Python - Pop element from for loop and append to end, modifying list while looping

Question

For every element in a for loop, I want to check if the element satisfies some condition. If yes, I want to do something to it; if no, I want to add it to the end of the list and do it later.

I'm aware that modifying loops while looping is bad, but I want to do it anyway, and want to know how to do it correctly.

for i in list:
    if something(i):
        do(i)
    else:
        #append to end of list, do later
        list.remove(i)
        list.append(i)

This piece of code mostly works, but causes me to skip the element after the removed i while iterating. How do I work around that?

@RomanPerekhrest I think `list` and `edges` are the same list. — Jean-François Fabre, Oct 23 '17 at 20:41

score 0 · Answer 1 · answered Oct 23 '17 at 20:41

0

Realized just after asking that I don't even need to remove if I'm only ever going to iterate through the list once - if I just append, I'll be able to operate on the element once everything else is finished.

In other words, this works:

for i in list:
    if something(i):
        do(i)
    else:
        #append to end of list, do later
        list.append(i)

answered Oct 23 '17 at 20:41

plzhalpplzxubtunu

53
1
6

1

And you don't care that the items you previously skipped stay in the list? – Reti43 Oct 23 '17 at 20:44
Memory will blow up, why not? – Yaroslav Surzhikov Oct 24 '17 at 01:09

score 0 · Accepted Answer · answered Oct 23 '17 at 20:46

I'm pretty sure there is no correct way to modify a list while looping over a list. In particular, removing objects from a list will cause issues (see strange result when removing item from a list).

I would consider making additional lists, one for your results, one temp list for the elements you have left to process. Note that in my example, tmp1 is your input list.

tmp1 = [values to process]
tmp2 = []
results = []
while(tmp1)
    for i in tmp1:
        if something(i):
            results.append(i)
            do(i)
        else:
            tmp2.append(i)
    tmp1 = tmp2
    tmp2 = []

Python - Pop element from for loop and append to end, modifying list while looping

2 Answers2