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

Jim J. Jewett jimjjewett at gmail.com
Mon Feb 27 23:50:35 CET 2012

In http://mail.python.org/pipermail/python-dev/2012-February/116955.html Victor Stinner proposed:

The blacklist implementation has a major issue: it is still possible to call write methods of the dict class (e.g. dict.set(myfrozendict, key, value)).

It is also possible to use ctypes and violate even more invariants. For most purposes, this falls under "consenting adults".

The whitelist implementation has an issue: frozendict and dict are not "compatible", dict is not a subclass of frozendict (and frozendict is not a subclass of dict).

And because of Liskov substitutability, they shouldn't be; they should be sibling children of a basedict that doesn't have the the mutating methods, but also doesn't promise not to mutate.

* frozendict values must be immutable, as dict keys

Why? That may be useful, but an immutable dict whose values might mutate is also useful; by forcing that choice, it starts to feel too specialized for a builtin.

* Add an hash field to the PyDictObject structure

That is another indication that it should really be a sibling class; most of the uses I have had for immutable dicts still didn't need hashing. It might be a worth adding anyhow, but only to immutable dicts -- not to every instance dict or keywords parameter.

* frozendict.hash computes hash(frozenset(self.items())) and caches the result is its private hash attribute

Why? hash(frozenset(selk.keys())) would still meet the hash contract, but it would be approximately twice as fast, and I can think of only one case where it wouldn't work just as well. (That case is wanting to store a dict of alternative configuration dicts (with no defaulting of values), but ALSO wanting to use the configurations themselves (as opposed to their names) as the dict keys.)

-jJ

--

If there are still threading problems with my replies, please email me with details, so that I can try to resolve them. -jJ

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