Lambda execution failed with status 200 due to customer function error - python-3.x

Question

I'm creating a simple Lambda function to serve a POST in AWS Gateway:
import json
import boto3
from datetime import datetime
def lambda_handler(event,context):
wf = event['WF']
if wf == 'start1':
body = 'Suceeded'
return {
"isBase64Encoded": False,
'statusCode': 200,
'body': json.dumps(body),
'headers': {
'Content-Type': 'application/json'
}
}
else:
body = 'Failed'
return {
"isBase64Encoded": False,
'statusCode': 400,
'body': json.dumps(body),
'headers': {
'Content-Type': 'application/json'
}
}
If I send a POST with this body is successful:
{
"WF": "start1",
"Context": "OK"
}
Returns:
Test Event Name
test
Response
{
"isBase64Encoded": false,
"statusCode": 200,
"body": "\"Suceeded\"",
"headers": {
"Content-Type": "application/json"
}
}
Function Logs
START RequestId: 91663a9d-d8b6-4aad-b8b6-eb2f15ff0354 Version: $LATEST
END RequestId: 91663a9d-d8b6-4aad-b8b6-eb2f15ff0354
REPORT RequestId: 91663a9d-d8b6-4aad-b8b6-eb2f15ff0354 Duration: 1.74 ms Billed Duration: 2 ms Memory Size: 128 MB Max Memory Used: 66 MB Init Duration: 300.73 ms
Request ID
91663a9d-d8b6-4aad-b8b6-eb2f15ff0354
But if I run a test in the API Gateway service in AWS I get:
Fri Jun 04 13:45:22 UTC 2021 : Sending request to https://XXXXXXX
Fri Jun 04 13:45:22 UTC 2021 : Received response. Status: 200, Integration latency: 15 ms
Fri Jun 04 13:45:22 UTC 2021 : Endpoint response headers: {Date=Fri, 04 Jun 2021 13:45:22 GMT, Content-Type=application/json, Content-Length=159, Connection=keep-alive, x-amzn-RequestId=d479fdac-6737-42fa-96a3-9e991056b48d, X-Amz-Function-Error=Unhandled, x-amzn-Remapped-Content-Length=0, X-Amz-Executed-Version=$LATEST, X-Amzn-Trace-Id=root=1-60ba2e72-ff62f051febbd21d6cc9d114;sampled=0}
Fri Jun 04 13:45:22 UTC 2021 : Endpoint response body before transformations: {"errorMessage": "'WF'", "errorType": "KeyError", "stackTrace": [" File \"/var/task/lambda_function.py\", line 7, in lambda_handler\n wf = event['WF']\n"]}
Fri Jun 04 13:45:22 UTC 2021 : Lambda execution failed with status 200 due to customer function error: 'WF'. Lambda request id: d479fdac-6737-42fa-96a3-9e991056b48d
Fri Jun 04 13:45:22 UTC 2021 : Method completed with status: 502
Why line 7 is wrong?
Is there another way to parse the body of the POST?

Answer 1

The solution was found by disabling Lambda Proxy Integration in the POST - Integration Request.

Answer 2

It looks like your API Gateway invocation is not populating the event parameter of your Lambda function. When you create a test event your lambda function's page, you get to create your own test event. API Gateway will not use this test event, it will instead send an empty event (probably null).
I.e.
import json
import boto3
from datetime import datetime
def lambda_handler(event,context): # Event is null here
wf = event['WF'] # Attempting to read the attribute 'WF' of null results in an error

Answer 3

In my instance, I had received this error when invoking my Lambda function via API gateway, it was also completely fine when invoking directly the lambda function expecting a simple JSON body.
my expected payload:
{"Email": "foo3#foo.com"}
It turns out Lambda Proxy Integration in the POST in the configuration API gateway changes the payload structure before passing into Lambda, and I was getting the error missing Key attribute Email. I noticed that line Endpoint request body after transformations: in cloudwatch. I learned that it was changing the payload.
How I debug this:
Invoke Lambda using test events ensuring the both the error scenario and success scenarios are working.
Ensure there are no permission issues via API gateway by making sure that your API gateway can invoke the Lambda and have appropriate IAM roles/policies
Ensure your lambda has the API gateway as the trigger
I tried removing all the security token first via API gateway to verify I am able to invoke the API
I tried both HTTP methods POST And GET, seeing the same error (Email was missing) was the key to figuring this out as I was already passing the payload in the request body in the POST method.