Can't modify list elements in a loop

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'

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.