dbm — Interfaces to Unix “databases”

Código-fonte: Lib/dbm/__init__.py

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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

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)

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.

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

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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

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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.