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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> | |
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