[Python-Dev] Add a frozendict builtin type (original) (raw)

Raymond Hettinger raymond.hettinger at gmail.com
Wed Feb 29 20:17:05 CET 2012

On Feb 27, 2012, at 10:53 AM, Victor Stinner wrote:

A frozendict type is a common request from users and there are various implementations.

ISTM, this request is never from someone who has a use case. Instead, it almost always comes from "completers", people who see that we have a frozenset type and think the core devs missed the ObviousThingToDo(tm). Frozendicts are trivial to implement, so that is why there are various implementations (i.e. the implementations are more fun to write than they are to use).

The frozenset type covers a niche case that is nice-to-have but rarely used. Many experienced Python users simply forget that we have a frozenset type. We don't get bug reports or feature requests about the type. When I do Python consulting work, I never see it in a client's codebase. It does occasionally get discussed in questions on StackOverflow but rarely gets offered as an answer (typically on variants of the "how do you make a set-of-sets" question). If Google's codesearch were still alive, we could add another datapoint showing how infrequently this type is used.

I wrote the C implementation for frozensets and the tests that demonstrate their use in problems involving sets-of-sets, yet I have needed the frozenset once in my career (for a NFA/DFA conversion algorithm).

From this experience, I conclude that adding a frozendict type would be a total waste (except that it would inspire more people to request frozen variante of other containers).

Raymond

P.S. The one advantage I can see for frozensets and frozendicts is that we have an opportunity to optimize them once they are built (optimizing insertion order to minimize collisions, increasing or decreasing density, eliminating dummy entries, etc). That being said, the same could be accomplished for regular sets and dicts by the addition of an optimize() method. I'm not really enamoured of that idea though because it breaks the abstraction and because people don't seem to need it (i.e. it has never been requested). -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20120229/e051c9b0/attachment.html>

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