For my use case, it was easy enough to wrap the `uid = d[obj]` in a try... catch... plus locking.

Using setdefault would cause the counter to be incremented every time.  In truth, here I did not care about having consecutive uids, so that would have worked just as well.

In real truth, it later turned out that I didn't really need numeric uids anyways; I could just use weakrefs to the object themselves instead.