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| 17 | <title>3.18 copy -- Shallow and deep copy operations</title> |
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| 54 | |
| 55 | <H1><A NAME="SECTION0051800000000000000000"> |
| 56 | 3.18 <tt class="module">copy</tt> -- |
| 57 | Shallow and deep copy operations</A> |
| 58 | </H1> |
| 59 | |
| 60 | <P> |
| 61 | <A NAME="module-copy"></A> |
| 62 | |
| 63 | <P> |
| 64 | This module provides generic (shallow and deep) copying operations. |
| 65 | <a id='l2h-675' xml:id='l2h-675'></a> |
| 66 | <P> |
| 67 | Interface summary: |
| 68 | |
| 69 | <P> |
| 70 | <div class="verbatim"><pre> |
| 71 | import copy |
| 72 | |
| 73 | x = copy.copy(y) # make a shallow copy of y |
| 74 | x = copy.deepcopy(y) # make a deep copy of y |
| 75 | </pre></div> |
| 76 | For module specific errors, <tt class="exception">copy.error</tt> is raised. |
| 77 | |
| 78 | <P> |
| 79 | The difference between shallow and deep copying is only relevant for |
| 80 | compound objects (objects that contain other objects, like lists or |
| 81 | class instances): |
| 82 | |
| 83 | <P> |
| 84 | |
| 85 | <UL> |
| 86 | <LI>A <em>shallow copy</em> constructs a new compound object and then (to the |
| 87 | extent possible) inserts <em>references</em> into it to the objects found |
| 88 | in the original. |
| 89 | |
| 90 | <P> |
| 91 | </LI> |
| 92 | <LI>A <em>deep copy</em> constructs a new compound object and then, |
| 93 | recursively, inserts <em>copies</em> into it of the objects found in the |
| 94 | original. |
| 95 | |
| 96 | <P> |
| 97 | </LI> |
| 98 | </UL> |
| 99 | |
| 100 | <P> |
| 101 | Two problems often exist with deep copy operations that don't exist |
| 102 | with shallow copy operations: |
| 103 | |
| 104 | <P> |
| 105 | |
| 106 | <UL> |
| 107 | <LI>Recursive objects (compound objects that, directly or indirectly, |
| 108 | contain a reference to themselves) may cause a recursive loop. |
| 109 | |
| 110 | <P> |
| 111 | </LI> |
| 112 | <LI>Because deep copy copies <em>everything</em> it may copy too much, |
| 113 | e.g., administrative data structures that should be shared even |
| 114 | between copies. |
| 115 | |
| 116 | <P> |
| 117 | </LI> |
| 118 | </UL> |
| 119 | |
| 120 | <P> |
| 121 | The <tt class="function">deepcopy()</tt> function avoids these problems by: |
| 122 | |
| 123 | <P> |
| 124 | |
| 125 | <UL> |
| 126 | <LI>keeping a ``memo'' dictionary of objects already copied during the current |
| 127 | copying pass; and |
| 128 | |
| 129 | <P> |
| 130 | </LI> |
| 131 | <LI>letting user-defined classes override the copying operation or the |
| 132 | set of components copied. |
| 133 | |
| 134 | <P> |
| 135 | </LI> |
| 136 | </UL> |
| 137 | |
| 138 | <P> |
| 139 | This version does not copy types like module, class, function, method, |
| 140 | stack trace, stack frame, file, socket, window, array, or any similar |
| 141 | types. |
| 142 | |
| 143 | <P> |
| 144 | Classes can use the same interfaces to control copying that they use |
| 145 | to control pickling. See the description of module |
| 146 | <tt class="module"><a href="module-pickle.html">pickle</a></tt><a id='l2h-678' xml:id='l2h-678'></a> for information on these |
| 147 | methods. The <tt class="module">copy</tt> module does not use the |
| 148 | <tt class="module"><a href="module-copyreg.html">copy_reg</a></tt> registration module. |
| 149 | |
| 150 | <P> |
| 151 | In order for a class to define its own copy implementation, it can |
| 152 | define special methods <tt class="method">__copy__()</tt> and |
| 153 | <tt class="method">__deepcopy__()</tt>. The former is called to implement the |
| 154 | shallow copy operation; no additional arguments are passed. The |
| 155 | latter is called to implement the deep copy operation; it is passed |
| 156 | one argument, the memo dictionary. If the <tt class="method">__deepcopy__()</tt> |
| 157 | implementation needs to make a deep copy of a component, it should |
| 158 | call the <tt class="function">deepcopy()</tt> function with the component as first |
| 159 | argument and the memo dictionary as second argument. |
| 160 | <a id='l2h-677' xml:id='l2h-677'></a> |
| 161 | <P> |
| 162 | <div class="seealso"> |
| 163 | <p class="heading">See Also:</p> |
| 164 | |
| 165 | <dl compact="compact" class="seemodule"> |
| 166 | <dt>Module <b><tt class="module"><a href="module-pickle.html">pickle</a></tt>:</b> |
| 167 | <dd>Discussion of the special methods used to |
| 168 | support object state retrieval and restoration. |
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