'isimmutable' and 'ImmutableNester'
robert.kern at gmail.com
Tue Nov 12 15:00:07 CET 2013
On 2013-11-12 11:14, Steven D'Aprano wrote:
> On Tue, 12 Nov 2013 18:12:43 +1100, Chris Angelico wrote:
>> def isimmutable(x):
>> return True
>> except TypeError:
>> return False
> I'm afraid that doesn't test for immutability. It tests for hashability,
> which is different.
I am going to nitpick below for nitpicking's sake, but I agree with this.
> No well-behaved mutable object can be hashable, but that's not to say
> that badly-behaved mutable objects won't be hashable.
That's not quite true. A well-behaved mutable may be (well-behaved) hashable as
long as the allowed mutations do not affect the equality comparison. For
example, in Python 2, all new classes are mutable by default, but they are also
well-behaved hashable by default because their equality comparison is identity
comparison. None of the mutations affect object identity, so the hash based on
identity remains well-behaved.
> And every immutable
> object should be hashable, but that's not to say that some immutable
> objects might choose, for their own reasons, not to be hashable.
I would also dispute this. A tuple itself is immutable, but it may not be
hashable because one of its contained objects is unhashable (whether due to
mutability or something else).
> So your function is subject to both false negatives and false positives.
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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