Why Does Assigning to a Python List Slice Modify the Original List?

Understanding the Mechanics of Assignment to List Slices

The Python documentation clearly states that slicing a list creates a new list. However, it can be puzzling to observe that assigning values to a list slice modifies the original list. This article unravels the mechanics behind this behavior and addresses the related questions:

1. Can Objects Returned by Slicing Appear on the Left Side of an Expression?

Slicing a list indeed produces a new list. Assigning to a slice, however, performs a different operation known as "slice assignment." This operation overwrites the specified slice of the original list with the assigned values.

2. Why is the Original List Modified if Slicing Creates a New List?

The slice assignment operation operates directly on the original list. Despite the creation of a new list during slicing, the assignment acts as a direct modification to the original list's elements within the specified range.

To clarify, let's consider the example:

a = [1, 2, 3]
a[0:2] = [4, 5]
print(a)

In this scenario, slicing a[0:2] creates a new list, but a[0:2] = [4, 5] replaces the elements at indices 0 and 1 of the original list a with 4 and 5, respectively. The output [4, 5, 3] confirms this modification to the original list.

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