List and tuple usage distinction??

[Jeremy Jones]

Ishwar Rattan wrote:

>I am a little confused about a list and a tuple.
>
>Both can have dissimilar data-type elements, can be returned
>by functions. The only difference that I see is that list is
>mutable and tuple is not (of course list have .append() etc.)
>
>What is a possible scenario where one is preferred over the other?
>
>-ishwar
>  
>
Lots of reasons ;-)

1.  Maybe you want to create a dict and key off of a list of items.  I 
haven't had to do that, but it's feasible.  Take a peek at this:

 >>> f = ('a', 'b', 'c')
 >>> g = list(f)
 >>> f
('a', 'b', 'c')
 >>> g
['a', 'b', 'c']
 >>> h = {}
 >>> h[f] = 1
 >>> h[g] = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: list objects are unhashable
 >>> h
{('a', 'b', 'c'): 1}

You can key off of a tuple (the tuple "f") and you can't off of a plain 
old list ("g").

2.  You don't need the list of items to change or similarly
3.  You want to make it more difficult for a list of items to change.  
You can still fairly easily change a tuple - sort of:
 >>> f = list(f)
 >>> f.append('d')
 >>> f = tuple(f)
 >>> f
('a', 'b', 'c', 'd')
 >>>
What you've actually done is change the tuple object that "f" is 
pointing to.

Someone else can answer better about memory usage, but I would think 
that a tuple would cost less memory than a comparably sized list, so 
that also may be a consideration.

Jeremy