Sets in Python

[Karthik Gurusamy]

On Sep 19, 3:06 pm, Paddy <paddy3... at googlemail.com> wrote:
> On Sep 19, 9:58 pm, Karthik Gurusamy <kar1... at gmail.com> wrote:
>
> > Since we know hashing is used, all that is needed is, a well-defined
> > way to construct a hash out of a mutable. "Given a sequence, how to
> > get a hash" is the problem. If later the given sequence is different,
> > that's not the dict's problem.
>
> Oh it is possible to construct a hash from a mutable. What is
> difficult is creating the same hash when the mutable mutates.

Why? There is no reason that the dict should maintain the same hash,
after all the user is calling with a different sequence as key (after
the mutation).

There seems to be an underlying assumption that the dictionary key-
>value mapping should somehow maintain the mapping even when the key
changes behind its back.

The contract could very well be, hey if you give me a different
sequence later (by mutating the one you added), don't expect me to
find it in the dictionary.


>Or
> indeed working out what it means when a hash key mutates and you
> access the dictionary.
> Ignoring this gives the programmer a big problem hence the limitation.
>
> I don't think you have a better solution.

But why would a programmer expect to find the match, when his/her code
has changed the sequence (or has somehow let the hash key mutate) from
the time of dictionary addition.

If I did, a = [10, 20] and I did d[a]= 'foo', then a.append(30).
If dict complains key error on d[a] now, I won't be surprised. If I do
d[[10, 20, 30]], I will be surprised if it doesn't find the item. Of
course, in today's behavior the above is syntax error.

Karthik


>
> - Paddy.