'isimmutable' and 'ImmutableNester'

Robert Kern 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):
>>      try:
>>          hash(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.


Robert Kern

"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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