Why are tuples immutable?

[Jp Calderone]

On Wed, 15 Dec 2004 14:18:21 GMT, Roel Schroeven <rschroev_nospam_ml at fastmail.fm> wrote:
>Antoon Pardon wrote:
> > Op 2004-12-15, Fredrik Lundh schreef <fredrik at pythonware.com>:
> >>sorry, but I don't understand your reply at all.  are you saying that dictionaries
> >>could support mutable keys (e.g lists) by making a copy of the key?  how would
> >>such a dictionary pick up changes to the original key object?  (I'm talking about
> >>the key stored in the dictionary, not the key you're using to look things up).
> > 
> > 
> > You want to mutate a key that is within a dictionary?
> 
> No, we don't want to mutate it; as far as I know, that is exactly the 
> reason why dictionaries don't support mutable keys.

  Dictionaries support mutable keys just find.  What they don't 
support is unhashable keys.  

  For some objects, this is an important distinction: lists are 
mutable but not hashable.

  For other objects, it is not: instances of user defined classes 
are mutable and hashable.  This is handy since the default hash is 
based on identity instead of the values of attributes.

  Mutating an object in a dictionary is completely reasonable.  What
is unreasonable is wanting to make a change that would change its
hash value.

  Jp