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| 54 | ||
| 55 | <H2><A NAME="SECTION0051410000000000000000"> | |
| 56 | 3.14.1 Relationship to other Python modules</A> | |
| 57 | </H2> | |
| 58 | ||
| 59 | <P> | |
| 60 | The <tt class="module">pickle</tt> module has an optimized cousin called the | |
| 61 | <tt class="module">cPickle</tt> module. As its name implies, <tt class="module">cPickle</tt> is | |
| 62 | written in C, so it can be up to 1000 times faster than | |
| 63 | <tt class="module">pickle</tt>. However it does not support subclassing of the | |
| 64 | <tt class="function">Pickler()</tt> and <tt class="function">Unpickler()</tt> classes, because in | |
| 65 | <tt class="module">cPickle</tt> these are functions, not classes. Most applications | |
| 66 | have no need for this functionality, and can benefit from the improved | |
| 67 | performance of <tt class="module">cPickle</tt>. Other than that, the interfaces of | |
| 68 | the two modules are nearly identical; the common interface is | |
| 69 | described in this manual and differences are pointed out where | |
| 70 | necessary. In the following discussions, we use the term ``pickle'' | |
| 71 | to collectively describe the <tt class="module">pickle</tt> and | |
| 72 | <tt class="module">cPickle</tt> modules. | |
| 73 | ||
| 74 | <P> | |
| 75 | The data streams the two modules produce are guaranteed to be | |
| 76 | interchangeable. | |
| 77 | ||
| 78 | <P> | |
| 79 | Python has a more primitive serialization module called | |
| 80 | <tt class="module"><a href="module-marshal.html">marshal</a></tt>, but in general | |
| 81 | <tt class="module">pickle</tt> should always be the preferred way to serialize Python | |
| 82 | objects. <tt class="module">marshal</tt> exists primarily to support Python's | |
| 83 | <span class="file">.pyc</span> files. | |
| 84 | ||
| 85 | <P> | |
| 86 | The <tt class="module">pickle</tt> module differs from <tt class="module"><a href="module-marshal.html">marshal</a></tt> several | |
| 87 | significant ways: | |
| 88 | ||
| 89 | <P> | |
| 90 | ||
| 91 | <UL> | |
| 92 | <LI>The <tt class="module">pickle</tt> module keeps track of the objects it has | |
| 93 | already serialized, so that later references to the same object | |
| 94 | won't be serialized again. <tt class="module">marshal</tt> doesn't do this. | |
| 95 | ||
| 96 | <P> | |
| 97 | This has implications both for recursive objects and object | |
| 98 | sharing. Recursive objects are objects that contain references | |
| 99 | to themselves. These are not handled by marshal, and in fact, | |
| 100 | attempting to marshal recursive objects will crash your Python | |
| 101 | interpreter. Object sharing happens when there are multiple | |
| 102 | references to the same object in different places in the object | |
| 103 | hierarchy being serialized. <tt class="module">pickle</tt> stores such objects | |
| 104 | only once, and ensures that all other references point to the | |
| 105 | master copy. Shared objects remain shared, which can be very | |
| 106 | important for mutable objects. | |
| 107 | ||
| 108 | <P> | |
| 109 | </LI> | |
| 110 | <LI><tt class="module">marshal</tt> cannot be used to serialize user-defined | |
| 111 | classes and their instances. <tt class="module">pickle</tt> can save and | |
| 112 | restore class instances transparently, however the class | |
| 113 | definition must be importable and live in the same module as | |
| 114 | when the object was stored. | |
| 115 | ||
| 116 | <P> | |
| 117 | </LI> | |
| 118 | <LI>The <tt class="module">marshal</tt> serialization format is not guaranteed to | |
| 119 | be portable across Python versions. Because its primary job in | |
| 120 | life is to support <span class="file">.pyc</span> files, the Python implementers | |
| 121 | reserve the right to change the serialization format in | |
| 122 | non-backwards compatible ways should the need arise. The | |
| 123 | <tt class="module">pickle</tt> serialization format is guaranteed to be | |
| 124 | backwards compatible across Python releases. | |
| 125 | ||
| 126 | <P> | |
| 127 | </LI> | |
| 128 | </UL> | |
| 129 | ||
| 130 | <P> | |
| 131 | <div class="warning"><b class="label">Warning:</b> | |
| 132 | ||
| 133 | The <tt class="module">pickle</tt> module is not intended to be secure against | |
| 134 | erroneous or maliciously constructed data. Never unpickle data | |
| 135 | received from an untrusted or unauthenticated source. | |
| 136 | </div> | |
| 137 | ||
| 138 | <P> | |
| 139 | Note that serialization is a more primitive notion than persistence; | |
| 140 | although | |
| 141 | <tt class="module">pickle</tt> reads and writes file objects, it does not handle the | |
| 142 | issue of naming persistent objects, nor the (even more complicated) | |
| 143 | issue of concurrent access to persistent objects. The <tt class="module">pickle</tt> | |
| 144 | module can transform a complex object into a byte stream and it can | |
| 145 | transform the byte stream into an object with the same internal | |
| 146 | structure. Perhaps the most obvious thing to do with these byte | |
| 147 | streams is to write them onto a file, but it is also conceivable to | |
| 148 | send them across a network or store them in a database. The module | |
| 149 | <tt class="module"><a href="module-shelve.html">shelve</a></tt> provides a simple interface | |
| 150 | to pickle and unpickle objects on DBM-style database files. | |
| 151 | ||
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| 188 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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