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

Understanding List Modification in Python Loops

When attempting to modify elements within a list while iterating over it using a loop, you may encounter an issue where the changes don't seem to persist. This behavior stems from the way Python handles variables assigned within loops.

Core Issue

In Python, when you assign a variable within a loop that references an element in a list (e.g., for i in li:), you're not modifying the list itself but rather creating a new reference to that element. So, when you attempt to modify i, you're not actually changing the value in the list.

Example

Consider the following code:

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

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

Although you've assigned "foo" to i within the loop, the value of li remains unchanged because i doesn't directly reference the elements in li but rather a copy of them.

Solutions

To modify list elements during a loop, you have several options:

1. List Comprehension: Create a new list with the modified elements, as you demonstrated in your example using [foo for i in li].
2. Indexed Loop: Use a loop that iterates through the indices of the list (like for idx in range(len(li))) and assign the modified value to li[idx].
3. Enumerate: Utilize enumerate to simultaneously iterate over indices and elements (e.g., for idx, item in enumerate(li)), allowing you to directly modify the list elements.

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