On Mon, Aug 10, 2020 at 9:05 AM Alex Hall <alex.mojaki@gmail.com> wrote:
Yes, we can implement things in Python that aren't allowed in formal mathematics and some fun questions arise. What should `set.UNIVERSAL in set.UNIVERSAL` return? Bertrand Russell thinks it's False.

Well... Russell thinks that the answer can neither be True nor False. :-)  That's why it's a paradox.

But without giving us more (and probably even if the OP does), this seems like a fun formal exercise that has no place in standard Python.

Anyone is perfectly free to define their own custom class (pun intended) `V` ... or maybe `L` for the intuitionists.  Just give it an `.__contains__()` method that behaves as you like, and you are done.  Maybe there are some more details, but it feels very custom, and not of wide usefulness.

The stuff about removing from V seems silly to me.  It's always going to be equivalent to ask `if x not in Vminus` as just asking `if x in excluded`.  And the latter we can do right now with regular sets.


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