On Thu, 26 Nov 2015 07:27 pm, Marko Rauhamaa wrote:
> 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.
Well, when you design your own language, you can make all the bad design
decisions you like :-)
Seriously, if you think *this* thread about mutable function defaults has
been long, can you imagine the bug reports and arguments if this was
possible in Python?
L = [1, 2]
d = {L: "found it"}
# much later, after L has been modified...
d[ [1, 2] ]
=> raises KeyError
But even worse:
a = [1, 2]
b = [1, 3]
d = {a: "spam", b: "ham"}
a[1] += 1
What will d[ [1, 3] ] return?
--
Steven
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