# `dbm` --- Unix "データベース" へのインターフェース

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()

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

  dumbdbm.close()

    Close the database.