On Sat, May 9, 2020 at 4:43 AM Dan Sommers <2qdxy4rzwzuui...@potatochowder.com> wrote: > > On Sat, 9 May 2020 03:01:15 +1000 > Steven D'Aprano <st...@pearwood.info> wrote: > > > On Thu, May 07, 2020 at 11:04:16AM -0400, Dan Sommers wrote: > > > On Thu, 7 May 2020 21:18:16 +1000 > > > Steven D'Aprano <st...@pearwood.info> wrote: > > > > > > > > The strongest equality is the "is" operator > > > > > > > > Please don't encourage the conceptual error of thinking of `is` as > > > > *equality*, not even a kind of equality. It doesn't check for equality, > > > > it checks for *identity* and we know that there is at least one object > > > > in Python where identical objects aren't equal: > > > > > > > > py> from math import nan > > > > py> nan is nan > > > > True > > > > py> nan == nan > > > > False > > > > > > We'd better agree to disagree on this one. > > > > Why? In what way is there any room for disagreement at all? > > I believe that the "is" operator is a test for some kind of equality, > and you apparently don't. > > > * some equal objects are not identical; > > * and some identical objects are not equal. > > > > It is a matter of fact that in Python `is` tests for object identity, > > not equality: > > > > https://docs.python.org/3/reference/expressions.html#is-not > > Section 6.10 is entitled Comparisons, and lists both "is" and "==" as > comparison operators. > > I admit that my use of the word "strongest" (to describe the "is" > operator) and my conceptual ordering of different kinds of equality > fails in the light of NaNs. Curse you, IEEE Floating Point! :-) > > Then again, that same documentation states "User-defined classes that > customize their comparison behavior should follow some consistency > rules, if possible." One of the consistency rules is "Equality > comparison should be reflexive. In other words, identical objects should > compare equal," and that rule is summarized as "x is y implies x == y." > > So I'm not the only one who thinks of "is" as a kind of equality. :-) >
The documentation doesn't say that "is" represents equality, but only that, in general, an object should be equal to itself. Identity is still a completely separate concept to equality. There's a concept of "container equality" that is expressed as "x is y or x == y", but that's still a form of equality check. "x is y" on its own is not an equality check. It's an identity check. Obviously it's a comparison, but so are many other things :) ChrisA _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/UP3ZX5GLQFTF6UFMH64KGUJIO5JVSFFV/ Code of Conduct: http://python.org/psf/codeofconduct/
