dbm --- Unix "databases" 的介面

原始碼：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.

回傳以下其中一個值：

  • 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

  • a string containing the required module name, such as 'dbm.ndbm' or 'dbm.gnu'

在 3.11 版的變更: filename accepts a path-like object.

dbm.open(file, flag='r', mode=0o666)

Open a database and return the corresponding database object.

參數:

  • file (path-like object) --

    要打開的資料庫檔案

    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 submodule listed above that can be imported is used.

  • flag (str) --

    • 'r' (default): Open existing database for reading only.

    • '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.

  • mode (int) -- The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

在 3.11 版的變更: file 接受一個類路徑物件。

The object returned by open() supports the same basic functionality as a dict; 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() methods.

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 3.2 版的變更: get() and setdefault() methods are now available for all dbm backends.

在 3.4 版的變更: Added native support for the context management protocol to the objects returned by open().

在 3.8 版的變更: Deleting a key from a read-only database raises a database module specific exception instead of KeyError.

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.

也參考

shelve 模組

Persistence module which stores non-string data.

The individual submodules are described in the following sections.

dbm.gnu --- GNU 資料庫管理器

原始碼：Lib/dbm/gnu.py

The dbm.gnu module provides an interface to the GDBM library, similar to the dbm.ndbm module, but with additional functionality like crash tolerance.

備註

The file formats created by dbm.gnu and dbm.ndbm are incompatible and can not be used interchangeably.

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='r', mode=0o666, /)

Open a GDBM database and return a gdbm object.

參數:

  • filename (path-like object) -- 要打開的資料庫檔案

  • flag (str) --

    • 'r' (default): Open existing database for reading only.

    • '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.

    The following additional characters may be appended to control how the database is opened:

    • 'f': Open the database in fast mode. Writes to the database will not be synchronized.

    • 's': Synchronized mode. Changes to the database will be written immediately to the file.

    • 'u': 不要鎖住資料庫。

    Not all flags are valid for all versions of GDBM. See the open_flags member for a list of supported flag characters.

  • mode (int) -- The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

引發:

error -- 如果一個無效的 flag 引數被傳入。

在 3.11 版的變更: filename accepts a path-like object.

dbm.gnu.open_flags

A string of characters the flag parameter of open() supports.

gdbm objects behave similar to mappings, but items() and values() methods are not supported. The following methods are also provided:

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 is not 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()

關閉 GDBM 資料庫。

dbm.ndbm --- 新資料庫管理器

原始碼：Lib/dbm/ndbm.py

The dbm.ndbm module provides an interface to the NDBM library. This module can be used with the "classic" NDBM interface or the GDBM compatibility interface.

備註

The file formats created by dbm.gnu and dbm.ndbm are incompatible and can not be used interchangeably.

警告

The NDBM library shipped as part of macOS has an undocumented limitation on the size of values, which can result in corrupted database files when storing values larger than this limit. Reading such corrupted files can result in a hard crash (segmentation fault).

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='r', mode=0o666, /)

Open an NDBM database and return an ndbm object.

參數:

  • filename (path-like object) -- The basename of the database file (without the .dir or .pag extensions).

  • flag (str) --

    • 'r' (default): Open existing database for reading only.

    • '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.

  • mode (int) -- The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

ndbm objects behave similar to mappings, but items() and values() methods are not supported. The following methods are also provided:

在 3.11 版的變更: Accepts path-like object for filename.

ndbm.close()

關閉 NDBM 資料庫。

dbm.dumb --- 可攜式 DBM 實作

原始碼：Lib/dbm/dumb.py

備註

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 dict-like interface which is written entirely in Python. Unlike other dbm backends, such as dbm.gnu, no external library is required.

dbm.dumb 模組定義了以下項目：

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='c', mode=0o666)

Open a dbm.dumb database. The returned database object behaves similar to a mapping, in addition to providing sync() and close() methods.

參數:

  • filename --

    The basename of the database file (without extensions). A new database creates the following files:

    • filename.dat

    • filename.dir

  • flag (str) --

    • 'r': Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c' (default): 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.

  • mode (int) -- The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

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 版的變更: open() always creates a new database when flag is 'n'.

在 3.8 版的變更: A database opened read-only if flag is 'r'. A database is not created if it does not exist if flag is 'r' or 'w'.

在 3.11 版的變更: filename accepts a path-like object.

In addition to the methods provided by the collections.abc.MutableMapping class, the following methods are provided:

dumbdbm.sync()

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

dumbdbm.close()

關閉資料庫。