database if it does notexist.

In addition to the methods provided by thecollections.abc.MutableMapping class, dumbdbm objectsprovide the following methods:

  • dumbdbm.sync()

  • Synchronize the on-disk directory and data files. This method is calledby the Shelve.sync() method.

  • dumbdbm.close()

  • Close the dumbdbm database.