After this lesson, you will be able to:

• Extract a sublist with start:stop while remembering that stop is exclusive.
• Use omitted bounds, negative bounds, and a positive step to select a useful window of items.
• Explain why assigning one list variable to another does not copy the list.
• Make an independent outer-list copy with [:] or .copy() before changing its elements.

A slice selects several items without manually indexing each one. With items[start:stop], start is included and stop is not. For example, items[1:4] returns items at indexes 1, 2, and 3.

A slice may omit either bound: items[:3] takes the first three items, and items[2:] takes everything from index 2 onward. Negative bounds count from the end, as you practiced in the previous lesson. items[::2] takes every second item using a positive step. Unlike indexing, a slice whose end goes beyond the list length simply stops at the available items.

Copying matters when data is mutable. candidate = baseline does not create a new list; both names refer to the same object. By contrast, candidate = baseline[:] or candidate = baseline.copy() creates a new outer list. Updating a top-level element in the copy will not change the original.

These are shallow copies: if the list contains mutable nested objects, those nested objects are still shared. We will handle deeper structures later.

1. Select a window with exclusive stop

The end position is not included.

stages = ["fetch", "build", "test", "package", "deploy"]
print(stages[1:4])

Output:

['build', 'test', 'package']

The element at index 4 is left out.

2. Use omitted and negative bounds

Leaving out the start or stop makes it easy to select items from either end.

regions = ["eu", "us", "apac", "sa", "ca"]
print(regions[:2])
print(regions[-2:])

Output:

['eu', 'us']
['sa', 'ca']

The negative start selects the final two items.

3. Sample at regular intervals

A third slice value chooses how far to move between selected items.

readings = [8, 9, 10, 11, 12, 13]
print(readings[::2])

Output:

[8, 10, 12]

The selection starts at the first element and advances by two.

4. Copy before editing

A slice copy gives a new list that can be updated independently.

baseline = ["queued", "running", "done"]
preview = baseline[:]
preview[1] = "paused"
print(baseline)
print(preview)

Output:

['queued', 'running', 'done']
['queued', 'paused', 'done']

baseline does not change. Calling baseline.copy() is another way to make this kind of copy.

1. Including the stop element: slicing excludes the stop index.
2. Assuming a slice past the end raises IndexError: it safely returns the available portion.
3. Using candidate = baseline as a copy: both names then point to the same list.
4. Assuming a shallow copy duplicates nested objects: [:] and .copy() only copy the outer list.