What is the proper way to return data with HTTP API + Lambda + DynamoDB as a json response? (added a Python example)

Your question and objective have multiple pieces:

• Q: How to handle encoding decimal values to JSON.
  • A: Use a custom encoder like how you are doing
  • Documentation: https://stackoverflow.com/questions/1960516/python-json-serialize-a-decimal-object
• Q: Avoiding reserved words
  • A: Use ProjectionExpression with ExpressionAttributeNames like how you are doing
  • Documentation: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/ReservedWords.html
• Q: Simplest way to return a get response from DynamoDB using python
  • A:

import json
import uuid
import os
import boto3
from datetime import datetime
from decimal import Decimal

class DecimalEncoder(json.JSONEncoder):
  def default(self, obj):
    if isinstance(obj, Decimal):
      return str(obj)
    return json.JSONEncoder.default(self, obj)
# Prepare DynamoDB client
USERS_TABLE = os.getenv('USERS_TABLE', None)
dynamodb = boto3.resource('dynamodb')
ddbTable = dynamodb.Table(USERS_TABLE)

def lambda_handler(event, context):
    route_key = f"{event['httpMethod']} {event['resource']}"

    # Set default response, override with data from DynamoDB if any
    response_body = {'Message': 'Unsupported route'}
    status_code = 400
    headers = {
        'Content-Type': 'application/json',
        'Access-Control-Allow-Origin': '*'
        }

    try:
        # Get a list of all Users
        if route_key == 'GET /users':
            ddb_response = ddbTable.scan(Select='ALL_ATTRIBUTES')
            # return list of items instead of full DynamoDB response
            response_body = ddb_response['Items']
            status_code = 200

        # CRUD operations for a single User
       
        # Read a user by ID
        if route_key == 'GET /users/{userid}':
            # get data from the database
            ddb_response = ddbTable.get_item(
                Key={'userid': event['pathParameters']['userid']}
            )
            # return single item instead of full DynamoDB response
            if 'Item' in ddb_response:
                response_body = ddb_response['Item']
            else:
                response_body = {}
            status_code = 200
        
    except Exception as err:
        status_code = 400
        response_body = {'Error:': str(err)}
        print(str(err))
    return {
        'statusCode': status_code,
        'body': json.dumps(response_body, cls=DecimalEncoder),
        'headers': headers
    }

Your mapped_items object is just remapping the same values as exist in the items response, so if you don't need to rename those values in the json, you don't need that step. For example, this is the body of your response with and without the mapped_items:

With mapped_items

[{\"col1\": \"1\", \"col2\": \"2\", \"col3\": \"3\", \"col4\": \"4\", \"col5\": \"5\", \"owner\": \"ddd\"}]

Without mapped_items (just returning response["Items"])

[{\"col3\": \"3\", \"col4\": \"4\", \"col5\": \"5\", \"owner\": \"ddd\", \"col1\": \"1\", \"col2\": \"2\"}]

Also, it is better practice to establish the dynamoDB connection outside of the request handler (this is demonstrated in the above code). That way, if a warm invocation occurs, that invocation will use the existing dynamodDB connection and complete quicker

References: https://catalog.workshops.aws/serverless-patterns/en-US