How to insert item at index in a DynamoDB list using update_item and Boto3

Question

I have a simple DynamoDB table structure:

{
 id: '123_456',
 number_list: ['a', 'c']
}

And I would like to add 'b' in the middle (or at some specific index) of this array.

What I know is that we can use list_append() to add it at the end such as:

table.update_item(
 Key={'id': '123_456'},
 UpdateExpression="SET number_list = list_append(number_list, :nl)"
 ExpressionAttributeValues={ ':nl': ['b'] }
)

This will result in ['a', 'c', 'b'] but I want ['a', 'b', 'c'] .

Is there a way to achieve this or should I get the stored list, add at index in python and store it again with:

SET list_number = :nln
{ ':nln': ['a', 'b', 'c'] }

I know we can delete at index with REMOVE list_number[1] which will result in ['a'] . I would like to find something similar.

Many thanks in advance.

Answer 1

You can construct new list from older one - if you know exactly where change should happen. Here is an example - search for "Appending to a list and updating a specific value at the same time". From that I would try with:

UpdateExpression=f"SET #nl[1] = :repl",
ExpressionAttributeNames={
    "#nl": "number_list",
},
ExpressionAttributeValues={
    ":repl": "b"
},