[Python-ideas] Have dict().update() return its own reference.

Masklinn masklinn at masklinn.net
Sat Apr 21 01:10:35 CEST 2012

On 2012-04-20, at 23:41 , Paul Moore wrote:

> On 20 April 2012 20:49, Masklinn <masklinn at masklinn.net> wrote:
>> If they're huge mappings, you probably don't want to go around copying
>> them either way[0] and would instead use more custom mappings, either
>> some sort of joining proxy or something out of Okasaki (a clojure-style
>> tree-based map with structural sharing for instance)
> Python 3.3 has collections.ChainMap for this sort of case.

Yeah, it's an example of the "joining proxy" thing. Though I'm not sure
I like it being editable, or the lookup order when providing a sequence
of maps (I haven't tested it but it appears the maps sequence is traversed
front-to-back, I'd have found the other way around more "obvious", as if
each sub-mapping was applied to a base through an update call).

An other potential weirdness of this solution — I don't know how ChainMap
behaves there, the documentation is unclear — is iteration over the map
and mapping.items() versus [(key, mapping[key]) for key in mapping]
potentially having very different behaviors/values since the former is
going to return all key:value pairs but the latter is only going to return
the key:(first value for key) pairs which may lead to significant repetitions
any time a key is present in multiple contexts.

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