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54 | <H3><A NAME="SECTION0051453000000000000000"> | |
55 | 3.14.5.3 Pickling and unpickling external objects</A> | |
56 | </H3> | |
57 | ||
58 | <P> | |
59 | For the benefit of object persistence, the <tt class="module">pickle</tt> module | |
60 | supports the notion of a reference to an object outside the pickled | |
61 | data stream. Such objects are referenced by a ``persistent id'', | |
62 | which is just an arbitrary string of printable ASCII characters. | |
63 | The resolution of such names is not defined by the <tt class="module">pickle</tt> | |
64 | module; it will delegate this resolution to user defined functions on | |
65 | the pickler and unpickler.<A NAME="tex2html20" | |
66 | HREF="#foot8985"><SUP>3.8</SUP></A> | |
67 | <P> | |
68 | To define external persistent id resolution, you need to set the | |
69 | <tt class="member">persistent_id</tt> attribute of the pickler object and the | |
70 | <tt class="member">persistent_load</tt> attribute of the unpickler object. | |
71 | ||
72 | <P> | |
73 | To pickle objects that have an external persistent id, the pickler | |
74 | must have a custom <tt class="function">persistent_id()</tt> method that takes an | |
75 | object as an argument and returns either <code>None</code> or the persistent | |
76 | id for that object. When <code>None</code> is returned, the pickler simply | |
77 | pickles the object as normal. When a persistent id string is | |
78 | returned, the pickler will pickle that string, along with a marker | |
79 | so that the unpickler will recognize the string as a persistent id. | |
80 | ||
81 | <P> | |
82 | To unpickle external objects, the unpickler must have a custom | |
83 | <tt class="function">persistent_load()</tt> function that takes a persistent id | |
84 | string and returns the referenced object. | |
85 | ||
86 | <P> | |
87 | Here's a silly example that <em>might</em> shed more light: | |
88 | ||
89 | <P> | |
90 | <div class="verbatim"><pre> | |
91 | import pickle | |
92 | from cStringIO import StringIO | |
93 | ||
94 | src = StringIO() | |
95 | p = pickle.Pickler(src) | |
96 | ||
97 | def persistent_id(obj): | |
98 | if hasattr(obj, 'x'): | |
99 | return 'the value %d' % obj.x | |
100 | else: | |
101 | return None | |
102 | ||
103 | p.persistent_id = persistent_id | |
104 | ||
105 | class Integer: | |
106 | def __init__(self, x): | |
107 | self.x = x | |
108 | def __str__(self): | |
109 | return 'My name is integer %d' % self.x | |
110 | ||
111 | i = Integer(7) | |
112 | print i | |
113 | p.dump(i) | |
114 | ||
115 | datastream = src.getvalue() | |
116 | print repr(datastream) | |
117 | dst = StringIO(datastream) | |
118 | ||
119 | up = pickle.Unpickler(dst) | |
120 | ||
121 | class FancyInteger(Integer): | |
122 | def __str__(self): | |
123 | return 'I am the integer %d' % self.x | |
124 | ||
125 | def persistent_load(persid): | |
126 | if persid.startswith('the value '): | |
127 | value = int(persid.split()[2]) | |
128 | return FancyInteger(value) | |
129 | else: | |
130 | raise pickle.UnpicklingError, 'Invalid persistent id' | |
131 | ||
132 | up.persistent_load = persistent_load | |
133 | ||
134 | j = up.load() | |
135 | print j | |
136 | </pre></div> | |
137 | ||
138 | <P> | |
139 | In the <tt class="module">cPickle</tt> module, the unpickler's | |
140 | <tt class="member">persistent_load</tt> attribute can also be set to a Python | |
141 | list, in which case, when the unpickler reaches a persistent id, the | |
142 | persistent id string will simply be appended to this list. This | |
143 | functionality exists so that a pickle data stream can be ``sniffed'' | |
144 | for object references without actually instantiating all the objects | |
145 | in a pickle.<A NAME="tex2html21" | |
146 | HREF="#foot8892"><SUP>3.9</SUP></A> Setting | |
147 | <tt class="member">persistent_load</tt> to a list is usually used in conjunction with | |
148 | the <tt class="method">noload()</tt> method on the Unpickler. | |
149 | ||
150 | <P> | |
151 | <BR><HR><H4>Footnotes</H4> | |
152 | <DL> | |
153 | <DT><A NAME="foot8985">... unpickler.</A><A | |
154 | HREF="node70.html#tex2html20"><SUP>3.8</SUP></A></DT> | |
155 | <DD>The actual mechanism for | |
156 | associating these user defined functions is slightly different for | |
157 | <tt class="module">pickle</tt> and <tt class="module">cPickle</tt>. The description given here | |
158 | works the same for both implementations. Users of the <tt class="module">pickle</tt> | |
159 | module could also use subclassing to effect the same results, | |
160 | overriding the <tt class="method">persistent_id()</tt> and <tt class="method">persistent_load()</tt> | |
161 | methods in the derived classes. | |
162 | ||
163 | </DD> | |
164 | <DT><A NAME="foot8892">... pickle.</A><A | |
165 | HREF="node70.html#tex2html21"><SUP>3.9</SUP></A></DT> | |
166 | <DD>We'll leave you with the image of Guido and Jim | |
167 | sitting around sniffing pickles in their living rooms. | |
168 | ||
169 | </DD> | |
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