[Python-Dev] Proposal: defaultdict (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Sat Feb 18 01:12:21 CET 2006

Ian Bicking wrote:

It is lazily incarnated for multidict, because there is no noticeable side effect -- if there is any internal side effects that is an implementation detail. However for defaultfactory=list, the result of .keys(), .haskey(), and .items() changes when you do d[somekey].

That's why I think has_key and in should return True for any key. This leaves keys(), items(), and values(). From a pure point of view, they should return infinite sets. Practicality beats purity, so yes, d[k] could be considered a modifying operation.

If you look carefully, you find that many access operations also have side effects. For example, .read() on a file not only returns some data, but also advances the file position. Queue.get not only returns the next item, but also removes it from the queue.

Regards, Martin

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