And yet another problem is that while Ruby classes require you to whitelist attributes (e.g. with attr_accessor) that will receive publicly-accessible getters/setters, Python classes by default allow the caller to "tack on" any instance variables they desire. (I've rarely seen __slots__ used in the wild, for instance.)

So the type of almost any class is really more like Struct{WhatYouThinkTheClassIs; ArbitraryDict;} ... and the temptation to misuse this, say for instance if you don't want to add arguments to function_used_lots_of_places(some_object), can be huge. And more generally, this openness means that there may be discrepancies between what's unit tested for a class, and how that class is used in the wild.