[Ping]
> Is a frozen list hashable?
[Guido]
> Yes -- that's what started this thread (using dicts as dict keys,
> actually).
Except this doesn't actually work unless list.freeze() recursively ensures
that all elements in the list are frozen too:
>>> hash((1, 2))
219750523
>>> hash((1, [2]))
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: unhashable type
>>>
That bothered me in Eric's original suggestion: unless x.freeze() does a
traversal of all objects reachable from x, it doesn't actually make x safe
against modification (except at the very topmost level). But doing such a
traversal isn't what *everyone* would want either (as with "const" in C, I
expect the primary benefit would be the chance to spend countless hours
worming around it in both directions <wink>).
[Skip]
> If you want immutable dicts or lists in order to use them as
> dictionary keys, just serialize them first:
>
> survey_says = {"spam": 14, "eggs": 42}
> sl = marshal.dumps(survey_says)
> dict[sl] = "spam"
marshal.dumps(dict) isn't canonical, though. That is, it may well be that
d1 == d2 but dumps(d1) != dumps(d2). Even materializing dict.values(), then
sorting it, then marshaling *that* isn't enough; e.g., consider {1: 1} and
{1: 1L}. The latter example applies to marshaling lists too.
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