Tuple question

[Mel Wilson]

In article <roy-D04C66.10354305092004 at reader1.panix.com>,
Roy Smith <roy at panix.com> wrote:
>So, to restate my original question, why should my mutable,
>content-based-eqality class instance be a valid dictionary key, when a
>list is not?  Which part of a list's behavior makes it inherently
>unusable as a key?  I'm not asking about design philosophy, I'm asking
>about observable behavior.

      a = [1,2,3,4,5]
      D = {a:'First five natural numbers'}

The hash of the list should be based on value, so that after

      b = [1,2,3,4,5]
      c = D[b]

c would be 'First five natural numbers', even though b is
not the same object as a.

Now suppose

      a.append(6)

What then happens on

      e = D[b]

b no longer equals a, so the existing dictionary item can't
match b as a dictionary key.  Moreover, the relevant
dictionary item is filed under hash([1,2,3,4,5]) and not
under hash([1,2,3,4,5,6]), so trying to access D with a key
equal to the appended-to value of a won't hash to the right
place to find a's item.  Trouble.

If you roll your own hashable class, it's assumed you have
thought about these issues.

        Regards.        Mel.