How to remove list elements in a for loop in Python

Question

I have a list  a     a    b    c    d    e     I want to remove elements in this list in a for loop like below   for item in a      print item     a remove item    But it doesn t work  What can I do

User · Answer

Probably a bit late to answer this but I just found this thread and I had created my own code for it previously...

    list = [1,2,3,4,5]
    deleteList = []
    processNo = 0
    for item in list:
        if condition:
            print item
            deleteList.insert(0, processNo)
        processNo += 1

    if len(deleteList) > 0:
        for item in deleteList:
            del list[item]

It may be a long way of doing it but seems to work well. I create a second list that only holds numbers that relate to the list item to delete. Note the "insert" inserts the list item number at position 0 and pushes the remainder along so when deleting the items, the list is deleted from the highest number back to the lowest number so the list stays in sequence.

User · Answer

As other answers have said  the best way to do this involves making a new list - either iterate over a copy  or construct a list with only the elements you want and assign it back to the same variable  The difference between these depends on your use case  since they affect other variables for the original list differently  or  rather  the first affects them  the second doesn t     If a copy isn t an option for some reason  you do have one other option that relies on an understanding of why modifying a list you re iterating breaks  List iteration works by keeping track of an index  incrementing it each time around the loop until it falls off the end of the list  So  if you remove at  or before  the current index  everything from that point until the end shifts one spot to the left  But the iterator doesn t know about this  and effectively skips the next element since it is now at the current index rather than the next one  However  removing things that are after the current index doesn t affect things    This implies that if you iterate the list back to front  if you remove an item at the current index  everything to it s right shifts left - but that doesn t matter  since you ve already dealt with all the elements to the right of the current position  and you re moving left - the next element to the left is unaffected by the change  and so the iterator gives you the element you expect    TL DR    gt  gt  gt  a   list range 5    gt  gt  gt  for b in reversed a       if b    3          a remove b   gt  gt  gt  a  0  1  2  4    However  making a copy is usually better in terms of making your code easy to read  I only mention this possibility for sake of completeness

User · Answer

You are not permitted to remove elements from the list while iterating over it using a for loop   The best way to rewrite the code depends on what it is you re trying to do   For example  your code is equivalent to   for item in a    print item a           Alternatively  you could use a while loop   while a    print a pop 0       I m trying to remove items if they match a condition  Then I go to next item     You could copy every element that doesn t match the condition into a second list   result      for item in a    if condition is False      result append item  a   result   Alternatively  you could use filter or a list comprehension and assign the result back to a   a   filter lambda item       a    or  a    item for item in a if        where     stands for the condition that you need to check

User · Answer

Iterate through a copy of the list    gt  gt  gt  a     a    b    c    d    e    gt  gt  gt  for item in a         print item     if item     b           a remove item   a b c d e  gt  gt  gt  print a   a    c    d    e

User · Answer

How about creating a new list and adding elements you want to that new list  You cannot remove elements while iterating through a list

User · Answer

import copy  a     a    b    c    d    e    b   copy copy a   for item in a      print item     b remove item  a   copy copy b    Works  to avoid changing the list you are iterating on  you make a copy of a  iterate over it and remove the items from b  Then you copy b  the altered copy  back to a

[python] How to remove list elements in a for loop in Python?

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