AWS Lambda parameter passing
You also have to include the query string parameter in the section Resources/Method Request.
I am creating an api with AWS API Gateway with Lambda functions. I want to be able to make an API call with the following criteria:

In the method request of the API i have specified the Query String: itemid
I want to be able to use this itemid value within my lambda function
I am using Python in Lambda

I have tried putting the following in the Mapping template under the Method execution, however get an error:
-{ "itemid": "$input.params('itemid')" }