AWS Lambda Python to handle actions defined in a configuration file over S3

main.py

import boto3
import os
import json

##############################################################
###################### Check context #########################
##############################################################

def check_context(event):
    if event["context"] in contexts.keys():
        return event["context"], True
    return "", False

##############################################################
####################### Check action #########################
##############################################################

def check_action(event):
    if event["action"] in contexts[event["context"]].keys():
        return event["action"], True
    return "", False

##############################################################
##################### Check parameters #######################
##############################################################

def check_parameters(event):
    if len(event["parameters"]) > 0:
        return event["parameters"], True
    return [], False

##############################################################
################## Method to execute action ##################
##############################################################

class VPCAction(object):
    
    @staticmethod
    def start_instances_action(parameters):
        ec2 = boto3.client("ec2", region_name=os.environ["REGION"])
        response = ec2.describe_instances(
            Filters=[
                {
                    "Name": "tag:Name",
                    "Values": parameters
                }
            ]
        )
        instances = [instance["Instances"][0]["InstanceId"] for instance in response["Reservations"]]
        response = ec2.start_instances(InstanceIds=instances)
        return { "text" : "start instances " + " ".join(p for p in parameters) }

    @staticmethod
    def stop_instances_action(parameters):
        ec2 = boto3.client("ec2", region_name=os.environ["REGION"])
        response = ec2.describe_instances(
            Filters=[
                {
                    "Name": "tag:Name",
                    "Values": parameters
                }
            ]
        )
        instances = [instance["Instances"][0]["InstanceId"] for instance in response["Reservations"]]
        response = ec2.stop_instances(InstanceIds=instances)
        return { "text" : "stop instances " + " ".join(p for p in parameters) }
    
    @staticmethod
    def no_action_defined(parameters):
        return { "text" : "no action defined" }

##############################################################
############### Context and allowed contexts ##################
##############################################################

def getActionConfiguration():
    s3 = boto3.resource("s3")
    content_object = s3.Object(os.environ["S3_BUCKET_NAME"], os.environ["S3_BUCKET_OBJECT"])
    file_content = content_object.get()["Body"].read().decode("utf-8")
    contexts = json.loads(file_content)
    vpcActions = VPCAction()
    for context in contexts.keys():
        for action_name in contexts[context].keys():
            try:
                contexts[context][action_name] = getattr(vpcActions, contexts[context][action_name])
            except AttributeError:
                contexts[context][action_name] = vpcActions.no_action_defined
    return contexts

##############################################################
################## Method to execute action ##################
##############################################################

contexts = getActionConfiguration()

def handler(events, cont):

    context    = ""
    action     = ""
    parameters = []
    messages   = { "text" : "Nothing to say" }
    responses  = {}
    
    if isinstance(events, (dict,)) and 'Records' in events.keys():
        events = json.loads(events['Records'][0]['Sns']['Message'])

    for event in events:
        
        context, context_ok = check_context(event)
        if not context_ok:
            messages = { "text" : "Invalid context provided"+ event["context"].encode("utf-8") +"'" }
            return messages
    
        action, action_ok = check_action(event)
        if not action_ok:
            messages = { "text" : "Invalid action for provided context: '" + event["action"].encode("utf-8") +"'" }
            return messages
    
        parameters, parameters_ok = check_parameters(event)
        if not parameters_ok:
            messages = { "text" : "Invalid parameters for provided context and action: '" + ' '.join(p for p in event["parameters"]) +"'" }
            return messages
    
        responses[event['context']] = contexts[context][action](parameters)
    
    return responses