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    <p>The accepted solution (as well as my first attempt) failed due to the ambiguity inherent in the specs: <code>'.'</code> may be "just a separator" <strong>or</strong> a part of the actual key string. Consider, for example, that <code>key</code> may be <code>'a.b.c.d.e.f'</code> and the actual key to use at the current level is <code>'a.b.c.d'</code> with <code>'e.f'</code> left over for the next-most-indented level. Also, the spec is ambiguous in another sense: if more than one dot-joined prefix of <code>'key'</code> is present, which one to use?</p> <p>Assume the intention is to try <em>every</em> such feasible prefix: this would possibly produce multiple solutions but we can arbitrarily return the first solution found in this case.</p> <pre><code>def getitem(key, context): stk = [(key.split('.'), context)] while stk: kl, ctx = stk.pop() if not kl: return ctx if kl[0].isdigit(): ik = int(kl[0]) try: stk.append((kl[1:], ctx[ik])) except LookupError: pass for i in range(1, len(kl) + 1): k = '.'.join(kl[:i]) if k in ctx: stk.append((kl[i:], ctx[k])) raise KeyError(key) </code></pre> <p>I was originally trying to avoid <em>all</em> <code>try/except</code>s (as well as recursion and introspection via <code>hasattr</code>, <code>isinstance</code>, etc), but one snuck back in: it's hard to check if an integer is an acceptable index/key into what might be either a dict or a list, without either some introspection to distinguish the cases, <em>or</em> (and it looks simpler here) a <code>try/except</code>, so I went fir te latter, simplicity being always near the top of my concerns. Anyway...</p> <p>I believe variants on this approach (where all the "possible continuation-context pairs" that might still be feasible at any point are kept around) are the only working way to deal with the ambiguities I've explained above (of course, one might choose to collect <em>all</em> possible solutions, arbitrarily pick one of them according to whatever heuristic criterion is desire, or maybe raise if the ambiguity is biting so there are multiple solutions, etc, etc, but these are minor variants of this general idea).</p>
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    1. USAlex Martelli
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    1. COThis will not work for two of the cases in the original code sample. The first is the case where the key contains a period `domain.name`. The second is the case where a part of the key is an index into a sequence e.g. `database.users.0`. There are no reasons for my complications, other than that was the way I thought of resolving the problem. I knew my solution smelled badly, that's why I asked how would other people solve it.
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      1. USJohn Keyes
    2. COThe `.0` as index into a sequence will work just fine (that's what the nested `try`/`except` is all about). You're right that it won't work if some or all of the dots are meant to be kept: neither will the answer you've selected in slightly more complex cases, e.g. when `key` is `'a.b.c.d'` and the leading `'a.b'` is to be taken as key at the current level, `'c.d'` as the one into the once-nested subdict thus chosen. The notation where `'.'` can stand for two completely different things is also ambiguous -- why not use a compound-key joiner char that can't appear in actual keys?
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    3. COI tried the .0 as index and it didn't work `KeyError: 'database.users.0'`. Of course you are correct regarding the `'a.b.c.d'` case. My only reason for including it was a 'just in case' approach. My config files won't include keys like this.
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