"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 (GNU dbm)
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 (New Database
Manager) library. This module can be used with the "classic" NDBM
interface or the GDBM (GNU dbm) 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.
