Why Doesn't Modifying `i` in a Python List Loop Change the List?

Modifying List Elements within Loops in Python

In Python, attempting to modify list elements while iterating over them using a loop often yields unexpected results. Understanding the underlying mechanics of this behavior is essential for effective list manipulation.

For instance, consider the following code:

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

print(li)
# Output: ["spam", "eggs"]

Despite assigning "foo" to i within the loop, the contents of li remain unchanged. This behavior stems from how Python iterates through lists.

Loop Mechanics

The loop for i in li behaves similarly to the following:

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

Therefore, assigning a new value to i does not alter the ith element of li. To modify list elements within a loop, alternative approaches are required.

Alternative Solutions

One solution is to use list comprehensions:

li = ["foo" for i in li]
print(li)
# Output: ["foo", "foo"]

Alternatively, iterate over the indices of the list:

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

print(li)
# Output: ["foo", "foo"]

Finally, enumerate can also be utilized:

for idx, item in enumerate(li):
    li[idx] = 'foo'

print(li)
# Output: ["foo", "foo"]

By understanding the loop mechanics and employing the appropriate methods, programmers can effectively modify list elements within loops in Python.

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