[Tutor] iterating over a changing list

Ed Owens eowens0124 at gmx.com
Wed Oct 10 21:52:15 CEST 2012

I'm trying to iterate over a list of elements, and make changes to the list
in front of the element I'm currently working with.  I can update the list,
but the  'for'  doesn't see the new element.  Here's the code:

import string

def add_element(items, point):
    items = items[:point+1][:] + [['new']] + items[point+1:]
    return items

def main():

itmlst = [['a'],['b']]
itmcntr = 0

for itm in itmlst:
    cmd = ''
    while True:
        cmd = raw_input('break, add, print:')
        if cmd == 'break':
        elif cmd == 'add':
            itmlst = add_element(itmlst,itmcntr)
        elif cmd == 'print':
            print 'current item:', itm
            print 'invalid'
    itmcntr += 1
    print 'finished with', itm, 'in', itmlst
print len(itmlst), 'total items in list'

If I provide the inputs: [print add print break print break] at the prompt,
I get this output

current item: ['a']
current item: ['a']
finished with ['a'] in [['a'], ['new'], ['b']]
current item: ['b']
finished with ['b'] in [['a'], ['new'], ['b']]
3 total items in list

The new element got added, but it wasn't used in the iteration over the list
of items. Other than setting up a counter and calling len() each loop, is
there some way to have the changing list recognized within the for loop?

Thanks in advance for any help.

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