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copied!<p>You can write an object that behaves like a dict quite easily with <a href="http://docs.python.org/library/abc.html" rel="noreferrer">ABC</a>s (Abstract Base Classes) from the <a href="http://docs.python.org/2/library/collections.html" rel="noreferrer">collections</a> module. It even tells you if you missed a method, so below is the minimal version that shuts the ABC up.</p> <pre><code>import collections class TransformedDict(collections.MutableMapping): """A dictionary that applies an arbitrary key-altering function before accessing the keys""" def __init__(self, *args, **kwargs): self.store = dict() self.update(dict(*args, **kwargs)) # use the free update to set keys def __getitem__(self, key): return self.store[self.__keytransform__(key)] def __setitem__(self, key, value): self.store[self.__keytransform__(key)] = value def __delitem__(self, key): del self.store[self.__keytransform__(key)] def __iter__(self): return iter(self.store) def __len__(self): return len(self.store) def __keytransform__(self, key): return key </code></pre> <p>You get a few free methods from the ABC:</p> <pre><code>class MyTransformedDict(TransformedDict): def __keytransform__(self, key): return key.lower() s = MyTransformedDict([('Test', 'test')]) assert s.get('TEST') is s['test'] # free get assert 'TeSt' in s # free __contains__ # free setdefault, __eq__, and so on import pickle assert pickle.loads(pickle.dumps(s)) == s # works too since we just use a normal dict </code></pre> <p>I wouldn't subclass <code>dict</code> (or other builtins) directly. It often makes no sense, because what you actually want to do is <em>implement the interface of a dict</em>. And that is exactly what ABCs are for.</p>

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