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Initial commit of OpenSPARC T2 design and verification files.
[OpenSPARC-T2-DV] / tools / src / nas,5.n2.os.2 / lib / python / html / python / lib / node70.html
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53
54<H3><A NAME="SECTION0051453000000000000000">
553.14.5.3 Pickling and unpickling external objects</A>
56</H3>
57
58<P>
59For the benefit of object persistence, the <tt class="module">pickle</tt> module
60supports the notion of a reference to an object outside the pickled
61data stream. Such objects are referenced by a ``persistent id'',
62which is just an arbitrary string of printable ASCII characters.
63The resolution of such names is not defined by the <tt class="module">pickle</tt>
64module; it will delegate this resolution to user defined functions on
65the pickler and unpickler.<A NAME="tex2html20"
66 HREF="#foot8985"><SUP>3.8</SUP></A>
67<P>
68To 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>
73To pickle objects that have an external persistent id, the pickler
74must have a custom <tt class="function">persistent_id()</tt> method that takes an
75object as an argument and returns either <code>None</code> or the persistent
76id for that object. When <code>None</code> is returned, the pickler simply
77pickles the object as normal. When a persistent id string is
78returned, the pickler will pickle that string, along with a marker
79so that the unpickler will recognize the string as a persistent id.
80
81<P>
82To unpickle external objects, the unpickler must have a custom
83<tt class="function">persistent_load()</tt> function that takes a persistent id
84string and returns the referenced object.
85
86<P>
87Here's a silly example that <em>might</em> shed more light:
88
89<P>
90<div class="verbatim"><pre>
91import pickle
92from cStringIO import StringIO
93
94src = StringIO()
95p = pickle.Pickler(src)
96
97def persistent_id(obj):
98 if hasattr(obj, 'x'):
99 return 'the value %d' % obj.x
100 else:
101 return None
102
103p.persistent_id = persistent_id
104
105class Integer:
106 def __init__(self, x):
107 self.x = x
108 def __str__(self):
109 return 'My name is integer %d' % self.x
110
111i = Integer(7)
112print i
113p.dump(i)
114
115datastream = src.getvalue()
116print repr(datastream)
117dst = StringIO(datastream)
118
119up = pickle.Unpickler(dst)
120
121class FancyInteger(Integer):
122 def __str__(self):
123 return 'I am the integer %d' % self.x
124
125def 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
132up.persistent_load = persistent_load
133
134j = up.load()
135print j
136</pre></div>
137
138<P>
139In 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
141list, in which case, when the unpickler reaches a persistent id, the
142persistent id string will simply be appended to this list. This
143functionality exists so that a pickle data stream can be ``sniffed''
144for object references without actually instantiating all the objects
145in 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
148the <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
156associating these user defined functions is slightly different for
157<tt class="module">pickle</tt> and <tt class="module">cPickle</tt>. The description given here
158works the same for both implementations. Users of the <tt class="module">pickle</tt>
159module could also use subclassing to effect the same results,
160overriding the <tt class="method">persistent_id()</tt> and <tt class="method">persistent_load()</tt>
161methods 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
167sitting around sniffing pickles in their living rooms.
168
169</DD>
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