[Tutor] Re: Tutor digest, Vol 1 #1594 - 15 msgs

[Jeff Shannon]

> Message: 12
> From: "Wesley Abbott" <wesleyabbott@hotmail.com>
> To: tutor@python.org
> Date: Wed, 24 Apr 2002 16:02:41 -0600
> Subject: [Tutor] Problems understanding List reverse()
>
> Hello everyody.
>
> I am having a slight problem with relation to 'list.reverse'. I am able ot
> get the reverse to work if a list is static, but if I have a user input
> information it does not seem to abe able to reverse.

It has nothing to do with the user input, it's because you're not using
list.reverse() quite right.  :)

As someone else mentioned, that reversel() function is dangerous.  Here's why:

>>> def reversel(alist):
...  alist.reverse()
...  return alist
...
>>> mylist = [1, 2, 3, 4]
>>> print mylist
[1, 2, 3, 4]
>>> otherlist = reversel(mylist)
>>> print otherlist
[4, 3, 2, 1]   # seems to work fine so far, but...
>>> print mylist
[4, 3, 2, 1]   # the original list is reversed, too!
>>>

That's probably not what was intended or expected.  As was pointed out, this can be
fixed by making a *copy* of the list before you reverse it:

>>> def reversel(alist):
...     listcopy = alist[:]
...     listcopy.reverse()
...     return listcopy
>>>

Now, as for your "user input" problem...

>>> print mylist
[4, 3, 2, 1]
>>> print mylist.reverse()
None    # It doesn't work!
>>> print mylist
[1, 2, 3, 4]    # Or does it?
>>>

You tried to print the result of list.reverse(), but since it reverses the list
*in-place*, there is *no* result.  It reversed the list all right, and then returned
None.  In fact, this was done deliberately so that the mistake you made with your
reversel() function would be less likely -- if list.reverse() returned the reversed
list, it would be easy to forget that it also *modified* the original list.

Hope that helps...

Jeff Shannon
Technician/Programmer
Credit International