dynamodb conditional update item (client)

Question

I am referring to this sample of update item: https://stackoverflow.com/a/62030403/13967222.

    for key, val in body.items():
        update_expression.append(f" {key} = :{key},")
        update_values[f":{key}"] = val

    return "".join(update_expression)[:-1], update_values

I am trying to achieve the same but using dynamodb client.

Is there way I add check if the value is available using dynamodb client?

Answer 1

You can check if the key attribute already exists passing ConditionExpression parameter in the request like that (based on the response in the answer you're referring to):

table.update_item(
        Key={'uuid':str(uuid)},
        UpdateExpression=a,
        ExpressionAttributeValues=dict(v),
        ConditionExpression="attribute_not_exists(uuid)"
        )

Your write will be executed only when ConditionExpression returns True. attribute_not_exists is a DynamoDb function that returns True when the specified attribute does not exist. There is also a attribute_exists function to achieve the opposite result. More about DDB condition functions in official documentation

Also, you can check the Python example of conditional writes in docs here