5.9 List Slicing (BT101CO)
List Slicing is a powerful feature in Python that allows you to extract a specific portion (a "slice") of a list to create a new list. While indexing retrieves a single item, slicing retrieves a sub-sequence.
1. The Three Parameters
The standard syntax for slicing is: list_name[start : stop : step]
start: The index where the slice begins (inclusive). If omitted, it defaults to 0.stop: The index where the slice ends (exclusive). The element at this index is not included. If omitted, it defaults to the end of the list.step: The increment between each index (optional). If omitted, it defaults to 1.
2. Basic Slicing Examples
Assume we have the following list: letters = ['A', 'B', 'C', 'D', 'E', 'F']
| Slice Code | Result | Explanation |
|---|---|---|
letters[1:4] | ['B', 'C', 'D'] | From index 1 to 3 (stops before 4). |
letters[:3] | ['A', 'B', 'C'] | From the beginning to index 2. |
letters[2:] | ['C', 'D', 'E', 'F'] | From index 2 to the very end. |
letters[:] | ['A', 'B', 'C', 'D', 'E', 'F'] | Creates a full copy of the list. |
3. Slicing with a step
The step value determines how many items to skip.
numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# Get every second element
print(numbers[::2]) # Output: [0, 2, 4, 6, 8]
# Get elements from index 1 to 7, skipping every other one
print(numbers[1:8:2]) # Output: [1, 3, 5, 7]4. Negative Slicing and Reversing
You can use negative numbers for any of the three parameters. This is particularly useful for working from the end of the list or reversing data.
- Last three items:
letters[-3:]→['D', 'E', 'F'] - Reverse the entire list:
letters[::-1]→['F', 'E', 'D', 'C', 'B', 'A'] - Everything except the last two:
letters[:-2]→['A', 'B', 'C', 'D']
5. Modifying Lists via Slicing
Because lists are mutable, you can actually use slicing on the left side of the assignment operator to replace multiple elements at once.
colors = ["red", "green", "blue", "yellow"]
colors[1:3] = ["pink", "black"]
print(colors) # Output: ['red', 'pink', 'black', 'yellow']Summary Checklist for Slicing
| Goal | Shortcut |
|---|---|
| First n elements | list[:n] |
| Last n elements | list[-n:] |
| Skip every n-th item | list[::n] |
| Reverse the list | list[::-1] |
Slicing is a core skill for data manipulation, allowing for concise and readable code.