How Does Python Handle Assignment to List Slices?

Understanding Assignment to List Slices

The Python documentation states that slicing a list produces a new list. This raises questions when assigning to a list slice, as exemplified below:

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

Left-Hand Side of Assignment

Despite slicing returning a new list, it can legally appear on the left-hand side of a slice assignment expression. This is because slicing not only returns a new list but also allows modification of the original list.

Modification of the Original List

Although a "new" list is returned from slicing, the original list is still referenced by the slice. As a result, assigning to the slice modifies both the original list and the returned slice.

To comprehend the mechanics behind this behavior, consider the following:

• Shallow Copy: When slicing a list, a shallow copy is created. This means a new list is allocated, but the underlying elements are not copied. They remain references to the elements in the original list.
• Slice Association: The slice on the left-hand side of an assignment expression refers directly to the underlying elements within the original list.
• Assignment: When assigning to a slice, the elements within the original list are re-assigned to the new values.

In essence, the syntax for slice assignment allows for both creating a new list and modifying elements within the original list simultaneously.

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