Python – Substitute prefix part of List

When working with lists, you may need to substitute the prefix (beginning) part of one list with another list. Python provides a simple approach using list slicing with the : operator and the len() function.

What is Prefix Substitution?

Prefix substitution means replacing the first N elements of a list with elements from another list, where N is the length of the replacement list.

Example

Here's how to substitute the prefix part of a list ?

# Define two lists
original_list = [29, 77, 19, 44, 26, 18]
replacement_list = [15, 44, 82]

print("Original list:")
print(original_list)

print("Replacement list:")
print(replacement_list)

# Sort both lists for demonstration
original_list.sort()
replacement_list.sort()

print("Original list after sorting:")
print(original_list)

print("Replacement list after sorting:")
print(replacement_list)

# Substitute prefix: take replacement list + remaining elements from original
result = replacement_list + original_list[len(replacement_list):]

print("Result after prefix substitution:")
print(result)

Original list:
[29, 77, 19, 44, 26, 18]
Replacement list:
[15, 44, 82]
Original list after sorting:
[18, 19, 26, 29, 44, 77]
Replacement list after sorting:
[15, 44, 82]
Result after prefix substitution:
[15, 44, 82, 29, 44, 77]

How It Works

The operation replacement_list + original_list[len(replacement_list):] works as follows:

• len(replacement_list) returns 3 (length of replacement list)

• original_list[3:] extracts elements from index 3 onwards: [29, 44, 77]

• Concatenation combines: [15, 44, 82] + [29, 44, 77] = [15, 44, 82, 29, 44, 77]

Alternative Approaches

You can also use list slicing assignment for in-place substitution ?

# In-place prefix substitution
original = [18, 19, 26, 29, 44, 77]
replacement = [15, 44, 82]

print("Before substitution:", original)

# Replace first 3 elements
original[:len(replacement)] = replacement

print("After substitution:", original)

Before substitution: [18, 19, 26, 29, 44, 77]
After substitution: [15, 44, 82, 29, 44, 77]

Conclusion

Use list concatenation with slicing to substitute the prefix part of a list. The key is using len(replacement_list) to determine where to start extracting from the original list.