How to Modify a List While Iterating in Python

Modifying a list while iterating can be tricky but there are several ways to do it safely. One simple way to modify a list while iterating is by using a for loop with the index of each item. This allows us to change specific items at certain positions.

a = [1, 2, 3, 4]
for i in range(len(a)):
    if a[i] % 2 == 0:
        a[i] = a[i] * 2
print(a)

[1, 4, 3, 8]

Let's look into other methods to Modify a List While Iterating in Python are:

Using List Comprehension

Create a new list based on the modifications and replace the original list if necessary. This avoids directly modifying the list during iteration.

a = [1, 2, 3, 4]
a = [x * 2 if x % 2 == 0 else x for x in a]
print(a)

[1, 4, 3, 8]

Using While Loop

In some cases, we might want more control over the iteration process. Using a while loop allows us to modify the list as we go, but we need to be careful to avoid skipping elements.

a = [1, 2, 3, 4]
i = 0
while i < len(a):
    if a[i] % 2 == 0:
        a[i] = a[i] * 2
    i += 1
print(a)

[1, 4, 3, 8]

Iterating Over a Copy

Iterate over a copy of the list (list.copy()) to avoid modifying the list during iteration.

a = [1, 2, 3, 4]
for item  in a.copy():
    if item % 2 != 0:
        a.remove(item)
print(a)

[2, 4]