12.5. dbm — Interfaces to Unix “databases”

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

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.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:

Value Meaning
'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().

Changed in version 3.2: get() and setdefault() are now available in all database modules.

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

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.

12.5.1. dbm.gnu — GNU’s reinterpretation of dbm

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

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[, 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:

Value Meaning
'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:

Value Meaning
'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)

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 != 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()

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

12.5.2. dbm.ndbm — Interface based on ndbm

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

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[, mode]])

Open a dbm database and return a dbm 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:

Value Meaning
'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).

12.5.3. dbm.dumb — Portable DBM implementation

Note

The dbm.dumb module is intended as a last resort fallback for the dbm module when a more robust module is not available. The dbm.dumb module is not written for speed and is not nearly as heavily used as the other database modules.

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.

The 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[, 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 is currently ignored; the database is always opened for update, and will be created if it does not exist.

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 methods provided by the collections.abc.MutableMapping class, dumbdbm objects provide the following method:

dumbdbm.sync()

Synchronize the on-disk directory and data files. This method is called by the Shelve.sync() method.