[Tutor] Questions about stdin, stdout, lists and tuples

[Daniel Yoo]

On Mon, 8 Jan 2001, D-Man wrote:

> | Also,
> | lists are like arrays, because I can reassign their values (they are not
> | immutable), whereas tuples are like const arrays, their values cannot be
> | altered?
> | 
> | Have I got that right?
> 
> Lists are like arrays, but much better.  They can grow, be sorted,
> reversed, and you can't overrun your memory bounds!
> 
> As far as semantics (except for index bounds errors) and syntax they
> are basically the same.

One thing that tuples can do that lists can't is act as dictionary
keys.  For example, since Python knows that tuples are immutable:

    city_distances = {}
    city_distances[ ('los angeles', 'san francisco') ] = 384

will work.  However,

    city_distances[ ['los angeles', 'san francisco'] ] = 384

won't.  More technically, Python can calculate hash numbers from tuples,
but not lists:

###
>>> mytuple = (1, 2)
>>> mylist = [1, 2]
>>> hash(mytuple)
219750523
>>> hash(mylist)
Traceback (innermost last):
  File "<stdin>", line 1, in ?
TypeError: unhashable type
###

and that's why they can't be used as keys.

Hope that makes some sort of sense... *grin*