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| 55 | <H1><A NAME="SECTION0010700000000000000000"> |
| 56 | 8.7 <tt class="module">gdbm</tt> -- |
| 57 | GNU's reinterpretation of dbm</A> |
| 58 | </H1> |
| 59 | |
| 60 | <P> |
| 61 | <A NAME="module-gdbm"></A> |
| 62 | <p class="availability">Availability: <span |
| 63 | class="platform">Unix</span>.</p> |
| 64 | |
| 65 | <P> |
| 66 | 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> |
| 67 | module, but uses <code>gdbm</code> instead to provide some additional |
| 68 | functionality. Please note that the file formats created by |
| 69 | <code>gdbm</code> and <code>dbm</code> are incompatible. |
| 70 | |
| 71 | <P> |
| 72 | The <tt class="module">gdbm</tt> module provides an interface to the GNU DBM |
| 73 | library. <code>gdbm</code> objects behave like mappings |
| 74 | (dictionaries), except that keys and values are always strings. |
| 75 | Printing a <code>gdbm</code> object doesn't print the keys and values, and |
| 76 | the <tt class="method">items()</tt> and <tt class="method">values()</tt> methods are not supported. |
| 77 | |
| 78 | <P> |
| 79 | The module defines the following constant and functions: |
| 80 | |
| 81 | <P> |
| 82 | <dl><dt><b><span class="typelabel">exception</span> <tt id='l2h-3000' xml:id='l2h-3000' class="exception">error</tt></b></dt> |
| 83 | <dd> |
| 84 | Raised on <code>gdbm</code>-specific errors, such as I/O errors. |
| 85 | <tt class="exception">KeyError</tt> is raised for general mapping errors like |
| 86 | specifying an incorrect key. |
| 87 | </dd></dl> |
| 88 | |
| 89 | <P> |
| 90 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 91 | <td><nobr><b><tt id='l2h-3001' xml:id='l2h-3001' class="function">open</tt></b>(</nobr></td> |
| 92 | <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> |
| 93 | <dd> |
| 94 | Open a <code>gdbm</code> database and return a <code>gdbm</code> object. The |
| 95 | <var>filename</var> argument is the name of the database file. |
| 96 | |
| 97 | <P> |
| 98 | The optional <var>flag</var> argument can be |
| 99 | <code>'r'</code> (to open an existing database for reading only -- default), |
| 100 | <code>'w'</code> (to open an existing database for reading and writing), |
| 101 | <code>'c'</code> (which creates the database if it doesn't exist), or |
| 102 | <code>'n'</code> (which always creates a new empty database). |
| 103 | |
| 104 | <P> |
| 105 | The following additional characters may be appended to the flag to |
| 106 | control how the database is opened: |
| 107 | |
| 108 | <P> |
| 109 | |
| 110 | <UL> |
| 111 | <LI><code>'f'</code> -- Open the database in fast mode. Writes to the database |
| 112 | will not be synchronized. |
| 113 | </LI> |
| 114 | <LI><code>'s'</code> -- Synchronized mode. This will cause changes to the database |
| 115 | will be immediately written to the file. |
| 116 | </LI> |
| 117 | <LI><code>'u'</code> -- Do not lock database. |
| 118 | </LI> |
| 119 | </UL> |
| 120 | |
| 121 | <P> |
| 122 | Not all flags are valid for all versions of <code>gdbm</code>. The |
| 123 | module constant <code>open_flags</code> is a string of supported flag |
| 124 | characters. The exception <tt class="exception">error</tt> is raised if an invalid |
| 125 | flag is specified. |
| 126 | |
| 127 | <P> |
| 128 | The optional <var>mode</var> argument is the <span class="Unix">Unix</span> mode of the file, used |
| 129 | only when the database has to be created. It defaults to octal |
| 130 | <code>0666</code>. |
| 131 | </dl> |
| 132 | |
| 133 | <P> |
| 134 | In addition to the dictionary-like methods, <code>gdbm</code> objects have the |
| 135 | following methods: |
| 136 | |
| 137 | <P> |
| 138 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 139 | <td><nobr><b><tt id='l2h-3002' xml:id='l2h-3002' class="function">firstkey</tt></b>(</nobr></td> |
| 140 | <td><var></var>)</td></tr></table></dt> |
| 141 | <dd> |
| 142 | It's possible to loop over every key in the database using this method |
| 143 | and the <tt class="method">nextkey()</tt> method. The traversal is ordered by |
| 144 | <code>gdbm</code>'s internal hash values, and won't be sorted by the key |
| 145 | values. This method returns the starting key. |
| 146 | </dl> |
| 147 | |
| 148 | <P> |
| 149 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 150 | <td><nobr><b><tt id='l2h-3003' xml:id='l2h-3003' class="function">nextkey</tt></b>(</nobr></td> |
| 151 | <td><var>key</var>)</td></tr></table></dt> |
| 152 | <dd> |
| 153 | Returns the key that follows <var>key</var> in the traversal. The |
| 154 | following code prints every key in the database <code>db</code>, without |
| 155 | having to create a list in memory that contains them all: |
| 156 | |
| 157 | <P> |
| 158 | <div class="verbatim"><pre> |
| 159 | k = db.firstkey() |
| 160 | while k != None: |
| 161 | print k |
| 162 | k = db.nextkey(k) |
| 163 | </pre></div> |
| 164 | </dl> |
| 165 | |
| 166 | <P> |
| 167 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 168 | <td><nobr><b><tt id='l2h-3004' xml:id='l2h-3004' class="function">reorganize</tt></b>(</nobr></td> |
| 169 | <td><var></var>)</td></tr></table></dt> |
| 170 | <dd> |
| 171 | If you have carried out a lot of deletions and would like to shrink |
| 172 | the space used by the <code>gdbm</code> file, this routine will reorganize |
| 173 | the database. <code>gdbm</code> will not shorten the length of a database |
| 174 | file except by using this reorganization; otherwise, deleted file |
| 175 | space will be kept and reused as new (key, value) pairs are added. |
| 176 | </dl> |
| 177 | |
| 178 | <P> |
| 179 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 180 | <td><nobr><b><tt id='l2h-3005' xml:id='l2h-3005' class="function">sync</tt></b>(</nobr></td> |
| 181 | <td><var></var>)</td></tr></table></dt> |
| 182 | <dd> |
| 183 | When the database has been opened in fast mode, this method forces any |
| 184 | unwritten data to be written to the disk. |
| 185 | </dl> |
| 186 | |
| 187 | <P> |
| 188 | <div class="seealso"> |
| 189 | <p class="heading">See Also:</p> |
| 190 | |
| 191 | <dl compact="compact" class="seemodule"> |
| 192 | <dt>Module <b><tt class="module"><a href="module-anydbm.html">anydbm</a></tt>:</b> |
| 193 | <dd>Generic interface to <code>dbm</code>-style databases. |
| 194 | </dl> |
| 195 | <dl compact="compact" class="seemodule"> |
| 196 | <dt>Module <b><tt class="module"><a href="module-whichdb.html">whichdb</a></tt>:</b> |
| 197 | <dd>Utility module used to determine the type of an |
| 198 | existing database. |
| 199 | </dl> |
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| 236 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> |
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