Process either S3 event notifications or SNS messages with python in AWS Lambda

lambda_handler.py

import urllib

def lambda_handler(event, context):
    for record in event['Records']:
        try:
            if 'aws:sns' == record['EventSource'] and record['Sns']['Message']:
                record = json.loads(record['Sns']['Message'])['Records'][0]
         except KeyError:
            pass
         bucket = record['s3']['bucket']['name']
         key = urllib.unquote(record['s3']['object']['key'])

When configuring AWS S3 events you can either choose to send to an SNS topic or invoke Lambda directly.
The above code snippet supports both types of invocation, so you can easily move from a direct invoke to a fan-out architecture without rearchitecting the code.