How do I conditionally insert an item into a dynamodb table using boto3

Question

If I have a table with a hash key of userId and a range key of productId, how do I put an item into that table only if it doesn't already exist using boto3's dynamodb bindings?

The normal call to put_item looks like this

table.put_item(Item={'userId': 1, 'productId': 2})

My call with a ConditionExpression looks like this:

table.put_item(
    Item={'userId': 1, 'productId': 2}, 
    ConditionExpression='userId <> :uid AND productId <> :pid', 
    ExpressionAttributeValues={':uid': 1, ':pid': 3}
)

But this raises a ConditionalCheckFailedException every time. Whether an item exists with the same productId or not.

Answer 1

The documentation for this unfortunately isn't super clear. I needed to accomplish something similar, and here's what worked for me, using boto3:

try:
    table.put_item(
        Item={
            'foo':1,
            'bar':2,
        },
        ConditionExpression='attribute_not_exists(foo) AND attribute_not_exists(bar)'
    )
except botocore.exceptions.ClientError as e:
    # Ignore the ConditionalCheckFailedException, bubble up
    # other exceptions.
    if e.response['Error']['Code'] != 'ConditionalCheckFailedException':
        raise

Similar to the other answer, the key is in the attribute_not_exists function, but it was unclear to me initially how to get that to work. After some experimentation, I was able to get it going with the above.