import boto3

# Enter the region your instances are in. Include only the region without specifying Availability Zone; e.g., 'us-east-1'
region = 'eu-west-1'

filters = [{'Name':'tag:Env', 'Values':['Dev']}]

def lambda_handler(event, context):
    ec2 = boto3.client('ec2', region_name=region)

    instances = [instance.id for instance in boto3.resource('ec2').instances.filter(Filters=filters)]
    ec2.stop_instances(InstanceIds=instances)
    print 'stopped your instances: ' + str(instances)

    """
    ec2.start_instances(InstanceIds=instances)
    print 'started your instances: ' + str(instances)
    """

if __name__ == "__main__":
    lambda_handler({},None)