# 13.9 `gdbm` -- GNU's reinterpretation of dbm

Availability: Unix.

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 `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).

  The following additional characters may be appended to the flag to control how the database is opened:

  - `'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 will 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:

`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.

See Also:

Module `anydbm`:
  Generic interface to `dbm`-style databases.

Module `whichdb`:
  Utility module used to determine the type of an existing database.