[Tutor] Changing lists in place

[Alan Gauld]

> I have a list that I want to change in a for loop. 

Thats nearly always a bad thing to do.
Its like the old cartoon of the guy vcutting off the 
branch of the tree while sitting on it. Python can 
get very confused.

Either make a copy and change the copy or use 
a while loop and an index.

However, as others have pointed out:

mylist = [ 'One    ', '   two', '   three   ' ]
print mylist
for element in mylist:

element is a copy of the value in mylist, it is not a 
reference to mylist[n]

    element = element.strip()
    print "<>" + element + "<>"

so changes to element are not changes to the 
list content.

If you want to change the list content use a while loop:

index  = 0
while index < len(mylist):
     mylist[index] = '<> + mylist[index].strip() + '<>'
     index += 1

Or a list comprehension:

mylist = ['<>' + n.strip() + '<>' for n in mylist]

But notice that the comprehension builds a new list it 
does not actually change mylist directly in the way the 
while loop does.

Alan G
Author of the learn to program web tutor
http://www.freenetpages.co.uk/hp/alan.gauld