On May 28, 4:41 pm, MRAB <pyt...@mrabarnett.plus.com> wrote:
> Here's a curiosity. float("nan") can occur multiple times in a set or as
> a key in a dict:Which is by design. NaNs intentionally have multiple possible instances (some implementations even include distinct payload values). Sets and dicts intentionally recognize an instance as being equal to itself (identity-implies-equality); otherwise, you could put a NaN in a set/dict but not be able to retrieve it. Basic invariants would fail -- such as: assert all(elem in container for elem in container). The interesting thing is that people experimenting with exotic objects (things with random hash functions, things with unusual equality or ordering relations, etc) are "surprised" when those objects display their exotic behaviors. To me, the "NaN curiousities" are among the least interesting. It's more fun to break sort algorithms with sets (which override the ordering relations with subset/superset relations) or with an object that mutates a list during the sort. Now, that is curious :-) Also, Dr Mertz wrote a Charming Python article full of these curiosities: http://gnosis.cx/publish/programming/charming_python_b25.txt IMO, equality and ordering are somewhat fundamental concepts. If a class is written that twists those concepts around a bit, then it should be no surprise if curious behavior emerges. Heck, I would venture to guess that something as simple as assuming the speed of light is constant might yield twin paradoxes and other curiousities ;-) Raymond -- http://mail.python.org/mailman/listinfo/python-list
