New submission from Vedran Čačić: I already wrote http://bugs.python.org/msg270548, but can't seem to reopen the issue, so I think the best thing is to report the bug separately.
So, in issue http://bugs.python.org/issue21136, performance enhancement https://hg.python.org/cpython/rev/91d7fadac271/ enabled a shortcut for some operations (__pow__ among them) to avoid reducing the result to lowest terms if it can be concluded it's already reduced. However, the logic has a corner case that was handled incorrectly. If a negative Fraction is raised to a negative integer, the result is a Fraction with a negative denominator, which is not normalized and in fact breaks many other operations (which rightly assume their operands to be normalized). >>> import fractions >>> fractions.Fraction(-1, 2) ** -1 Fraction(2, -1) >>> _ == -2 False Of course, the easy fix would be to change line 52 of fractions.py to _normalize=True - but that would reintroduce the slowness talked about in http://bugs.python.org/issue21136#msg215409. Alternative fix is to branch depending on the sign of the base, which might be messy if we intend to be completely general -- for example, in finite quotient rings, fractions don't have well-defined signs. [BTW I'm not quite sure why the code in fractions.py is so general, with many weird cases trying to support every imaginable construction - maybe someone really wanted such a level of generality. I don't, so I'd be fine with this working only on ordinary int/int Fractions.] ---------- components: Library (Lib) messages: 270616 nosy: Vedran.Čačić priority: normal severity: normal status: open title: negative Fraction ** negative int not normalized type: behavior versions: Python 3.5, Python 3.6 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue27539> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
