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<title>3.17.1 Restrictions
</title>
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<H2><A NAME=
"SECTION0051710000000000000000">
<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>
<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.
<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.
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <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>
A subclass of
<tt class=
"class">UserDict.DictMixin
</tt> which stores pickled values in the
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:
parameter was added. The
<var>binary
</var> parameter is deprecated and
provided for backwards compatibility only.
</span>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><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <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>
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.
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <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>
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.
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<span class=
"release-info">Release
2.4.2, documentation updated on
28 September
2005.
</span>
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