8.7 Built-in Module gdbm

 

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:

error

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

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 'r' (to open an existing database for reading only -- default), 'w' (to open an existing database for reading and writing), 'c' (which creates the database if it doesn't exist), or 'n' (which always creates a new empty database).

Appending f to the flag opens the database in fast mode; altered data will not automatically be written to the disk after every change. This results in faster writes to the database, but may result in an inconsistent database if the program crashes while the database is still open. Use the sync() method to force any unwritten data to be written to the disk.

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:

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.

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)

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.

sync ()

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