11.9. gdbm — GNU’s reinterpretation of dbm

This module is quite similar to the dbm module, but uses gdbm instead to provide some additional functionality. Please note that the file formats created by gdbm and dbm are incompatible.

The gdbm module provides an interface to the GNU DBM library. gdbm objects behave like mappings (dictionaries), except that keys and values are always strings. Printing a gdbm object doesn’t print the keys and values, and the items() and values() methods are not supported.

The module defines the following constant and functions:

exception gdbm.error

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

gdbm.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 0666.

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 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.