This is a simple python script to post to Slack from a SNS Topic (CloudWatch Trigger).

SNSPostToSlack.py

#!/usr/bin/env python
# Written by:   Robert J.
# Email:        [email protected]

import boto3
import json
import logging
import os, sys

from urllib2 import Request, urlopen, URLError, HTTPError

HOOK_URL        = os.environ['HOOK_URL']
SLACK_CHANNEL   = os.environ['SLACK_CHANNEL']
ENVIRONMENT     = os.environ['EnvType']
ICON_EMOJI      = ':cloudwatch:'
USERNAME        = 'CloudWatch Bot'

logger = logging.getLogger()
logger.setLevel(logging.INFO)


def main(event, context):
    env_blacklist = ['beta']

    if ENVIRONMENT in env_blacklist:
        logger.info("Ignoring CloudWatch event since it is part of the %s stack." % (ENVIRONMENT))
    else:
        message         = json.loads(event['Records'][0]['Sns']['Message'])
        json_payload    = generate_payload(message)
        
        post_payload(json_payload)
    
    logger.info("Exiting Lambda Function.")


def error(message, code=1):
    logger.error(message)
    sys.exit(code)


def generate_payload(message):
    try:
        alarm_name      = message['AlarmName']
        old_state       = message['OldStateValue']
        new_state       = message['NewStateValue']
        reason          = message['NewStateReason']
        metric_name     = message['Trigger']['MetricName']
        resource        = message['Trigger']['Dimensions'][0]['value']
    except KeyError as e:
        error("Key Missing: " + str(e))

    if new_state == "ALARM":
        color = "#A30200"
    elif new_state == "OK":
        color = "#2eb886"
    else:
        color = "#dddddd"

    slack_message   = {
        "attachments": [
           {
                "fallback": reason,
                "color": color,
                "title": alarm_name,
                "title_link": "https://us-west-2.console.aws.amazon.com/cloudwatch/home?region=us-west-2#alarm:alarmFilter=ANY",
                "fields": [
                    {
                        "title": "State",
                        "value": "%s -> %s" % (old_state, new_state),
                        "short": False
                    },
                    {
                        "title": "Reason",
                        "value": reason,
                        "short": False
                    },
                    {
                        "title": "Metric Name",
                        "value": metric_name,
                        "short": False
                    },
                    {
                        "title": "Affected Resource",
                        "value": resource,
                        "short": False
                    },
                ]
            }
        ],
        'channel': SLACK_CHANNEL,
        'username': USERNAME,
        'icon_emoji': ICON_EMOJI
    }

    return slack_message


def post_payload(json_payload):
    req = Request(HOOK_URL, json.dumps(json_payload))

    try:
        response = urlopen(req)
        response.read()
        logger.info("Message posted to %s", json_payload['channel'])
    except HTTPError as e:
        error("Request failed: %d %s", e.code, e.reason)
    except URLError as e:
        error("Server connection failed: %s", e.reason)


def lambda_handler(event, context):
    logger.info(json.dumps(event, indent=4))
    try:
        main(event, context)
    except Exception as msg:
        error(str(msg))