for loop ignores some elements of my list

Question

Possible Duplicate:
Remove items from a list while iterating in Python

I want to remove all the dates in a list which are later than a given date. I can't see why my method only removes some items of the list. Here's what I've got:

import datetime
import numpy as np
import os

list_dates = [datetime.date(2012,1,3), datetime.date(2012,1,1), datetime.date(2012,1,5), datetime.date(2013,1,3), datetime.date(2013,1,1)]

for item in list_dates:
    if item > datetime.date(2012,1,1):
        list_dates.remove(item)

print list_dates

returns

[datetime.date(2012, 1, 1), datetime.date(2013, 1, 3)]

It's probably because you're modifying the list as you traverse it. — Ben James, Sep 05 '11 at 16:12
You cannot iterate through a list and simultaneously remove items from it. — GWW, Sep 05 '11 at 16:13
@GWW: The problem is, you *can* - it just doesn't work and is rarely useful. — , Sep 05 '11 at 16:39
Agreeing with @delnan. This kind of thing really should throw a syntax error. Actually, I thought it *did*, but maybe I'm thinking of Java. — andronikus, Sep 05 '11 at 18:45

score 2 · Accepted Answer · answered Sep 05 '11 at 16:22

James and GWW are right, you cant modify the list while you are iterating through the same. Instead copy the items which you want to keep to a new list.

list_dates = [datetime.date(2012,1,3), datetime.date(2012,1,1), datetime.date(2012,1,5), datetime.date(2013,1,3), datetime.date(2013,1,1)]        

for item in list_dates:
            if item < datetime.date(2012,1,1):
                new_list_dates.add(item); // move this item (which you want to keep) to a new list.

    print new_list_dates

You could also use a list comprehension for this (if you see the question I marked this as a duplicate of) — GWW, Sep 05 '11 at 16:40

warvariuc · Answer 2 · 2011-09-05T17:59:01.393

Or modify in place like this:

import datetime

list_dates = [datetime.date(2012,1,3), datetime.date(2012,1,1), datetime.date(2012,1,5), datetime.date(2013,1,3), datetime.date(2013,1,1)]

for i in range(len(list_dates) - 1, -1, -1):
    item = list_dates[i]
    if item > datetime.date(2012,1,1):
        del list_dates[i]

print list_dates

or

import datetime


list_dates = [datetime.date(2012,1,3), datetime.date(2012,1,1), datetime.date(2012,1,5), datetime.date(2013,1,3), datetime.date(2013,1,1)]

i = 0
while i < len(list_dates):
    item = list_dates[i]
    if item > datetime.date(2012,1,1):
        del list_dates[i]
    else:
        i += 1

print list_dates

for loop ignores some elements of my list

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