Why Doesn't Modifying List Elements Directly in a Python Loop Work?

Why Modifying List Elements in a Python Loop Proves Fruitless

While traversing a list in Python, many programmers encounter an unexpected obstacle: the inability to modify its elements through traditional loops. To illustrate:

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

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

Despite the apparent assignment of "foo" to list elements, the list remains unchanged. This perplexing behavior stems from the way Python handles loops with lists.

Behind the scenes, Python loops operate by iterating over the indices of the list. In the example above, for each index (corresponding to an item in the list), a new temporary variable i is created. Assignments to i only affect this local variable, leaving the actual list elements untouched.

To successfully modify list elements, one must either employ a list comprehension as in the provided example:

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

Alternatively, iterate over the indices directly:

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

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

Alternatively, leverage the enumerate() function:

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

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

By embracing these solutions, programmers can confidently modify list elements within loops, avoiding fruitless assignments that leave their lists unaltered.

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