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POLimited deep copy of an instance with a container of containers as an attribute
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  1. POLimited deep copy of an instance with a container of containers as an attribute
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    copied!<p>I have a class</p> <ul> <li>whose instances have attributes that are containers <ul> <li>which themselves contain containers, each containing many items</li> </ul></li> <li>has an expensive initialization of these containers</li> </ul> <p>I want to create copies of instances such that</p> <ol> <li>the container attributes are copied, rather than shared as references, but</li> <li>the containers within each container are not deeply copied, but are shared references</li> <li>a call to the class's expensive <code>__init__()</code> method is avoided if possible</li> </ol> <p>For an example, let's use the class <code>SetDict</code>, below, which, when creating an instance, initializes a dictionary-like data structure as an attribute, <code>d</code>. <code>d</code> stores integers as keys and sets as values.</p> <pre><code>import collections class SetDict(object): def __init__(self, size): self.d = collections.defaultdict(set) # Do some initialization; if size is large, this is expensive for i in range(size): self.d[i].add(1) </code></pre> <p>I would like to copy instances of <code>SetDict</code>, such that <code>d</code> is itself copied, but the sets that are its values are <em>not</em> deep-copied, and are instead only references to the sets.</p> <p>For example, consider the following behavior currently for this class, where <code>copy.copy</code> doesn't copy the attribute <code>d</code> to the new copy, but <code>copy.deepcopy</code> creates completely new copies of the sets that are values of <code>d</code>.</p> <pre><code>&gt;&gt;&gt; import copy &gt;&gt;&gt; s = SetDict(3) &gt;&gt;&gt; s.d defaultdict(&lt;type 'set'&gt;, {0: set([1]), 1: set([1]), 2: set([1])}) &gt;&gt;&gt; # Try a basic copy &gt;&gt;&gt; t = copy.copy(s) &gt;&gt;&gt; # Add a new key, value pair in t.d &gt;&gt;&gt; t.d[3] = set([2]) &gt;&gt;&gt; t.d defaultdict(&lt;type 'set'&gt;, {0: set([1]), 1: set([1]), 2: set([1]), 3: set([2])}) &gt;&gt;&gt; # But oh no! We unintentionally also added the new key to s.d! &gt;&gt;&gt; s.d defaultdict(&lt;type 'set'&gt;, {0: set([1]), 1: set([1]), 2: set([1]), 3: set([2])}) &gt;&gt;&gt; &gt;&gt;&gt; s = SetDict(3) &gt;&gt;&gt; # Try a deep copy &gt;&gt;&gt; u = copy.deepcopy(s) &gt;&gt;&gt; u.d[0].add(2) &gt;&gt;&gt; u.d[0] set([1, 2]) &gt;&gt;&gt; # But oh no! 2 didn't get added to s.d[0]'s set &gt;&gt;&gt; s.d[0] set([1]) </code></pre> <p>The behavior I'd like to see instead would be the following:</p> <pre><code>&gt;&gt;&gt; s = SetDict(3) &gt;&gt;&gt; s.d defaultdict(&lt;type 'set'&gt;, {0: set([1]), 1: set([1]), 2: set([1])}) &gt;&gt;&gt; t = copy.copy(s) &gt;&gt;&gt; # Add a new key, value pair in t.d &gt;&gt;&gt; t.d[3] = set([2]) &gt;&gt;&gt; t.d defaultdict(&lt;type 'set'&gt;, {0: set([1]), 1: set([1]), 2: set([1]), 3: set([2])}) &gt;&gt;&gt; # s.d retains the same key-value pairs &gt;&gt;&gt; s.d defaultdict(&lt;type 'set'&gt;, {0: set([1]), 1: set([1]), 2: set([1])}) &gt;&gt;&gt; t.d[0].add(2) &gt;&gt;&gt; t.d[0] set([1, 2]) &gt;&gt;&gt; # s.d[0] also had 2 added to its set &gt;&gt;&gt; s.d[0] set([1, 2]) </code></pre> <p>This was my first attempt to create a class that would do this, but it fails due to infinite recursion:</p> <pre><code>class CopiableSetDict(SetDict): def __copy__(self): import copy # This version gives infinite recursion, but conveys what we # intend to do. # # First, create a shallow copy of this instance other = copy.copy(self) # Then create a separate shallow copy of the d # attribute other.d = copy.copy(self.d) return other </code></pre> <p>I'm not sure how to properly override the <code>copy.copy</code> (or <code>copy.deepcopy</code>) behavior to achieve this. I'm also not entirely sure if I should be overriding <code>copy.copy</code> or <code>copy.deepcopy</code>. How can I go about getting the desired copy behavior?</p>
 

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