On 08/07/2010 09:44 AM, Gabriel Genellina wrote: > En Sat, 07 Aug 2010 04:04:06 -0300, Stefan Schwarzer > <sschwar...@sschwarzer.net> escribió: >> On 2010-08-07 00:28, Steven D'Aprano wrote: > >>> Actually, yes, equality is implemented with a short-cut > that checks for >>> identity first. That makes something like: >>> [...] >> >> Oops, I didn't realize that the OP had mentioned the >> identity check as an optimization in case the objects are >> the same. I thought he was confusing the operator with `is`. >> >>> s = "abc"*1000*1000*10 >>> s == s >>> >>> nice and quick, as Python can immediately recognise that a string is >>> always equal to itself without having to walk the entire string >>> comparing >>> each character with itself. >> >> Yes, that definitely makes sense. I guess I would have >> implemented it this way as well. :) > > For strings and other internal types this optimization certainly makes > sense. For user-defined types it gets in the way and prevents defining > an object such x==x is False (like NANs). >
That's probably why this optimisation doesn't exist for user-defined types: Python 3.1.2 (release31-maint, Jul 8 2010, 09:18:08) [GCC 4.4.4] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> >>> class A: ... def __eq__(self, x): ... if self is x: return False ... else: return True ... >>> >>> A() == A() True >>> a = A() >>> a == a False >>> -- http://mail.python.org/mailman/listinfo/python-list
