In this case, I was inadvertently preserving the state of prior calls in properties of the class, e.g., self.filtered_items, which is a python list. I am using Flask for the front end, and it evidently is preserving the state of the class I use to retrieve the data between requests from the web browser. To solve this problem, I am just emptying the list in the property before it repopulates: self.filtered_items = [].