| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> |
| 2 | <html> |
| 3 | <head> |
| 4 | <link rel="STYLESHEET" href="lib.css" type='text/css' /> |
| 5 | <link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" /> |
| 6 | <link rel='start' href='../index.html' title='Python Documentation Index' /> |
| 7 | <link rel="first" href="lib.html" title='Python Library Reference' /> |
| 8 | <link rel='contents' href='contents.html' title="Contents" /> |
| 9 | <link rel='index' href='genindex.html' title='Index' /> |
| 10 | <link rel='last' href='about.html' title='About this document...' /> |
| 11 | <link rel='help' href='about.html' title='About this document...' /> |
| 12 | <link rel="next" href="node77.html" /> |
| 13 | <link rel="prev" href="module-shelve.html" /> |
| 14 | <link rel="parent" href="module-shelve.html" /> |
| 15 | <link rel="next" href="node77.html" /> |
| 16 | <meta name='aesop' content='information' /> |
| 17 | <title>3.17.1 Restrictions</title> |
| 18 | </head> |
| 19 | <body> |
| 20 | <DIV CLASS="navigation"> |
| 21 | <div id='top-navigation-panel' xml:id='top-navigation-panel'> |
| 22 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> |
| 23 | <tr> |
| 24 | <td class='online-navigation'><a rel="prev" title="3.17 shelve " |
| 25 | href="module-shelve.html"><img src='../icons/previous.png' |
| 26 | border='0' height='32' alt='Previous Page' width='32' /></A></td> |
| 27 | <td class='online-navigation'><a rel="parent" title="3.17 shelve " |
| 28 | href="module-shelve.html"><img src='../icons/up.png' |
| 29 | border='0' height='32' alt='Up One Level' width='32' /></A></td> |
| 30 | <td class='online-navigation'><a rel="next" title="3.17.2 Example" |
| 31 | href="node77.html"><img src='../icons/next.png' |
| 32 | border='0' height='32' alt='Next Page' width='32' /></A></td> |
| 33 | <td align="center" width="100%">Python Library Reference</td> |
| 34 | <td class='online-navigation'><a rel="contents" title="Table of Contents" |
| 35 | href="contents.html"><img src='../icons/contents.png' |
| 36 | border='0' height='32' alt='Contents' width='32' /></A></td> |
| 37 | <td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png' |
| 38 | border='0' height='32' alt='Module Index' width='32' /></a></td> |
| 39 | <td class='online-navigation'><a rel="index" title="Index" |
| 40 | href="genindex.html"><img src='../icons/index.png' |
| 41 | border='0' height='32' alt='Index' width='32' /></A></td> |
| 42 | </tr></table> |
| 43 | <div class='online-navigation'> |
| 44 | <b class="navlabel">Previous:</b> |
| 45 | <a class="sectref" rel="prev" href="module-shelve.html">3.17 shelve </A> |
| 46 | <b class="navlabel">Up:</b> |
| 47 | <a class="sectref" rel="parent" href="module-shelve.html">3.17 shelve </A> |
| 48 | <b class="navlabel">Next:</b> |
| 49 | <a class="sectref" rel="next" href="node77.html">3.17.2 Example</A> |
| 50 | </div> |
| 51 | <hr /></div> |
| 52 | </DIV> |
| 53 | <!--End of Navigation Panel--> |
| 54 | |
| 55 | <H2><A NAME="SECTION0051710000000000000000"> |
| 56 | 3.17.1 Restrictions</A> |
| 57 | </H2> |
| 58 | |
| 59 | <P> |
| 60 | |
| 61 | <UL> |
| 62 | <LI>The choice of which database package will be used |
| 63 | (such as <tt class="module"><a href="module-dbm.html">dbm</a></tt>, <tt class="module"><a href="module-gdbm.html">gdbm</a></tt> or <tt class="module"><a href="module-bsddb.html">bsddb</a></tt>) depends on |
| 64 | which interface is available. Therefore it is not safe to open the database |
| 65 | directly using <tt class="module"><a href="module-dbm.html">dbm</a></tt>. The database is also (unfortunately) subject |
| 66 | to the limitations of <tt class="module"><a href="module-dbm.html">dbm</a></tt>, if it is used -- this means |
| 67 | that (the pickled representation of) the objects stored in the |
| 68 | database should be fairly small, and in rare cases key collisions may |
| 69 | cause the database to refuse updates. |
| 70 | <a id='l2h-672' xml:id='l2h-672'></a> |
| 71 | |
| 72 | <P> |
| 73 | </LI> |
| 74 | <LI>Depending on the implementation, closing a persistent dictionary may |
| 75 | or may not be necessary to flush changes to disk. The <tt class="method">__del__</tt> |
| 76 | method of the <tt class="class">Shelf</tt> class calls the <tt class="method">close</tt> method, so the |
| 77 | programmer generally need not do this explicitly. |
| 78 | |
| 79 | <P> |
| 80 | </LI> |
| 81 | <LI>The <tt class="module">shelve</tt> module does not support <em>concurrent</em> read/write |
| 82 | access to shelved objects. (Multiple simultaneous read accesses are |
| 83 | safe.) When a program has a shelf open for writing, no other program |
| 84 | should have it open for reading or writing. <span class="Unix">Unix</span> file locking can |
| 85 | be used to solve this, but this differs across <span class="Unix">Unix</span> versions and |
| 86 | requires knowledge about the database implementation used. |
| 87 | |
| 88 | <P> |
| 89 | </LI> |
| 90 | </UL> |
| 91 | |
| 92 | <P> |
| 93 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 94 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-669' xml:id='l2h-669' class="class">Shelf</tt></b>(</nobr></td> |
| 95 | <td><var>dict</var><big>[</big><var>, protocol=None</var><big>[</big><var>, writeback=False</var><big>[</big><var>, binary=None</var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt> |
| 96 | <dd> |
| 97 | A subclass of <tt class="class">UserDict.DictMixin</tt> which stores pickled values in the |
| 98 | <var>dict</var> object. |
| 99 | |
| 100 | <P> |
| 101 | By default, version 0 pickles are used to serialize values. The |
| 102 | version of the pickle protocol can be specified with the |
| 103 | <var>protocol</var> parameter. See the <tt class="module">pickle</tt> documentation for a |
| 104 | discussion of the pickle protocols. |
| 105 | <span class="versionnote">Changed in version 2.3: |
| 106 | The <var>protocol</var> |
| 107 | parameter was added. The <var>binary</var> parameter is deprecated and |
| 108 | provided for backwards compatibility only.</span> |
| 109 | |
| 110 | <P> |
| 111 | If the <var>writeback</var> parameter is <code>True</code>, the object will hold a |
| 112 | cache of all entries accessed and write them back to the <var>dict</var> at |
| 113 | sync and close times. This allows natural operations on mutable entries, |
| 114 | but can consume much more memory and make sync and close take a long time. |
| 115 | </dl> |
| 116 | |
| 117 | <P> |
| 118 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 119 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-670' xml:id='l2h-670' class="class">BsdDbShelf</tt></b>(</nobr></td> |
| 120 | <td><var>dict</var><big>[</big><var>, protocol=None</var><big>[</big><var>, writeback=False</var><big>[</big><var>, binary=None</var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt> |
| 121 | <dd> |
| 122 | |
| 123 | <P> |
| 124 | A subclass of <tt class="class">Shelf</tt> which exposes <tt class="method">first</tt>, |
| 125 | <tt class="method">next</tt>, <tt class="method">previous</tt>, <tt class="method">last</tt> and |
| 126 | <tt class="method">set_location</tt> which are available in the <tt class="module">bsddb</tt> module |
| 127 | but not in other database modules. The <var>dict</var> object passed to |
| 128 | the constructor must support those methods. This is generally |
| 129 | accomplished by calling one of <tt class="function">bsddb.hashopen</tt>, |
| 130 | <tt class="function">bsddb.btopen</tt> or <tt class="function">bsddb.rnopen</tt>. The optional |
| 131 | <var>protocol</var>, <var>writeback</var>, and <var>binary</var> parameters have the |
| 132 | same interpretation as for the <tt class="class">Shelf</tt> class. |
| 133 | |
| 134 | <P> |
| 135 | </dl> |
| 136 | |
| 137 | <P> |
| 138 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 139 | <td><nobr><b><span class="typelabel">class</span> <tt id='l2h-671' xml:id='l2h-671' class="class">DbfilenameShelf</tt></b>(</nobr></td> |
| 140 | <td><var>filename</var><big>[</big><var>, flag='c'</var><big>[</big><var>, protocol=None</var><big>[</big><var>, writeback=False</var><big>[</big><var>, binary=None</var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt> |
| 141 | <dd> |
| 142 | |
| 143 | <P> |
| 144 | A subclass of <tt class="class">Shelf</tt> which accepts a <var>filename</var> instead of |
| 145 | a dict-like object. The underlying file will be opened using |
| 146 | <tt class="function">anydbm.open</tt>. By default, the file will be created and |
| 147 | opened for both read and write. The optional <var>flag</var> parameter has |
| 148 | the same interpretation as for the <tt class="function">open</tt> function. The |
| 149 | optional <var>protocol</var>, <var>writeback</var>, and <var>binary</var> parameters |
| 150 | have the same interpretation as for the <tt class="class">Shelf</tt> class. |
| 151 | |
| 152 | <P> |
| 153 | </dl> |
| 154 | |
| 155 | <P> |
| 156 | |
| 157 | <DIV CLASS="navigation"> |
| 158 | <div class='online-navigation'> |
| 159 | <p></p><hr /> |
| 160 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> |
| 161 | <tr> |
| 162 | <td class='online-navigation'><a rel="prev" title="3.17 shelve " |
| 163 | href="module-shelve.html"><img src='../icons/previous.png' |
| 164 | border='0' height='32' alt='Previous Page' width='32' /></A></td> |
| 165 | <td class='online-navigation'><a rel="parent" title="3.17 shelve " |
| 166 | href="module-shelve.html"><img src='../icons/up.png' |
| 167 | border='0' height='32' alt='Up One Level' width='32' /></A></td> |
| 168 | <td class='online-navigation'><a rel="next" title="3.17.2 Example" |
| 169 | href="node77.html"><img src='../icons/next.png' |
| 170 | border='0' height='32' alt='Next Page' width='32' /></A></td> |
| 171 | <td align="center" width="100%">Python Library Reference</td> |
| 172 | <td class='online-navigation'><a rel="contents" title="Table of Contents" |
| 173 | href="contents.html"><img src='../icons/contents.png' |
| 174 | border='0' height='32' alt='Contents' width='32' /></A></td> |
| 175 | <td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png' |
| 176 | border='0' height='32' alt='Module Index' width='32' /></a></td> |
| 177 | <td class='online-navigation'><a rel="index" title="Index" |
| 178 | href="genindex.html"><img src='../icons/index.png' |
| 179 | border='0' height='32' alt='Index' width='32' /></A></td> |
| 180 | </tr></table> |
| 181 | <div class='online-navigation'> |
| 182 | <b class="navlabel">Previous:</b> |
| 183 | <a class="sectref" rel="prev" href="module-shelve.html">3.17 shelve </A> |
| 184 | <b class="navlabel">Up:</b> |
| 185 | <a class="sectref" rel="parent" href="module-shelve.html">3.17 shelve </A> |
| 186 | <b class="navlabel">Next:</b> |
| 187 | <a class="sectref" rel="next" href="node77.html">3.17.2 Example</A> |
| 188 | </div> |
| 189 | </div> |
| 190 | <hr /> |
| 191 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> |
| 192 | </DIV> |
| 193 | <!--End of Navigation Panel--> |
| 194 | <ADDRESS> |
| 195 | See <i><a href="about.html">About this document...</a></i> for information on suggesting changes. |
| 196 | </ADDRESS> |
| 197 | </BODY> |
| 198 | </HTML> |