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From: Antoon Pardon <antoon.pardon@rece.vub.ac.be>
Newsgroups: comp.lang.python
Subject: Re: Late-binding of function defaults (was Re: What is a function parameter =[] for?)
Date: Thu, 26 Nov 2015 09:45:46 +0100
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Xref: csiph.com comp.lang.python:99545

Op 26-11-15 om 09:27 schreef Marko Rauhamaa:
> Chris Angelico <rosuav@gmail.com>:
>
>> On Thu, Nov 26, 2015 at 5:52 PM, Marko Rauhamaa <marko@pacujo.net> wrote:
>>> Nothing prevents using mutable objects as keys in Python.
>> Sure, you _can_. But if the key's hash changes between dict insertion
>> and retrieval, all manner of invariants will break, and likewise if
>> two equal objects have different hashes. From which you can deduce
>> logically that any object used as a key must remain (not) equal to
>> everything that it was (not) equal to from that time until it is
>> looked up... which basically means its value mustn't change. It must
>> be immutable.
> What I'm saying is that Python does not prevent mutable keys but tries
> to do that with lists and tuples.
>
> I think Python should stop trying.
>
> I have wanted to use lists as keys, and there should be no reason to
> allow mutable tuples. It should be enough to say that the behavior of a
> dictionary is undefined if a key should mutate on the fly.

Maybe python could introduce a dictionary that copies the keys in and
out the dictionary. In that case keys can be mutable without that being
a problem for the dictionary, because the dictionary works with it's own
copy of the key that it doesn't mutate.

-- 
Antoon.