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, the slow-but-simple implementation in module dbm.dumb will be used. There is a third party interface to the Oracle Berkeley DB.

exception dbm.error

A tuple containing the exceptions that can be raised by each of the supported modules, with a unique exception also named dbm.error as the first item --- the latter is used when dbm.error is raised.

dbm.whichdb(filename)

This function attempts to guess which of the several simple database modules available --- dbm.gnu, dbm.ndbm or dbm.dumb --- should be used to open a given file.

Returns one of the following values: None if the file can't be opened because it's unreadable or doesn't exist; the empty string ('') if the file's format can't be guessed; or a string containing the required module name, 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 to determine 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 the database has to be created. It defaults to octal 0o666 (and will be modified by the prevailing umask).

The object returned by open() supports the same basic functionality as dictionaries; keys and their corresponding values can be stored, retrieved, and deleted, and the in operator and the keys() method are available, 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 error instead of KeyError.

Key and values are always stored as bytes. This means that when strings are used they are implicitly converted to the default encoding before being stored.

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

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

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

import dbm

# Open database, creating it if necessary.
with dbm.open('cache', 'c') as db:

    # Record some values
    db[b'hello'] = b'there'
    db['www.python.org'] = 'Python Website'
    db['www.cnn.com'] = 'Cable News Network'

    # Note that the keys are considered bytes now.
    assert db[b'www.python.org'] == b'Python Website'
    # Notice how the value is now in bytes.
    assert db['www.cnn.com'] == b'Cable News Network'

    # Often-used methods of the dict interface work too.
    print(db.get('python.org', b'not present'))

    # Storing a non-string key or value will raise an exception (most
    # likely a TypeError).
    db['www.yahoo.com'] = 4

# 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 library gdbm instead to provide some additional functionality. Please note that the file 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 and values are always converted to bytes before storing. Printing a gdbm object doesn't print the keys and values, and the items() and values() methods are not supported.

exception dbm.gnu.error

Raised on dbm.gnu-specific errors, such as I/O errors. KeyError is raised 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 control how 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 constant open_flags is a string of supported flag characters. The exception error is raised if an invalid flag is specified.

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

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

gdbm.firstkey()

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

gdbm.nextkey(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.firstkey()
while k != None:
    print(k)
    k = db.nextkey(k)

gdbm.reorganize()

If you have carried out a lot of deletions and would like to shrink the space used by the gdbm file, this routine will reorganize the database. gdbm objects will not shorten the length of a database file except by using this reorganization; 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 any unwritten 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 are always stored as bytes. Printing a dbm object doesn't print the keys and values, and the items() and values() methods are not supported.

This module can be used with the "classic" ndbm interface or the GNU GDBM compatibility interface. On Unix, the configure script will attempt to 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 raised for 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 the name 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 the database has to be created. It defaults to octal 0o666 (and will be modified by the prevailing umask).

In addition to the dictionary-like methods, ndbm objects provide 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 the dbm module when a more robust module is not available. The dbm.dumb module is not written for speed and is not nearly as heavily used as the other database modules.

---

The dbm.dumb module provides a persistent dictionary-like interface which is written entirely in Python. Unlike other modules such as dbm.gnu no external library is required. As with other persistent mappings, the keys and values 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 is raised 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 is the basename of the database file (without any specific extensions). When a dumbdbm database is created, files with .dat and .dir extensions are 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 the database has to be created. It defaults to octal 0o666 (and will be modified by the prevailing umask).

警告

It is possible to crash the Python interpreter when loading a database with a sufficiently large/complex entry due to stack depth limitations in Python'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 with flags 'r' and 'w' no longer creates a database if it does not exist.

In addition to the methods provided by the collections.abc.MutableMapping class, dumbdbm objects provide the following methods:

dumbdbm.sync()

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

dumbdbm.close()

Close the dumbdbm database.