AWS: Publish SNS message for Lambda function via boto3 (Python2)

you need to add a default key to your message payload, and specify MessageStructure:

import json
import boto3

message = {"foo": "bar"}
client = boto3.client('sns')
response = client.publish(
    TargetArn=arn,
    Message=json.dumps({'default': json.dumps(message)}),
    MessageStructure='json'
)

Just in case you want to have different messages for sms and email subscribers:

import json
import boto3

message = {"foo": "bar"}
client = boto3.client('sns')
response = client.publish(
    TargetArn=arn,
    Message=json.dumps({'default': json.dumps(message),
                        'sms': 'here a short version of the message',
                        'email': 'here a longer version of the message'}),
    Subject='a short subject for your message',
    MessageStructure='json'
)