Dan Sommers wrote: > On Thu, 09 Jun 2005 15:50:42 +1200, > Greg Ewing <[EMAIL PROTECTED]> wrote: > > >>Rocco Moretti wrote: >> >>>The main problem is that Python is trying to stick at least three >>>different concepts onto the same set of operators: equivalence (are >>>these two objects the same?), ordering (in a sorted list, which comes >>>first?), and mathematical "size". > >>>This gives the wacky world where >>>"[(1,2), (3,4)].sort()" works, whereas "[1+2j, 3+4j].sort()" doesn't. > > > Python inherits that wackiness directly from (often wacky) world of > Mathematics. > > IMO, the true wackiness is that > > [ AssertionError, (vars, unicode), __name__, apply ].sort( ) > > "works," too. Python refusing to sort my list of complex numbers is a > Good Thing.
The "wackyness" I refered to wasn't that a list of complex numbers isn't sortable, but the inconsistent behaviour of list sorting. As you mentioned, an arbitraty collection of objects in a list is sortable, but as soon as you throw a complex number in there, you get an exception. One way to handle that is to refuse to sort anything that doesn't have a "natural" order. But as I understand it, Guido decided that being able to sort arbitrary lists is a feature, not a bug. But you can't sort ones with complex numbers in them, because you also want '1+3j<3+1j' to raise an error. > Four separate classes of __comparison__ methods in a language that > doesn't (and can't and shouldn't) preclude or warn about rules regarding > which methods "conflict" with which other methods? I do not claim to be > an expert, but that doesn't seem very Pythonic to me. What "conflict"? Where are you getting the doesn't/can't/shouldn't prescription from? Which method you use depends on what you want to achieve: (Hypothetical Scheme) Object Identity? - use 'is' Mathematical Ordering? - use '__eq__' & friends Object Equivalence? - use '__equiv__' Arbitrary Ordering? (e.g. for list sorting) - use '__order__' The only caveat is to define sensible defaults for the cases where one fuction is not defined. But that shouldn't be too hard. __eqiv__ -> __eq__ -> is __order__ -> __lt__/__cmp__ > AIUI, __cmp__ exists for backwards compatibility, and __eq__ and friends > are flexible enough to cover any possible comparison scheme. Except if you want the situation where "[1+2j, 3+4j].sort()" works, and '1+3j < 3+1j' fails. I think the issue is you thinking along the lines of Mathematical numbers, where the four different comparisons colapse to one. Object identity? There is only one 'two' - heck, in pure mathematics, there isn't even a 'float two'/'int two' difference. Equivalence *is* mathematical equality, and the "arbitrary ordering" is easily defined as "true" ordering. It's only when you break away from mathematics do you see the divergance in behavior. -- http://mail.python.org/mailman/listinfo/python-list
