update_item.py

table.update_item(
    Key={
        'scatter_gather_id': scatter_gather_id
    },
    UpdateExpression='SET finished_processes = finished_processes + :val, #updated_at = :updated_at',
    ExpressionAttributeValues={
        ':val': value_to_sum,
        ':updated_at': datetime_now
    },
    ExpressionAttributeNames={
        '#updated_at': 'updated_at'
    },
    ReturnValues='ALL_NEW'
)