11.9. "gdbm" — GNU’s reinterpretation of dbm
********************************************

Note: The "gdbm" module has been renamed to "dbm.gnu" in Python 3.
  The *2to3* tool will automatically adapt imports when converting
  your sources to Python 3.

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.

See also:

  Module "anydbm"
     Generic interface to "dbm"-style databases.

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