Why Can't I Modify a Python List Directly Within a `for` Loop?

Python List Modification Conundrum

When traversing a list in Python using a for loop, users may encounter an unexpected inability to alter its elements without employing list comprehensions.

Why the Inaccessibility?

The crux of the issue lies in the behavior of the for i in li loop construct. Internally, it iterates as follows:

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

As seen, i is initially assigned a reference to an element in the list. Any subsequent modification to i only affects the local variable, not the list item itself.

Resolving the Dilemma

To address this, one can either leverage list comprehensions:

li = ["foo" for i in li]

Or, traverse the indices explicitly:

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

Alternatively, enumerate can be employed to simplify the process:

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

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