"dbm" --- Interfaces to Unix "databases"
****************************************

**Código-fonte:** 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

   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

======================================================================

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.
