@rhettinger
Apologies. You're totally right but I wasn't expecting this to become a lengthy conversation.

So, to get closer to the initial issue, I believe that this ticket could be closed provided that the documentation is improved by making developers aware of the potential side effects of isinstance. I was doing some more experiments and it looks like

def _isinstance(obj, classinfo):
    return issubclass(type(obj), classinfo)

might be considered a "side-effects-free" version of isinstance. So perhaps one thing to mention in the documentation is that `isinstance(obj, classinfo) != issubclass(type(obj), classinfo)` in general.