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Why are tuples immutable?

Scott David Daniels Scott.Daniels at Acm.Org
Wed Dec 29 11:28:16 EST 2004 
Antoon Pardon wrote:
> Op 2004-12-23, Scott David Daniels schreef <Scott.Daniels at Acm.Org>:
>>This is half the problem.  In the period where an element is in the
>>wrong hash bucket, a new entry for the same value can be created in
>>the proper hash bucket.  Then the code will have to determine how to
>>merge two entries at rehash time.
> 
> But similar problems can happen in a sorted list, where the sort
> is done on a "key" of the data and where you don't want duplicate
> "keys".
> 
> If you then mutate a "key", it may be possible to insert a duplicate
> "key" in the sorted list because the list isn't sorted anymore. If
> you then resort your list you also have to determine how to merge
> the two items with the same "key"
I'd say this is a stretch.  The "key" argument to sort is very new, and
it is a function from data to a value.  The "key" can be mutated only if
the key function is picking out a mutable part of a data element.

> This just to show that repairing sorted lists can be just as
> troublesome as repairing dictionaries. People who think 
> sorted lists of mutable objects is less bothersome as dictionaries
> with mutable keys, haven't thought things through IMO.

But Python has no "sorted list" data type, so it is perfectly reasonable
to expect such lists to go through transitional representations.  A set
should not briefly have duplicate elements, a list should not vary in 
length when elements are replaced, an integer being continuously
incremented should not be viewable from a separate thread as anything
but monotonicly increasing; dictionaries are primitives and should not
have observable transitional states.

--Scott David Daniels
Scott.Daniels at Acm.Org
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