# `dbm` — Interfaces to Unix “databases”

Código-fonte: Lib/dbm/__init__.py

`dbm` é uma interface genérica para variantes do banco de dados DBM — `dbm.gnu` ou `dbm.ndbm`. Se nenhum desses módulos estiver instalado, a implementação lenta, mas simples, no módulo `dbm.dumb` será usada. Há uma interface de terceiros para o Oracle Berkeley DB.

exception `dbm.``error`

  Uma tupla contendo as exceções que podem ser levantadas por cada um dos módulos suportados, com a única exceção também chamada `dbm.error` como o primeiro item — o último é usado quando `dbm.error` é levantada.

`dbm.``whichdb`(filename)

  Esta função tenta adivinhar qual dos vários módulos de banco de dados simples disponíveis — `dbm.gnu`, `dbm.ndbm` ou `dbm.dumb` — deve ser usado para abrir o arquivo fornecido.

  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.

  The optional flag argument can be:

  Valor | Significado
  --- | ---
  `'r'` | Open existing database for reading only (default)
  `'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 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()`.

Alterado na versão 3.2: `get()` and `setdefault()` are now available in all database modules.

Alterado na versão 3.8: 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.

Estes objetos também oferecem suporte a serem usados em uma instrução `with`, que vai fechá-los automaticamente quando estiver concluída.

Alterado na versão 3.4: Added native support for the context management protocol to the objects returned by `open()`.

Os seguintes exemplos registram alguns hostnames e um título correspondente, e então exibe o conteúdo do banco de dados:

    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.

Ver também

Módulo `shelve`

  Módulo persistente que armazena dados não-string.

Os submódulos individuais são descritos nas seções a seguir.

## `dbm.gnu` — GNU’s reinterpretation of dbm

Código-fonte: 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`

  Levantada em erros específicos do `dbm.gnu`, como erros de E/S. `KeyError` é levantada para erros gerais de mapeamento, como especificar uma chave incorreta.

`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.

  The optional flag argument can be:

  Valor | Significado
  --- | ---
  `'r'` | Open existing database for reading only (default)
  `'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 the flag to control how the database is opened:

  Valor | Significado
  --- | ---
  `'f'` | Open the database in fast mode. Writes to the database will not be synchronized.
  `'s'` | Synchronized mode. This will cause changes to the database to be immediately written to the file.
  `'u'` | Do not lock database.

  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)

    Retorna a chave que segue key na travessia. O código a seguir imprime cada chave no banco de dados `db`, sem precisar criar uma lista na memória que contenha todas elas:

        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`()

    Quando o banco de dados é aberto no modo rápido, esse método força a gravação de todos os dados não gravados no disco.

  `gdbm.``close`()

    Fecha o banco de dados `gdbm`.

## `dbm.ndbm` — Interface based on ndbm

Código-fonte: 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`

  Levantada em erros específicos de `dbm.ndbm`, como erros de E/S. `KeyError` é levantada para erros gerais de mapeamento, como especificar uma chave incorreta.

`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:

  Valor | Significado
  --- | ---
  `'r'` | Open existing database for reading only (default)
  `'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 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`()

    Fecha o banco de dados `ndbm`.

## `dbm.dumb` — Implementação portátil do DBM

Código-fonte: Lib/dbm/dumb.py

Nota

O módulo `dbm.dumb` é pensado como um último recurso alternativo para o módulo `dbm` quando um módulo mais robusto não está disponível. O módulo `dbm.dumb` não é escrito para velocidade e não é tão usado quanto os outros módulos de banco de dados.

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.

O módulo define o seguinte:

exception `dbm.dumb.``error`

  Levantada em erros específicos de `dbm.dumb`, como erros de E/S. `KeyError` é levantada para erros gerais de mapeamento, como especificar uma chave incorreta.

`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.

  The optional flag argument can be:

  Valor | Significado
  --- | ---
  `'r'` | Open existing database for reading only (default)
  `'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 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).

  Aviso

  É possível travar o interpretador Python ao carregar um banco de dados com uma entrada suficientemente grande/complexa devido a limitações de profundidade de pilha no compilador AST do Python.

  Alterado na versão 3.5: `open()` always creates a new database when the flag has the value `'n'`.

  Alterado na versão 3.8: 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.