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Date: Tue, 12 Nov 2013 10:12:22 +0100
Subject: Re: 'isimmutable' and 'ImmutableNester'
From: =?UTF-8?Q?Frank=2DRene_Sch=C3=A4fer?= <fschaef@gmail.com>
To: random832@fastmail.us
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Xref: csiph.com comp.lang.python:59165

(1) hash()-ability != immutability (!)

Proof:

class X:
    def __hash__(self): return 0

def pseudo_isimmutable(this):
    try:
        hash(this)
        return True
    except TypeError:
        return False

shapeshifter = (1, 2, X())
print pseudo_isimmutable(shapeshifter)
shapeshifter[2].changed = 4711


(2) The intended scenario is not described by a fragment such as:

      if isimmutable(obj): x = obj
      else:                     x = copy.copy(obj)
      function_that_might_modify(x)

But instead, a more characteristic scenario is

     assert isimmutable(obj)
     # What happens behind the curtain may rely on referencing
     things_behind_the_curtain(obj)

Or,

    def let_me_know():
         obj = get_what_is_wanted()
         assert isimmutable(obj)
         # The caller may do with it what he wants without risking consistency
         return obj

where lots of copying





2013/11/11  <random832@fastmail.us>:
>> A built-in function 'isimmutable()' shall tell efficiently whether the
>> object
>> of concern is mutable or not.
>
> What's the benefit over attempting to hash() the object?
>
> copy.deepcopy already has special case for int, string, and tuples
> (including tuples that do and do not have mutable members) - could what
> you need be accomplished by overriding __copy__ and __deepcopy__ in your
> custom class to return itself if it is immutable?

2013/11/11  <random832@fastmail.us>:
>> A built-in function 'isimmutable()' shall tell efficiently whether the
>> object
>> of concern is mutable or not.
>
> What's the benefit over attempting to hash() the object?
>
> copy.deepcopy already has special case for int, string, and tuples
> (including tuples that do and do not have mutable members) - could what
> you need be accomplished by overriding __copy__ and __deepcopy__ in your
> custom class to return itself if it is immutable?