Missing handler error in AWS Lambda

Move your code inside a python function. You can give it any name but that will become your handler. Go to lambda function basic settings and change the default handler to <yourfilename>_<yourfunctionname>. By default when you create a lambda function, the file name will be lambda_function_name.py (you can change it) and your handler method will be lambda_handler (you can change it), so the entry point is lambda_function_name.lambda_handler.

You need to define a function in your code. The code is missing the function named lambda_handler. Your code should look like:

    import botocore
    import boto3

    def lambda_handler(event, context):
        s3 = boto3.resource('s3')
        bucket = s3.Bucket('bucketname')
        exists = True

        try:
            s3.meta.client.head_bucket(Bucket='bucketname')
        except botocore.exceptions.ClientError as e:
            # If a client error is thrown, then check that it was a 404 error.
            # If it was a 404 error, then the bucket does not exist.
            error_code = int(e.response['Error']['Code'])
            if error_code == 404:
                exists = False