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<title>8.7 gdbm -- GNU's reinterpretation of dbm</title>
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<H1><A NAME="SECTION0010700000000000000000">
8.7 <tt class="module">gdbm</tt> --
         GNU's reinterpretation of dbm</A>
</H1>

<P>
<A NAME="module-gdbm"></A>
<p class="availability">Availability: <span
 class="platform">Unix</span>.</p>

<P>
This module is quite similar to the <tt class="module"><a href="module-dbm.html">dbm</a></tt><a id='l2h-3006' xml:id='l2h-3006'></a>
module, but uses <code>gdbm</code> instead to provide some additional
functionality.  Please note that the file formats created by
<code>gdbm</code> and <code>dbm</code> are incompatible.

<P>
The <tt class="module">gdbm</tt> module provides an interface to the GNU DBM
library.  <code>gdbm</code> objects behave like mappings
(dictionaries), except that keys and values are always strings.
Printing a <code>gdbm</code> object doesn't print the keys and values, and
the <tt class="method">items()</tt> and <tt class="method">values()</tt> methods are not supported.

<P>
The module defines the following constant and functions:

<P>
<dl><dt><b><span class="typelabel">exception</span>&nbsp;<tt id='l2h-3000' xml:id='l2h-3000' class="exception">error</tt></b></dt>
<dd>
Raised on <code>gdbm</code>-specific errors, such as I/O errors.
<tt class="exception">KeyError</tt> is raised for general mapping errors like
specifying an incorrect key.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3001' xml:id='l2h-3001' class="function">open</tt></b>(</nobr></td>
  <td><var>filename, </var><big>[</big><var>flag, </var><big>[</big><var>mode</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Open a <code>gdbm</code> database and return a <code>gdbm</code> object.  The
<var>filename</var> argument is the name of the database file.

<P>
The optional <var>flag</var> argument can be
<code>'r'</code> (to open an existing database for reading only -- default),
<code>'w'</code> (to open an existing database for reading and writing),
<code>'c'</code> (which creates the database if it doesn't exist), or
<code>'n'</code> (which always creates a new empty database).

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

<P>

<UL>
<LI><code>'f'</code> -- Open the database in fast mode.  Writes to the database
                     will not be synchronized.
</LI>
<LI><code>'s'</code> -- Synchronized mode. This will cause changes to the database
                     will be immediately written to the file.
</LI>
<LI><code>'u'</code> -- Do not lock database.
</LI>
</UL>

<P>
Not all flags are valid for all versions of <code>gdbm</code>.  The
module constant <code>open_flags</code> is a string of supported flag
characters.  The exception <tt class="exception">error</tt> is raised if an invalid
flag is specified.

<P>
The optional <var>mode</var> argument is the <span class="Unix">Unix</span> mode of the file, used
only when the database has to be created.  It defaults to octal
<code>0666</code>.
</dl>

<P>
In addition to the dictionary-like methods, <code>gdbm</code> objects have the
following methods:

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3002' xml:id='l2h-3002' class="function">firstkey</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
It's possible to loop over every key in the database using this method
and the <tt class="method">nextkey()</tt> method.  The traversal is ordered by
<code>gdbm</code>'s internal hash values, and won't be sorted by the key
values.  This method returns the starting key.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3003' xml:id='l2h-3003' class="function">nextkey</tt></b>(</nobr></td>
  <td><var>key</var>)</td></tr></table></dt>
<dd>
Returns the key that follows <var>key</var> in the traversal.  The
following code prints every key in the database <code>db</code>, without
having to create a list in memory that contains them all:

<P>
<div class="verbatim"><pre>
k = db.firstkey()
while k != None:
    print k
    k = db.nextkey(k)
</pre></div>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3004' xml:id='l2h-3004' class="function">reorganize</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
If you have carried out a lot of deletions and would like to shrink
the space used by the <code>gdbm</code> file, this routine will reorganize
the database.  <code>gdbm</code> 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.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3005' xml:id='l2h-3005' class="function">sync</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
When the database has been opened in fast mode, this method forces any
unwritten data to be written to the disk.
</dl>

<P>
<div class="seealso">
  <p class="heading">See Also:</p>

  <dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-anydbm.html">anydbm</a></tt>:</b>
    <dd>Generic interface to <code>dbm</code>-style databases.
  </dl>
  <dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-whichdb.html">whichdb</a></tt>:</b>
    <dd>Utility module used to determine the type of an
                      existing database.
  </dl>
</div>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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