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Date: Tue, 12 Nov 2013 20:21:00 +1100
Subject: Re: 'isimmutable' and 'ImmutableNester'
From: Chris Angelico <rosuav@gmail.com>
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Xref: csiph.com comp.lang.python:59168

On Tue, Nov 12, 2013 at 8:12 PM, Frank-Rene Sch=E4fer <fschaef@gmail.com> w=
rote:
> (1) hash()-ability !=3D immutability (!)
>
> Proof:
>
> class X:
>     def __hash__(self): return 0
>

x =3D=3D y !=3D y =3D=3D x

Proof:

class X:
   def __eq__(self,other): return True
class Y:
   def __eq__(self,other): return False

All you've done is proven that you can subvert things. By fiddling
with __hash__, __eq__, and so on, you can make sets and dicts behave
very oddly. Means nothing.

Fundamentally, your mutability check is going to need some form of
assistance from user-defined classes. That means a class can break
your rules.

ChrisA