dbm --- Interfaces to Unix "databases"

ソースコード: 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

サポートされているモジュールそれぞれによって送出される可能性のある例外を含むタプル。これにはユニークな例外があり、最初の要素として同じく dbm.error という名前の例外が含まれます --- dbm.error が送出される場合、後者(訳注:タプルの dbm.error ではなく例外 dbm.error)が使用されます。

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.

Return 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

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

バージョン 3.11 で変更: filename accepts a path-like object.

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

Open a database and return the corresponding database object.

パラメータ:

  • file (path-like object) --

    The database file to open.

    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.

バージョン 3.11 で変更: file accepts a path-like object.

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.

これらのオブジェクトは、 with 文での使用にも対応しています。with 文を使用した場合、終了時に自動的に閉じられます。

バージョン 3.2 で変更: get() and setdefault() methods are now available for all dbm backends.

バージョン 3.4 で変更: Added native support for the context management protocol to the objects returned by open().

バージョン 3.8 で変更: Deleting a key from a read-only database raises a database module specific exception instead of KeyError.

以下の例ではホスト名と対応するタイトルをいくつか記録し、データベースの内容を出力します:

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.

参考

shelve モジュール

非文字列データを記録する永続化モジュール。

個々のサブモジュールは以降の節で説明されます。

dbm.gnu --- GNU database manager

ソースコード: 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.

注釈

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

exception dbm.gnu.error

I/O エラーのような dbm.gnu 特有のエラーで送出されます。誤ったキーの指定のように、一般的なマップ型のエラーに対しては KeyError が送出されます。

dbm.gnu.open(filename, flag='r', mode=0o666, /)

Open a GDBM database and return a gdbm object.

パラメータ:

  • filename (path-like object) -- The database file to open.

  • 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': Do not lock database.

    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.

例外:

error -- If an invalid flag argument is passed.

バージョン 3.11 で変更: 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)

データベースの順方向探索において、key よりも後に来るキーを返します。以下のコードはデータベース db について、キー全てを含むリストをメモリ上に生成することなく全てのキーを出力します:

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()

データベースが高速モードで開かれていた場合、このメソッドはディスクにまだ書き込まれていないデータを全て書き込ませます。

gdbm.close()

Close the GDBM database.

dbm.ndbm --- New Database Manager

ソースコード: 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.

注釈

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

I/O エラーのような dbm.ndbm 特有のエラーで送出されます。誤ったキーの指定のように、一般的なマップ型のエラーに対しては KeyError が送出されます。

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.

パラメータ:

  • 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:

バージョン 3.11 で変更: Accepts path-like object for filename.

ndbm.close()

Close the NDBM database.

dbm.dumb --- 可搬性のある DBM 実装

ソースコード: Lib/dbm/dumb.py

注釈

dbm.dumb モジュールは、 dbm が頑健なモジュールを他に見つけることができなかった際の最後の手段とされています。 dbm.dumb モジュールは速度を重視して書かれているわけではなく、他のデータベースモジュールのように重い使い方をするためのものではありません。

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.

The dbm.dumb module defines the following:

exception dbm.dumb.error

I/O エラーのような dbm.dumb 特有のエラーの際に送出されます。不正なキーを指定したときのような、一般的な対応付けエラーの際には KeyError が送出されます。

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.

パラメータ:

  • 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

十分に大きかったり複雑だったりするエントリーのあるデータベースを読み込んでいるときに、 Python の抽象構文木コンパイラのスタックの深さの限界を越えるせいで、 Python インタプリタをクラッシュさせることができます。

バージョン 3.5 で変更: open() always creates a new database when flag is 'n'.

バージョン 3.8 で変更: 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'.

バージョン 3.11 で変更: 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()

ディスク上の辞書とデータファイルを同期します。このメソッドは Shelve.sync() メソッドから呼び出されます。

dumbdbm.close()

Close the database.