Python - Replace sublist with others in list

Replacing a sublist (portion of a list) with another list is a common operation in Python. This article explores multiple approaches to replace elements at specific index ranges with new values.

Let's understand the problem with an example ?

original_list = [1, 2, 3, 4, 5, 6]
new_items = [7, 8, 9]

# Replace elements from index 2 to 5 (exclusive) with new_items
# Result should be: [1, 2, 7, 8, 9, 6]

Using List Slicing (Recommended)

The simplest and most Pythonic approach uses slice assignment to replace a sublist ?

def replace_sublist(original_list, start, end, new_items):
    original_list[start:end] = new_items
    return original_list

original_list = [1, 2, 3, 4, 5, 6]
new_items = [7, 8, 9]
result = replace_sublist(original_list.copy(), 2, 5, new_items)

print("Original list:", [1, 2, 3, 4, 5, 6])
print("New items:", new_items)
print("Result:", result)

Original list: [1, 2, 3, 4, 5, 6]
New items: [7, 8, 9]
Result: [1, 2, 7, 8, 9, 6]

Using List Concatenation

Create a new list by concatenating three parts: before, replacement, and after ?

def replace_sublist(original_list, start, end, new_items):
    return original_list[:start] + new_items + original_list[end:]

original_list = [1, 2, 3, 4, 5, 6]
new_items = [7, 8, 9]
result = replace_sublist(original_list, 2, 5, new_items)

print("Original list:", original_list)
print("New items:", new_items)
print("Result:", result)

Original list: [1, 2, 3, 4, 5, 6]
New items: [7, 8, 9]
Result: [1, 2, 7, 8, 9, 6]

Using List Comprehension

Build a new list conditionally replacing elements based on their index position ?

def replace_sublist(original_list, start, end, new_items):
    return [new_items[i - start] if start <= i < end and i - start < len(new_items) 
            else element for i, element in enumerate(original_list)]

original_list = [1, 2, 3, 4, 5, 6]
new_items = [7, 8, 9]
result = replace_sublist(original_list, 2, 5, new_items)

print("Original list:", original_list)
print("New items:", new_items)
print("Result:", result)

Original list: [1, 2, 3, 4, 5, 6]
New items: [7, 8, 9]
Result: [1, 2, 7, 8, 9, 6]

Using Index-Based Replacement

Replace elements one by one using a loop and direct indexing ?

def replace_sublist(original_list, start, end, new_items):
    result = original_list.copy()
    for i in range(start, min(end, start + len(new_items))):
        if i - start < len(new_items):
            result[i] = new_items[i - start]
    return result

original_list = [1, 2, 3, 4, 5, 6]
new_items = [7, 8, 9]
result = replace_sublist(original_list, 2, 5, new_items)

print("Original list:", original_list)
print("New items:", new_items)
print("Result:", result)

Original list: [1, 2, 3, 4, 5, 6]
New items: [7, 8, 9]
Result: [1, 2, 7, 8, 9, 6]

Comparison

Conclusion

List slicing (`list[start:end] = new_items`) is the most efficient and readable method for replacing sublists. Use concatenation when you need to preserve the original list unchanged.