> I'm finding it rather hard to guess what the rules are for what works and what doesn't

So now that I've looked at the source: anything with an `__int__` method works, except that `float` instances are explicitly excluded. So this explains for example:

>>> sorted([1, 2, 3], reverse=numpy.float64(0.5))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: integer argument expected, got float
>>> sorted([1, 2, 3], reverse=numpy.float32(0.5))
[1, 2, 3]