DynamoDB append list to nested list with boto3

Question

I have a DynamoDB object which has a list with lists:

options: [
  ['fish', 'cat', 'dog', 'cow']
]

I want to append another list to it so it looks like this:

options: [
  ['fish', 'cat', 'dog', 'cow'],
  ['bird', 'sheep', 'snake', 'bee']
]

I tried it with the append_list expression SET options = list_append(options, :opt) but the result is a single list merged together of the existing list and the one I am trying to append.

Answer 1

Like this:

options= [['dog', 'cat']]
try:
    resp = table.update_item(
        Key={
            'pk': id
        },
        UpdateExpression="SET #options = list_append(if_not_exists(#options, :empty_list), :options)",
        ExpressionAttributeValues={ ":options": options, ":empty_list": []},
        ExpressionAttributeNames={"#options": "options"},
    )
    print(resp['ResponseMetadata'])
except Exception as e:
    print(e)

You have to append a list, the client will extract the elements from the list, so we nest the parameters within another list.

I also use if_not_exists as you would get a validation exceptions if you try to append to a list that does not exist already.