dbm — Interfaces to Unix “databases” (original) (raw)

Source code: Lib/dbm/__init__.py

[dbm](#module-dbm "dbm: Interfaces to various Unix "database" formats.") is a generic interface to variants of the DBM database:

If none of these modules are 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

A tuple containing the exceptions that can be raised by each of the supported modules, with a unique exception also named dbm.error as the first item — the latter is used when dbm.error is raised.

dbm.whichdb(filename)

This function attempts to guess which of the several simple database modules available — dbm.sqlite3, dbm.gnu, dbm.ndbm, or dbm.dumb — should be used to open a given file.

Return one of the following values:

dbm.open(file, flag='r', mode=0o666)

Open a database and return the corresponding database object.

Parameters:

The object returned by open() supports the same basic functionality as adict; 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.

These objects also support being used in a with statement, which will automatically close them when done.

Changed in version 3.2: get() and setdefault() methods are now available for all[dbm](#module-dbm "dbm: Interfaces to various Unix "database" formats.") backends.

Changed in version 3.4: Added native support for the context management protocol to the objects returned by open().

Changed in version 3.8: Deleting a key from a read-only database raises a database module specific exception instead of KeyError.

The following example records some hostnames and a corresponding title, and then prints out the contents of the database:

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.

See also

Module shelve

Persistence module which stores non-string data.

The individual submodules are described in the following sections.

dbm.sqlite3 — SQLite backend for dbm

Added in version 3.13.

Source code: Lib/dbm/sqlite3.py

This module uses the standard library sqlite3 module to provide an SQLite backend for the [dbm](#module-dbm "dbm: Interfaces to various Unix "database" formats.") module. The files created by dbm.sqlite3 can thus be opened by sqlite3, or any other SQLite browser, including the SQLite CLI.

dbm.sqlite3.open(filename, /, flag='r', mode=0o666)

Open an SQLite database. The returned object behaves like a mapping, implements a close() method, and supports a “closing” context manager via the with keyword.

Parameters:

dbm.gnu — GNU database manager

Source code: Lib/dbm/gnu.py

The dbm.gnu module provides an interface to the GDBMlibrary, similar to the dbm.ndbm module, but with additional functionality like crash tolerance.

Note

The file formats created by dbm.gnu and dbm.ndbm are incompatible and can not be used interchangeably.

exception dbm.gnu.error

Raised on dbm.gnu-specific errors, such as I/O errors. KeyError is raised for general mapping errors like specifying an incorrect key.

dbm.gnu.open(filename, flag='r', mode=0o666, /)

Open a GDBM database and return a gdbm object.

Parameters:

Raises:

error – If an invalid flag argument is passed.

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 thenextkey() 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)

Returns the key that follows key in the traversal. The following code prints every key in the database db, without having to create a list in memory that contains them all:

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. gdbmobjects 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()

When the database has been opened in fast mode, this method forces any unwritten data to be written to the disk.

gdbm.close()

Close the GDBM database.

gdbm.clear()

Remove all items from the GDBM database.

Added in version 3.13.

dbm.ndbm — New Database Manager

Source code: Lib/dbm/ndbm.py

The dbm.ndbm module provides an interface to theNDBM library. This module can be used with the “classic” NDBM interface or theGDBM compatibility interface.

Note

The file formats created by dbm.gnu and dbm.ndbm are incompatible and can not be used interchangeably.

Warning

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

Raised on dbm.ndbm-specific errors, such as I/O errors. KeyError is raised for general mapping errors like specifying an incorrect key.

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.

Parameters:

ndbm objects behave similar to mappings, but items() and values() methods are not supported. The following methods are also provided:

ndbm.close()

Close the NDBM database.

ndbm.clear()

Remove all items from the NDBM database.

Added in version 3.13.

dbm.dumb — Portable DBM implementation

Source code: Lib/dbm/dumb.py

Note

The dbm.dumb module is intended as a last resort fallback for the[dbm](#module-dbm "dbm: Interfaces to various Unix "database" formats.") module when a more robust module is not available. The dbm.dumbmodule is not written for speed and is not nearly as heavily used as the other database modules.

The dbm.dumb module provides a persistent dict-like interface which is written entirely in Python. Unlike other [dbm](#module-dbm "dbm: Interfaces to various Unix "database" formats.") backends, such as dbm.gnu, no external library is required.

The dbm.dumb module defines the following:

exception dbm.dumb.error

Raised on dbm.dumb-specific errors, such as I/O errors. KeyError is raised for general mapping errors like specifying an incorrect key.

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.

Parameters:

Warning

It is possible to crash the Python interpreter when loading a database with a sufficiently large/complex entry due to stack depth limitations in Python’s AST compiler.

Changed in version 3.5: open() always creates a new database when flag is 'n'.

Changed in version 3.8: 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'.

In addition to the methods provided by thecollections.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.