When are immutable tuples *essential*? Why can't you just use lists *everywhere* instead?

[Steve Holden]

Neil Cerutti wrote:
> On 2007-04-21, Steven D'Aprano <steve at REMOVE.THIS.cybersource.com.au> wrote:
>> On Fri, 20 Apr 2007 15:36:00 -0700, seberino at spawar.navy.mil wrote:
>>>> The article explains that, amongst other things, tuples are
>>>> faster than lists, so if you are working with constant values
>>>> (inmutables) they are more indicated than lists.
>>> Thanks.  I thought Python's design wasn't so concerned with
>>> optimizations. Adding a new type "just" for optimization
>>> reasons seems perhaps unnecessary.  I could be wrong.
>> It's times like this I want to cry...
>>
>>
>>>>> adict = {(1,2): "parrot"}
>> Try replacing that tuple with a list. "Just optimization" my eye!
> 
> So the question becomes: Why do Python dictionaries require keys
> to be of an immutable type?
> 
Because otherwise people would expect to be able to use a list to select 
a dictionary entry even after they'd modified elements of the list after 
creating the dictionary entry. Which they couldn't. So Python doesn't 
let them use lists.

regards
  Steve
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