2

Im trying to implement a function that will get the event from cloudwatch and print the results. I am able to get the event but I want to extract one particular key from that JSON.

Here is my function:

import json
    def lambda_handler(event, context):
        print("Received event: " + json.dumps(event, indent=2)) 
        message = event['Records'][0]['Sns']['Message']
        print(message)

The event got from Cloudwatch:

"Records": [
{
"EventVersion": "1.0", 
"EventSubscriptionArn": "arn:aws:sns:us-east-1:xxxxxxxxxxxxx:bhuvi:XXXXXXXXXXXXXXXXXXXXXXXXXX", 
"EventSource": "aws:sns", 
"Sns": {
"SignatureVersion": "1", 
"Timestamp": "2018-01-13T19:18:44.369Z", 
"Signature": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", 
"SigningCertUrl": "https://sns.us-east-1.amazonaws.com/SimpleNotificationService-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.pem", 
"MessageId": "4b76b0ea-5e0f-502f-81ec-e23e03dbaf01", 
"Message": "{\"AlarmName\":\"test\",\"AlarmDescription\":\"test\",\"AWSAccountId\":\"xxxxxxxxxxxxx\",\"NewStateValue\":\"ALARM\",\"NewStateReason\":\"Threshold Crossed: 1 out of the last 1 datapoints [2.6260535333900545 (13/01/18 19:13:00)] was greater than or equal to the threshold (1.0) (minimum 1 datapoint for OK -> ALARM transition).\",\"StateChangeTime\":\"2018-01-13T19:18:44.312+0000\",\"Region\":\"US East (N. Virginia)\",\"OldStateValue\":\"OK\",\"Trigger\":{\"MetricName\":\"CPUUtilization\",\"Namespace\":\"AWS/RDS\",\"StatisticType\":\"Statistic\",\"Statistic\":\"AVERAGE\",\"Unit\":null,\"Dimensions\":[{\"name\":\"DBInstanceIdentifier\",\"value\":\"myrds\"}],\"Period\":300,\"EvaluationPeriods\":1,\"ComparisonOperator\":\"GreaterThanOrEqualToThreshold\",\"Threshold\":1.0,\"TreatMissingData\":\"\",\"EvaluateLowSampleCountPercentile\":\"\"}}", 
"MessageAttributes":
{}
, 
"Type": "Notification", 
"UnsubscribeUrl": "https://sns.us-east-1.amazonaws.com/?xcsgagrgrwgwrg", 
"TopicArn": "arn:aws:sns:us-east-1:xxxxxxxxxxxxx:bhuvi", 
"Subject": "ALARM: \"test\" in US East (N. Virginia)"
}
}
]
}

My extract command(Upto message) and its result:

message = event['Records'][0]['Sns']['Message']
        print(message)

Result

{
    "AlarmName": "test",
    "AlarmDescription": "test",
    "AWSAccountId": "xxxxxxxxxxxxx",
    "NewStateValue": "ALARM",
    "NewStateReason": "Threshold Crossed: 1 out of the last 1 datapoints [2.6260535333900545 (13/01/18 19:13:00)] was greater than or equal to the threshold (1.0) (minimum 1 datapoint for OK -> ALARM transition).",
    "StateChangeTime": "2018-01-13T19:18:44.312+0000",
    "Region": "US East (N. Virginia)",
    "OldStateValue": "OK",
    "Trigger": {
        "MetricName": "CPUUtilization",
        "Namespace": "AWS/RDS",
        "StatisticType": "Statistic",
        "Statistic": "AVERAGE",
        "Unit": null,
        "Dimensions": [
            {
                "name": "DBInstanceIdentifier",
                "value": "myrds"
            }
        ],
        "Period": 300,
        "EvaluationPeriods": 1,
        "ComparisonOperator": "GreaterThanOrEqualToThreshold",
        "Threshold": 1,
        "TreatMissingData": "",
        "EvaluateLowSampleCountPercentile": ""
    }

I want to extract some values from this message pane.

For eg: I want to extract name. So I tried the below command, but unfortunately its not working. Can anyone help me on this? enter image description here

my code for this:

message = event['Records'][0]['Sns']['Message']['Trigger']['Dimensions']['name']
    print(message)

ERROR:

{
  "stackTrace": [
    [
      "/var/task/lambda_function.py",
      14,
      "lambda_handler",
      "message = event['Records'][0]['Sns']['Message']['Trigger']['Dimensions']['name']"
    ]
  ],
  "errorType": "TypeError",
  "errorMessage": "string indices must be integers"
}
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2 Answers 2

Reset to default
6

So there are 3 problems:

Problem 1: In your example event, ['Records'][0]['Sns']['Message'] is a str in JSON format. That means that you need to parse to a dict like this:

 message = event['Records'][0]['Sns']['Message']
 message = json.loads(message)

Problem 2: message['Trigger']['Dimensions'] is a list but you are trying to access it like if it were a dict. So you only need to change your code to:

message = message['Trigger']['Dimensions'][0]['name']

Problem 3: Message is a str that means that you need to verify that is a plain str or json str (otherwise you are going to have problems with multiple structures and types). For that your code could look like:

 message = event['Records'][0]['Sns']['Message']
    if isinstance(message, str):
        try:
            message = json.loads(message)
        except Exception as e:
            print(e) # Or do nothing, this is just to log the error
    elif isinstance(message, list):
        message = message[0]
    # Maybe evaluate bool, tuple, etc other types

    print('RESPONSE', message['Trigger']['Dimensions'][0]['name'] if isinstance(message, dict) else message)

However I would also recommend to make it more extensible iterating the elements that you know are list. And for safety reasons (trying to avoid null pointer exceptions), use the get() function with a default value. http://www.tutorialspoint.com/python/dictionary_get.htm . Try maybe to create a function to parse structures and make it reusable.

Good luck!

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23 Comments

Ohhh I think that now I see it, Message is a str with JSON content (is not a dict, it think that the print was a little bit confusing). In that case you just need to transform it from json to dict. You can do this: import json message = json.loads(event['Records'][0]['Sns']['Message']) print(message['Trigger']['Dimensions'][0]['name'])
let me try this one.
Can you do something? can you show me this print? print(event['Records'][0]['Sns']['Message'].__class__.__name__)
The captured JSON contents are like this, "Message": "{\"AlarmName\":\"test\". Some " and / characters are there. Was this causing the error?
Now you changed the structure of your event, now message is just a plain str, evaluate that case. I commented your gist with that solution. Please always verify the structure of your message using the .get() function. Maybe you should create kind of a parser to verify multiple structures
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1

Just as Records is a list, so you use ['Records'][0]['Sns']..., so is Dimensions, so again you need to access the first element.

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1 Comment

Sorry I didn't get you, can you please give me what should I give in the message parameter?

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