> But what about the first argument? Couldn't you make the case that the pythonic way of handling it is to only care about if the object responds to __getitem__(0)?

Mmm no? Here, what he cares about is that it's a single-element collection, he doesn't just want the collection's first item. If he did, foo[0] would do a better job.

So the object responding to __getitem__(0) is not a sufficient condition.

In fact, it's entirely wrong as sets do _not_ implement __getitem__. Worse, __getitem__(0) does a very different thing than unpacking on a dict. Unpacking is more ducky than __getitem__ for this case, because it expresses the following: the right-hand is a single-element iterable. Any iterable will work as long as it only has a single item, it doesn't have to be a sequence, a mapping, or even a collection (generators or callable_iterators will work just as well)