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55 | <H2><A NAME="SECTION0051460000000000000000"></A><A NAME="pickle-sub"></A> | |
56 | <BR> | |
57 | 3.14.6 Subclassing Unpicklers | |
58 | </H2> | |
59 | ||
60 | <P> | |
61 | By default, unpickling will import any class that it finds in the | |
62 | pickle data. You can control exactly what gets unpickled and what | |
63 | gets called by customizing your unpickler. Unfortunately, exactly how | |
64 | you do this is different depending on whether you're using | |
65 | <tt class="module">pickle</tt> or <tt class="module">cPickle</tt>.<A NAME="tex2html22" | |
66 | HREF="#foot8987"><SUP>3.10</SUP></A> | |
67 | <P> | |
68 | In the <tt class="module">pickle</tt> module, you need to derive a subclass from | |
69 | <tt class="class">Unpickler</tt>, overriding the <tt class="method">load_global()</tt> | |
70 | method. <tt class="method">load_global()</tt> should read two lines from the pickle | |
71 | data stream where the first line will the name of the module | |
72 | containing the class and the second line will be the name of the | |
73 | instance's class. It then looks up the class, possibly importing the | |
74 | module and digging out the attribute, then it appends what it finds to | |
75 | the unpickler's stack. Later on, this class will be assigned to the | |
76 | <tt class="member">__class__</tt> attribute of an empty class, as a way of magically | |
77 | creating an instance without calling its class's <tt class="method">__init__()</tt>. | |
78 | Your job (should you choose to accept it), would be to have | |
79 | <tt class="method">load_global()</tt> push onto the unpickler's stack, a known safe | |
80 | version of any class you deem safe to unpickle. It is up to you to | |
81 | produce such a class. Or you could raise an error if you want to | |
82 | disallow all unpickling of instances. If this sounds like a hack, | |
83 | you're right. Refer to the source code to make this work. | |
84 | ||
85 | <P> | |
86 | Things are a little cleaner with <tt class="module">cPickle</tt>, but not by much. | |
87 | To control what gets unpickled, you can set the unpickler's | |
88 | <tt class="member">find_global</tt> attribute to a function or <code>None</code>. If it is | |
89 | <code>None</code> then any attempts to unpickle instances will raise an | |
90 | <tt class="exception">UnpicklingError</tt>. If it is a function, | |
91 | then it should accept a module name and a class name, and return the | |
92 | corresponding class object. It is responsible for looking up the | |
93 | class and performing any necessary imports, and it may raise an | |
94 | error to prevent instances of the class from being unpickled. | |
95 | ||
96 | <P> | |
97 | The moral of the story is that you should be really careful about the | |
98 | source of the strings your application unpickles. | |
99 | ||
100 | <P> | |
101 | <BR><HR><H4>Footnotes</H4> | |
102 | <DL> | |
103 | <DT><A NAME="foot8987">...cPickle.</A><A | |
104 | href="pickle-sub.html#tex2html22"><SUP>3.10</SUP></A></DT> | |
105 | <DD>A word of caution: the | |
106 | mechanisms described here use internal attributes and methods, which | |
107 | are subject to change in future versions of Python. We intend to | |
108 | someday provide a common interface for controlling this behavior, | |
109 | which will work in either <tt class="module">pickle</tt> or <tt class="module">cPickle</tt>. | |
110 | ||
111 | </DD> | |
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