How to send and return a JSON response through a lambda function aws

Question

I have an a closure by Amazon webservices called lambda function with delegation defined as following:

def lambda_handler(event, context):
    logger.info('Ev° %s', event)

    if event['action'] == "getPosition":
        getPosition(event, context)

def getPosition(event, context):

    # read position from file
    positionLoaded = readPosition(pId=int(json.loads(str(event['body']['id']))))

    # build response
    response = {
        "statusCode": 200,
        "body": json.dumps(positionLoaded, indent=4),
        "message": "OK",
        "headers": {
            "Content-Type": "application/json",
        }
    }

return response


# Reads position in json file from s3 Bucket with pId
def readPosition(pId: int):
    positionFromBucketJSON = s3_client.get_object(Bucket="bucketName", Key=str(pId) + ".json")
    return json.loads(positionFromBucketJSON['Body'].read().decode('utf-8'))

When I send a request to the lambda function as following:

{
  "action": "getPosition",
  "body": {
    "id": 2021152530123456
  }
}

I get an error response from the lambda function as following:

{"errorMessage": "'Not Found'", "errorType": "KeyError", "stackTrace": ["  File \"/var/task/positionService.py\", line 58, in lambda_handler\n    getPosition(event, context, callback=None)\n", "  File \"/var/task/positionService.py\", line 76, in getPosition\n    message = "not existing at all!"}

I don't know why since JSON-File 2021152530123456.json do exist directly in s3 bucket.

Could you help to figure out what the error could be in this case?

jarmod · Accepted Answer · 2021-08-22 19:22:14Z

2

The Python Lambda function handler takes 2 parameters (event, context), not 3 parameters (event, context, callback). You may have been reading JavaScript documentation, where a Lambda function handler has a 3rd parameter (callback).

Here is an example of a Python Lambda function that returns a JSON payload:

import json
        
def lambda_handler(event, context):
    return {
        "statusCode": 200,
        "headers": {
            "Content-Type": "application/json"
        },
        "body": json.dumps({
            "id": 12,
            "description": "hoverboard"
        })
    }

In your original code, you have two additional problems:

your Lambda handler doesn't return anything (it should return getPosition(event, context))
your getPosition() function's return statement is mis-indented (correct the indentation)

edited Aug 22, 2021 at 19:22

answered Aug 22, 2021 at 15:19

jarmod

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7 Comments

Manifest Man Over a year ago

i,m going to check it out, and come back, thanks for the details, i have read that documentation also from many other ones. See you later then

Manifest Man Over a year ago

i,m getting now null as return after update!

jarmod Over a year ago

What does your current code look like? It's going to be difficult to diagnose something we can't see.

Manifest Man Over a year ago

i just updated the question with newer code :)

jarmod Over a year ago

Your Lambda handler, as written, doesn't return anything. And your getPosition() function's return statement is mis-indented.

|

Dharman · Accepted Answer · 2021-08-28 19:32:23Z

-1

I solve my issue by adding a return to the main lambda function while delegating to the getPosition(...) function, it look then like following:

def lambda_handler(event, context):
    logger.info('Ev° %s', event)

    if event['action'] == "getPosition":
        return getPosition(event, context)

edited Aug 28, 2021 at 19:32

Dharman♦

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answered Aug 22, 2021 at 19:02

Manifest Man

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