We will assert the variable is defined, that is, the expected input binding was actually supplied by the calling context: I used to be beating my head on this for quite some time prior to getting what I feel is The solution. Python @properties can be a subset of descriptors, https://propertyingurugram.in/emaar-india-plans-to-invest-a-billion-dollars-over-the-next-4-to-5-years-to-develop-real-estate-projects-in-the-country/