Check unique hash_key in dynamodb - python

Question

i'm trying to check existence of an hash_key in dynamodb with boto (i can not update my filed if exist) i've try with query

   for i in self.table.query(hask_key=[value]):
      print i['url']

But i miss an argument (???). I've try with scan but i need to check my hash_key, not attributes.

I've try with get_item but i cannot manage the response if hash_key does not exist.

Any advice?

I've play with redis and it have exist method.

score 7 · Accepted Answer · answered Apr 03 '13 at 11:36

Rather than doing a query operation, you should just attempt to retrieve the item using get_item. If it exists, the item will be returned. If not, the call will raise a DynamoDBKeyNotFoundError which you can catch. So, something like:

def exists(hash_key):
    try:
        item = self.table.get_item(hash_key=<hash key>...)
    except boto.dynamodb.exceptions.DynamoDBKeyNotFoundError:
        item = None
    return item

would return None if the hash didn't exist and would return the item if it did exist.

Is there a cheaper way to do this than to pay the full cost of retrieving the item? — jocull, May 13 '21 at 15:19

score 4 · Answer 2 · answered Mar 07 '20 at 22:12

Just ended up on this page after a google search. Here's a 2020 update:

The API behaviour has changed and is returning either 'the item' or 'nothing', but won't raise an exception: https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/dynamodb.html#DynamoDB.Client.get_item

Here is an example of how to use get_item to query for a single item: https://boto3.amazonaws.com/v1/documentation/api/latest/guide/dynamodb.html#getting-an-item

And this SOF answer addresses the specific problem of checking for the existence of an item in a DDB table: https://stackoverflow.com/a/48589589/512155

Check unique hash_key in dynamodb - python

2 Answers2