dynamodb table update item not detecting attribute

Question

when updating dynamodb table i get this error:

ERROR:root:------- here: An error occurred (ValidationException) when calling the 
 UpdateItem operation: The provided expression refers to an attribute that does not exist 
 in the item

I think its the expression in update_vote function that causes trouble

#!/usr/bin/env python3

import boto3
import json
import logging
import sys

logging.basicConfig(stream=sys.stdout, level=logging.INFO)

queue = boto3.resource('sqs', region_name='us-east-1').get_queue_by_name(QueueName="erjan")
table = boto3.resource('dynamodb', region_name='us-east-1').Table('Votes')

def process_message(message):
    try:
        
        payload = message.message_attributes
        voter = payload['voter']['StringValue']
        vote  = payload['vote']['StringValue']

        logging.info("Voter: %s, Vote: %s", voter, vote)
        store_vote(voter, vote)
        update_count(vote)
        message.delete()
    except Exception as e:
        print('-------')
        print('message.attr')
        print(message.message_attributes)
        try:
            vote = payload['vote']['StringValue']
            logging.error("Failed to process message")
            logging.error('vote %d' % vote)
        except TypeError:           
            logging.error("yes error catched")

def store_vote(voter, vote):
    try:
        logging.info('table put item.......')
        print('table put item......')
        response = table.put_item(
           Item={'voter': voter, 'vote': vote}
        )
    except:
        logging.error(" Failed to store message")
        raise


#this function causes error i think

def update_count(vote):
    logging.info('update count....')
    print('update count....')
    table.update_item(
        Key={'voter': 'default_voter'},
        UpdateExpression="set #vote = #vote + :incr", #this update expression
            ExpressionAttributeNames={'#vote': vote},
            ExpressionAttributeValues={':incr': 1}
    )

if __name__ == "__main__":
    while True:
        try:
            messages = queue.receive_messages(MessageAttributeNames=['vote','voter'])
        except KeyboardInterrupt:
           logging.info("Stopping...")
           break
        except:
            logging.error(sys.exc_info()[0])
            continue
        for message in messages:
            process_message(message)

dynamodb table has 1 partition key - 'voter'. it should store counts for how many clicks "a" or "b" received.

Answer 1

The reason you are seeing this exception is due to the fact no vote attribute exists in your item currently, causing the exception:

The provided expression refers to an attribute that does not exist in the item

---

1. Use ADD instead of SET

This is the recommended approach. It automatically sets the value to 0 should it not exist, then adds your provided value. The only time you would use option #2 is when the attribute is stored within a nested value, for which ADD will not work.

Use the ADD action in an update expression to add a new attribute and its values to an item. DOCS

• ADD path value

table.update_item(
    Key={'voter': 'default_voter'},
    UpdateExpression="ADD #vote :incr",
    ExpressionAttributeNames={'#vote': vote},
    ExpressionAttributeValues={':incr': 1}
)

2. Use if_not_exists Function

If the item does not contain an attribute at the specified path, if_not_exists evaluates to value; otherwise, it evaluates to path.

This approach requires you to set a value which is initially set if the attribute should not exist: :empty=0. This is more of a workaround than how you are recommended to add values in DynamoDB.

• if_not_exists (path, value)

table.update_item(
    Key={'voter': 'default_voter'},
    UpdateExpression="set #vote = if_not_exists(#vote, :empty) + :incr",
    ExpressionAttributeNames={'#vote': vote},
    ExpressionAttributeValues={':incr': 1, ':empty': 0}
)

Answer 2

You may want to set a default value if currently there is no value stored to be incremented:

def update_count(vote):
    logging.info('update count....')
    print('update count....')
    table.update_item(
        Key={'voter': 'default_voter'},
        UpdateExpression="set #vote = if_not_exists(#vote, :start) + :incr",  
        ExpressionAttributeNames={'#vote': vote},
        ExpressionAttributeValues={':incr': 1, ':start': 0}
    )

Notice the if_not_exists which will set the initial value for the votes to 0.