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copied!<p>A class is a callable. When you call <code>SetDict(3)</code>, <code>SetDict.__call__</code> first calls the constructor <code>SetDict.__new__(SetDict)</code> and then calls the initializer <code>__init__(3)</code> on the return value of <code>__new__</code> if it's an instance of <code>SetDict</code>. So you can get a new instance of <code>SetDict</code> (or any other class) without calling its initializer by just calling its constructor directly.</p> <p>After that, you have an instance of your type and you can simply add regular copies of any container attributes and return it. Something like this should do the trick.</p> <pre><code>import collections import copy 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) def __copy__(self): other = SetDict.__new__(SetDict) other.d = self.d.copy() return other </code></pre> <p><code>__new__</code> is a static method and requires the class to be constructed as its first argument. It should be as simple as this unless you're overriding <code>__new__</code> to do something in which case you should show what it is so that this can be modified. Here's the test code do demonstrate the behavior that you want.</p> <pre><code>t = SetDict(3) print t.d # defaultdict(<type 'set'>, {0: set([1]), 1: set([1]), 2: set([1])}) s = copy.copy(t) print s.d # defaultdict(<type 'set'>, {0: set([1]), 1: set([1]), 2: set([1])}) t.d[3].add(1) print t.d # defaultdict(<type 'set'>, {0: set([1]), 1: set([1]), 2: set([1]), 3: set([1])}) print s.d # defaultdict(<type 'set'>, {0: set([1]), 1: set([1]), 2: set([1])}) s.d[0].add(2) print t.d[0] # set([1, 2]) print s.d[0] # set([1, 2]) </code></pre>

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