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| 54 | |
| 55 | <H3><A NAME="SECTION0051451000000000000000"></A><A NAME="pickle-inst"></A> |
| 56 | <BR> |
| 57 | 3.14.5.1 Pickling and unpickling normal class |
| 58 | instances |
| 59 | </H3> |
| 60 | |
| 61 | <P> |
| 62 | When a pickled class instance is unpickled, its <tt class="method">__init__()</tt> |
| 63 | method is normally <em>not</em> invoked. If it is desirable that the |
| 64 | <tt class="method">__init__()</tt> method be called on unpickling, an old-style class |
| 65 | can define a method <tt class="method">__getinitargs__()</tt>, which should return a |
| 66 | <em>tuple</em> containing the arguments to be passed to the class |
| 67 | constructor (i.e. <tt class="method">__init__()</tt>). The |
| 68 | <tt class="method">__getinitargs__()</tt> method is called at |
| 69 | pickle time; the tuple it returns is incorporated in the pickle for |
| 70 | the instance. |
| 71 | <a id='l2h-648' xml:id='l2h-648'></a><a id='l2h-650' xml:id='l2h-650'></a> |
| 72 | <P> |
| 73 | <a id='l2h-652' xml:id='l2h-652'></a> |
| 74 | <P> |
| 75 | New-style types can provide a <tt class="method">__getnewargs__()</tt> method that is |
| 76 | used for protocol 2. Implementing this method is needed if the type |
| 77 | establishes some internal invariants when the instance is created, or |
| 78 | if the memory allocation is affected by the values passed to the |
| 79 | <tt class="method">__new__()</tt> method for the type (as it is for tuples and |
| 80 | strings). Instances of a new-style type <tt class="class">C</tt> are created using |
| 81 | |
| 82 | <P> |
| 83 | <div class="verbatim"><pre><TT> |
| 84 | obj = C.__new__(C, *<var>args</var>) |
| 85 | </TT></pre></div> |
| 86 | |
| 87 | <P> |
| 88 | where <var>args</var> is the result of calling <tt class="method">__getnewargs__()</tt> on |
| 89 | the original object; if there is no <tt class="method">__getnewargs__()</tt>, an |
| 90 | empty tuple is assumed. |
| 91 | |
| 92 | <P> |
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| 94 | <P> |
| 95 | Classes can further influence how their instances are pickled; if the |
| 96 | class defines the method <tt class="method">__getstate__()</tt>, it is called and the |
| 97 | return state is pickled as the contents for the instance, instead of |
| 98 | the contents of the instance's dictionary. If there is no |
| 99 | <tt class="method">__getstate__()</tt> method, the instance's <tt class="member">__dict__</tt> is |
| 100 | pickled. |
| 101 | |
| 102 | <P> |
| 103 | Upon unpickling, if the class also defines the method |
| 104 | <tt class="method">__setstate__()</tt>, it is called with the unpickled |
| 105 | state.<A NAME="tex2html18" |
| 106 | HREF="#foot8816"><SUP>3.6</SUP></A> If there is no <tt class="method">__setstate__()</tt> method, the |
| 107 | pickled state must be a dictionary and its items are assigned to the |
| 108 | new instance's dictionary. If a class defines both |
| 109 | <tt class="method">__getstate__()</tt> and <tt class="method">__setstate__()</tt>, the state object |
| 110 | needn't be a dictionary and these methods can do what they |
| 111 | want.<A NAME="tex2html19" |
| 112 | HREF="#foot8979"><SUP>3.7</SUP></A> |
| 113 | <P> |
| 114 | <div class="warning"><b class="label">Warning:</b> |
| 115 | |
| 116 | For new-style classes, if <tt class="method">__getstate__()</tt> returns a false |
| 117 | value, the <tt class="method">__setstate__()</tt> method will not be called. |
| 118 | </div> |
| 119 | |
| 120 | <P> |
| 121 | <BR><HR><H4>Footnotes</H4> |
| 122 | <DL> |
| 123 | <DT><A NAME="foot8816">... |
| 124 | state.</A><A |
| 125 | href="pickle-inst.html#tex2html18"><SUP>3.6</SUP></A></DT> |
| 126 | <DD>These methods can also be used to implement copying |
| 127 | class instances. |
| 128 | |
| 129 | </DD> |
| 130 | <DT><A NAME="foot8979">... |
| 131 | want.</A><A |
| 132 | href="pickle-inst.html#tex2html19"><SUP>3.7</SUP></A></DT> |
| 133 | <DD>This protocol is also used by the shallow and deep |
| 134 | copying operations defined in the |
| 135 | <tt class="module"><a href="module-copy.html">copy</a></tt> module. |
| 136 | |
| 137 | </DD> |
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