In place insertion into list (or array)

Question

I'm running a script in Python, where I need to insert new numbers into an array (or list) at certain index locations. The problem is that obviously as I insert new numbers, the index locations are invalidated. Is there a clever way to insert the new values at the index locations all at once? Or is the only solution to increment the index number (first value of the pair) as I add?

Sample test code snippets:

original_list = [0, 1, 2, 3, 4, 5, 6, 7]
insertion_indices = [1, 4, 5]
new_numbers = [8, 9, 10]
pairs = [(insertion_indices[i], new_numbers[i]) for i in range(len(insertion_indices))]

for pair in pairs:
    original_list.insert(pair[0], pair[1])

Results in:

[0, 8, 1, 2, 9, 10, 3, 4, 5, 6, 7]

whereas I want:

[0, 8, 1, 2, 3, 9, 4, 10, 5, 6, 7]

Answer 1

Insert those values in backwards order. Like so:

original_list = [0, 1, 2, 3, 4, 5, 6, 7]
insertion_indices = [1, 4, 5]
new_numbers = [8, 9, 10]

new = zip(insertion_indices, new_numbers)
new.sort(reverse=True)

for i, x in new:
    original_list.insert(i, x)

---

The reason this works is based on the following observation:

Inserting a value at the beginning of the list offsets the indexes of all other values by 1. Inserting a value at the end though, and the indexes remain unchanged. As a consequence, if you start by inserting the value with the largest index (10) and continue "backwards" you would not have to update any indexes.