Python can't read body in API Gateway request?

Question

I'm trying to code a Lambda function that is invoked via a hit to an API Gateway endpoint. Simply returning the event, I can see that there is a body in the response:

def lambda_handler(event, context): return str(event)

Response:

{ 'version': '2.0', 'routeKey': 'ANY /identify', 'rawPath': '/default/identify', 'rawQueryString': '', 'headers': { ... }, 'requestContext': { ... }, 'body': '<base64 encoded string is here>', 'isBase64Encoded': True }

However, as soon as I try to return just the body, I get the following error (multiple examples included which all return the same error).

def lambda_handler(event, context): return str(event['body'])

def lambda_handler(event, context): return json.loads(event['body'])

def lambda_handler(event, context): params = parse_qs(event["body"])

def lambda_handler(event, context): return event['body']

{ "errorMessage": "'body'", "errorType": "KeyError", "requestId": "5acbcc66-da05-429f-baa9-6c8d83801b4f", "stackTrace": [ " File \"/var/task/lambda_function.py\", line 10, in lambda_handler\n return json.loads(event['body'])\n" ] }

Any ideas?

Might be relevant: amazon web services - Getting json body in aws Lambda via API gateway - Stack Overflow — John Rotenstein
– John Rotenstein, Commented Jan 11, 2022 at 3:24
<base64 encoded string is here> are you even decoding the base64? What is the outcome of the decoding procedure? — Marcin
– Marcin, Commented Jan 11, 2022 at 3:50

obelix15012 · Accepted Answer · 2022-01-13 06:14:10Z

KeyError indicates that you're trying to access a key in a Python dictionary that doesn't exist.

According to the documentation: "API Gateway invokes your function with an event that contains a JSON representation of the HTTP request."

I think that what is happening is when you're returning str(event) it just returns the stringified version of the json event. But when you try to access event[body] in your handler without running json.loads() on the json first, you get the KeyError above instead. I typically run json.loads() first in all my handlers and then assign that to a data object (which is a Python dictionary) as below:

data = json.loads(event["body"])

Then I can use that body:

print("[DEBUG] event['body']: {}".format(data))

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