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Can't modify list elements in a loop Python


Question

While looping over a list in Python, I was unable to modify the elements without a list comprehension. For reference:

li = ["spam", "eggs"]
for i in li:
    i = "foo"

li
["spam", "eggs"]

li = ["foo" for i in li]
li 
["foo", "foo"]

So, why can't I modify elements through a loop in Python? There's definitely something I'm missing, but I don't know what. I'm sure this is a duplicate, but I couldn't find a question about this, and if there is a link, that would be more than enough. Thank you in advance!

2013/10/10
1
36
10/10/2013 8:29:51 AM

Accepted Answer

Because the way for i in li works is something like this:

for idx in range(len(li)):
    i = li[idx]
    i = 'foo'

So if you assign anything to i, it won't affect li[idx].

The solution is either what you have proposed, or looping through the indices:

for idx in range(len(li)):
    li[idx] = 'foo'

or use enumerate:

for idx, item in enumerate(li):
    li[idx] = 'foo'
2013/10/10
41
10/10/2013 8:34:04 AM

In fact with list comprehension you are not modifying the list, you are creating a new list and then assigning it to the variable that contained the previous one.

Anyway, when you do for i in li you are getting a copy of each value of li in variable i, you don't get the reference to a position in li, so you are not modifying any value in li.

If you want to modify your list you can do it with enumerate:

>>> li = ["spam", "eggs"]
>>> for i,_ in enumerate(li):
        li[i] = "foo"
>>> li
 ['foo', 'foo']

or with xrange (in Python 2.7, use range in python 3):

>>> for i in xrange(len(li)):
        li[i] = "foo"
>>> li 
 ['foo', 'foo']

or with the list comprehension you showed in your question.

2013/10/10

I'm able to modify a list while looping:

lst = range(10)  // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

for i, elem in enumerate(lst):
    lst[i] = 0   // [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
2013/10/10

Maybe using dictionaries might be helpful.

>>> li = {0: "spam", 1:"eggs"}
    for k, v in li.iteritems():
        li[k] = "foo"

>>> li
{0: 'foo', 1: 'foo'}
2013/10/10

for element in li - returns you a copy of element, not the element itself.

Solution for your case would be:

for i in range(len(li)):
    li[i] = 'foo'
2013/10/10