It's not stated anywhere in the code, but the idea is that for any commodity (let's say a chair), you'd subclass the Commodity class and define those methods for that specific commodity.

Basically, Commodity is an abstract class for the kind of thing a commodity is. If you want to describe an actual commodity, you have to provide info specific to that commodity.

I think a traits-based language would probably represent this better, thougg. Per Marx, commodities do have properties, but they're primarily defined through social relations, so you'd want to be able to represent things like, "commodities are comparable through X process".