Python Basics — List Slicing

List Slicing

List slicing works exactly like string slicing — but since lists are mutable, you can also use slicing to modify, insert, and delete elements.

Basic Slicing

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

print(numbers[2:5])    # [2, 3, 4]
print(numbers[:3])     # [0, 1, 2]
print(numbers[7:])     # [7, 8, 9]
print(numbers[-3:])    # [7, 8, 9]
print(numbers[::2])    # [0, 2, 4, 6, 8]
print(numbers[::-1])   # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Shallow Copy via Slicing

original = [1, 2, 3, 4, 5]
copy = original[:]

copy.append(6)
print(original)  # [1, 2, 3, 4, 5]  (unchanged)
print(copy)      # [1, 2, 3, 4, 5, 6]

Assigning to Slices

You can replace a section of a list with new elements:

numbers = [0, 1, 2, 3, 4, 5]

# Replace elements 2-4
numbers[2:5] = [20, 30, 40]
print(numbers)  # [0, 1, 20, 30, 40, 5]

Insert Elements via Slice

numbers = [0, 1, 4, 5]

# Insert at position 2 (empty slice)
numbers[2:2] = [2, 3]
print(numbers)  # [0, 1, 2, 3, 4, 5]

Replace with Different Length

numbers = [0, 1, 2, 3, 4, 5]

# Replace 3 elements with 1
numbers[1:4] = [99]
print(numbers)  # [0, 99, 4, 5]

numbers = [0, 1, 2, 3, 4, 5]

# Replace 1 element with 3
numbers[2:3] = [20, 21, 22]
print(numbers)  # [0, 1, 20, 21, 22, 3, 4, 5]

Deleting via Slices

numbers = [0, 1, 2, 3, 4, 5]

# Delete elements 2-4
del numbers[2:5]
print(numbers)  # [0, 1, 5]

numbers = [0, 1, 2, 3, 4, 5]

# Delete by assigning empty list
numbers[2:5] = []
print(numbers)  # [0, 1, 5]

Slice vs Copy Behavior

When you slice a list, you get a shallow copy:

# Simple values — fully independent
original = [1, 2, 3]
copy = original[:]
copy[0] = 99
print(original)  # [1, 2, 3]  (unchanged)

# Nested objects — inner objects are shared!
original = [[1, 2], [3, 4]]
copy = original[:]
copy[0][0] = 99
print(original)  # [[99, 2], [3, 4]]  (inner list changed!)

For truly independent copies of nested lists, use copy.deepcopy():

import copy

original = [[1, 2], [3, 4]]
deep = copy.deepcopy(original)
deep[0][0] = 99
print(original)  # [[1, 2], [3, 4]]  (unchanged)

Step Slicing with Lists

numbers = list(range(10))

# Every other element
print(numbers[::2])    # [0, 2, 4, 6, 8]

# Every third element
print(numbers[::3])    # [0, 3, 6, 9]

# Reverse
print(numbers[::-1])   # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

# Reverse every other
print(numbers[::-2])   # [9, 7, 5, 3, 1]

Practical Examples

Splitting a List into Chunks

def chunk(lst, size):
    return [lst[i:i + size] for i in range(0, len(lst), size)]

data = list(range(1, 11))
print(chunk(data, 3))
# [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10]]

Rotating a List

def rotate(lst, n):
    n = n % len(lst)
    return lst[n:] + lst[:n]

items = [1, 2, 3, 4, 5]
print(rotate(items, 2))   # [3, 4, 5, 1, 2]
print(rotate(items, -1))  # [5, 1, 2, 3, 4]

Summary

• List slicing: list[start:stop:step]
• Slicing returns a new (shallow copy) list
• You can assign to slices to replace, insert, or delete elements
• del list[start:stop] removes elements
• Shallow copies share nested objects — use copy.deepcopy() for full independence
• list[:] creates a shallow copy of the entire list

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Next up: List Comprehensions →