Insert Element in list during iteration

Question

I need to insert element during iteration of list and did in following way. But I feel it can be written in better. Here B's dictionary which contain A element Length

_leftcell = leftcell[:]
index = 1
for A in leftcell:
    if B[A].length  % 140 != 0:
        _leftcell.insert(index, 2)
        index +=2
leftcell= _leftcell[:]

Answer 1

I would do something like:

for item in leftcell[:]:
    if B[item].length % 140:
        leftcell.insert(leftcell.index(item), 2)

Assuming I've correctly understood what you're trying to achieve.

Answer 2

iterate over the list in reverse so you don't have to worry about changes at the end of the list

left_len = len(leftcell)
for i in xrange(left_len-1,0,-1):
    if B[leftcell[i]].length  % 140 != 0:
        leftcell.insert(i, 2)