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<H2><A NAME="SECTION0051710000000000000000">
3.17.1 Restrictions</A>
</H2>

<P>

<UL>
<LI>The choice of which database package will be used
(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
which interface is available.  Therefore it is not safe to open the database
directly using <tt class="module"><a href="module-dbm.html">dbm</a></tt>.  The database is also (unfortunately) subject
to the limitations of <tt class="module"><a href="module-dbm.html">dbm</a></tt>, if it is used -- this means
that (the pickled representation of) the objects stored in the
database should be fairly small, and in rare cases key collisions may
cause the database to refuse updates.
<a id='l2h-672' xml:id='l2h-672'></a>

<P>
</LI>
<LI>Depending on the implementation, closing a persistent dictionary may
or may not be necessary to flush changes to disk.  The <tt class="method">__del__</tt>
method of the <tt class="class">Shelf</tt> class calls the <tt class="method">close</tt> method, so the
programmer generally need not do this explicitly.

<P>
</LI>
<LI>The <tt class="module">shelve</tt> module does not support <em>concurrent</em> read/write
access to shelved objects.  (Multiple simultaneous read accesses are
safe.)  When a program has a shelf open for writing, no other program
should have it open for reading or writing.  <span class="Unix">Unix</span> file locking can
be used to solve this, but this differs across <span class="Unix">Unix</span> versions and
requires knowledge about the database implementation used.

<P>
</LI>
</UL>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-669' xml:id='l2h-669' class="class">Shelf</tt></b>(</nobr></td>
  <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>
<dd>
A subclass of <tt class="class">UserDict.DictMixin</tt> which stores pickled values in the
<var>dict</var> object.  

<P>
By default, version 0 pickles are used to serialize values.  The
version of the pickle protocol can be specified with the
<var>protocol</var> parameter. See the <tt class="module">pickle</tt> documentation for a
discussion of the pickle protocols. 
<span class="versionnote">Changed in version 2.3:
The <var>protocol</var>
parameter was added. The <var>binary</var> parameter is deprecated and
provided for backwards compatibility only.</span>

<P>
If the <var>writeback</var> parameter is <code>True</code>, the object will hold a
cache of all entries accessed and write them back to the <var>dict</var> at
sync and close times.  This allows natural operations on mutable entries,
but can consume much more memory and make sync and close take a long time.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-670' xml:id='l2h-670' class="class">BsdDbShelf</tt></b>(</nobr></td>
  <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>
<dd>

<P>
A subclass of <tt class="class">Shelf</tt> which exposes <tt class="method">first</tt>,
<tt class="method">next</tt>, <tt class="method">previous</tt>, <tt class="method">last</tt> and
<tt class="method">set_location</tt> which are available in the <tt class="module">bsddb</tt> module
but not in other database modules.  The <var>dict</var> object passed to
the constructor must support those methods.  This is generally
accomplished by calling one of <tt class="function">bsddb.hashopen</tt>,
<tt class="function">bsddb.btopen</tt> or <tt class="function">bsddb.rnopen</tt>.  The optional
<var>protocol</var>, <var>writeback</var>, and <var>binary</var> parameters have the
same interpretation as for the <tt class="class">Shelf</tt> class.

<P>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-671' xml:id='l2h-671' class="class">DbfilenameShelf</tt></b>(</nobr></td>
  <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>
<dd>

<P>
A subclass of <tt class="class">Shelf</tt> which accepts a <var>filename</var> instead of
a dict-like object.  The underlying file will be opened using
<tt class="function">anydbm.open</tt>.  By default, the file will be created and
opened for both read and write.  The optional <var>flag</var> parameter has
the same interpretation as for the <tt class="function">open</tt> function.  The
optional <var>protocol</var>, <var>writeback</var>, and <var>binary</var> parameters
have the same interpretation as for the <tt class="class">Shelf</tt> class.

<P>
</dl>

<P>

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	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
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158	<div class='online-navigation'>
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