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.
The general rule of thumb is that you don't modify a collection/array/list while iterating over it. Use a secondary list to store the items you want to act upon and execute that logic in a loop after your initial 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.
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]
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With