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POIn python, how can you retrieve a key from a dictionary?
 

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  1. POIn python, how can you retrieve a key from a dictionary?
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    <p>I have a hashable identifier for putting things in a dictionary:</p> <pre><code>class identifier(): def __init__(self, d): self.my_dict = d self.my_frozenset = frozenset(d.items()) def __getitem__(self, item): return self.my_dict[item] def __hash__(self): return hash(self.my_frozenset) def __eq__(self, rhs): return self.my_frozenset == rhs.my_frozenset def __ne__(self, rhs): return not self == rhs </code></pre> <p>I have a node type that encapsulates identifer for purposes of hashing and equality:</p> <pre><code>class node: def __init__(self, id, value): # id is of type identifier self.id = id self.value = value # define other data here... def __hash__(self): return hash(self.id) def __eq__(self, rhs): if isinstance(rhs, node): return self.id == rhs.id ### for the case when rhs is an identifier; this allows dictionary ### node lookup of a key without wrapping it in a node return self.id == rhs def __ne__(self, rhs): return not self == rhs </code></pre> <p>I put some nodes into a dictionary:</p> <pre><code>d = {} n1 = node(identifier({'name':'Bob'}), value=1) n2 = node(identifier({'name':'Alex'}), value=2) n3 = node(identifier({'name':'Alex', 'nationality':'Japanese'}), value=3) d[n1] = 'Node 1' d[n2] = 'Node 2' d[n3] = 'Node 3' </code></pre> <p>Some time later, I have only an identifier:</p> <pre><code>my_id = identifier({'name':'Alex'}) </code></pre> <p>Is there any way to efficiently lookup the node that has been stored with this identifier in this dictionary?</p> <p>Please note that this is a little trickier than it sounds; I know that I can trivially use <code>d[my_id]</code> to retrieve the associated item <code>'Node 2'</code>, but <strong>I want to efficiently return a reference to <code>n2</code></strong>.</p> <p>I know that I could do it by looking at every element in <code>d</code>, but I've tried that and it's much too slow (the dictionary has thousands of items in it and I do this a fair number of times).</p> <p>I know that internally <code>dict</code> is using the <code>hash</code> and <code>eq</code> operators for that identifier to store node <code>n2</code> and its associated item, <code>'Node 2'</code>. In fact, using <code>my_id</code> to lookup <code>'Node 2'</code> actually needs to lookup <code>n2</code> as an intermediate step, so <strong>this should definitely be possible.</strong> </p> <p>I am using this to store data in a graph. The nodes have a lot of additional data (where I put <code>value</code>) that is not used in the hash. I didn't create the graph package I'm using (networkX), but I can see the dictionary that stores my nodes. I could also keep an extra dictionary around of identifiers to nodes, but this would be a pain (I'd need to wrap the graph class and rewrite all add node, remove node, add nodes from list, remove nodes from list, add edge, etc. type functions to keep that dictionary up to date).</p> <p><strong>This is quite the puzzle. Any help would be really appreciated!</strong></p>
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    1. COThe later versions of NetworkX actually do keep an internal dictionary of "node attributes" which you might be able to use. Try G.add_node(id,name='Bob',value=2) and then inspect G.node[id].
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    2. CO+1 Good comment. I was using this to store the extra things I was using, but changed to a more object-oriented design because there were methods and members that all `node` types should have; it is not just `value`; there are a lot of things I store inside `node`.
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