Here's the explanation. I had thought that check=False just stops the check that the equations are satisfied (I posted an examples where there were no equations which perhaps hid my point; I found this while constructing points on a curve in P^2). But in fact it checks nothing at all.
All I would want to do is change the line self._coords = v to self._coords = list(v) at the appropriate place. If that failed, then it's the user's false. In this case it did not fail and almost everything worked, except that I got inconsistencies in sorting points since the sort function tearted a point with _coords as list differently from points with _coords as tuple! John On 5 February 2010 16:05, William Stein <wst...@gmail.com> wrote: > On Fri, Feb 5, 2010 at 6:44 AM, John Cremona <john.crem...@gmail.com> wrote: >> First example, nothing unexpected. Note that P is created with a >> tuple and Q with a list. >> >> sage: P2 = ProjectiveSpace(GF(2),2) >> sage: P = P2.point((0,0,1)) >> sage: Q = P2.point([0,0,1]) >> sage: P >> (0 : 0 : 1) >> sage: Q >> (0 : 0 : 1) >> sage: P==Q >> True >> sage: P._coords >> [0, 0, 1] >> sage: Q._coords >> [0, 0, 1] >> >> >> Now the same but with "check=False": >> >> sage: P = P2.point((0,0,1),check=False) >> sage: Q = P2.point([0,0,1],check=False) >> sage: P >> (0 : 0 : 1) >> sage: Q >> (0 : 0 : 1) >> sage: P==Q >> False >> sage: P._coords >> (0, 0, 1) >> sage: Q._coords >> [0, 0, 1] >> >> The point is that on creation of the point, valid tuple input is >> converted to a list, unless check=False in which case tuples are left >> as tuples. This can result in wrong results. >> >> I know that users should only use "check=False" if they know what they >> are doing! This caught me out while writing a patch for #8193 (which >> I used to show a new student the ropes, but testing it led us to this >> weirdness!). I suggest that the point-creation function takes the >> trouble to always store _coords in a consistent way, presumably as a >> list as at present (thought surely a tuple would be better?) > > I don't understand. The entire point of check=False is to not check, > and you're suggesting that check=False does check. I'm confused > despite your length explanation. If you call a function with > check=False and give invalid input, you shouldn't expect anything to > work after that. Maybe you want another option that isn't > "check=False"? > >> >> John >> >> -- >> To post to this group, send an email to sage-devel@googlegroups.com >> To unsubscribe from this group, send an email to >> sage-devel+unsubscr...@googlegroups.com >> For more options, visit this group at >> http://groups.google.com/group/sage-devel >> URL: http://www.sagemath.org >> > > > > -- > William Stein > Associate Professor of Mathematics > University of Washington > http://wstein.org > > -- > To post to this group, send an email to sage-devel@googlegroups.com > To unsubscribe from this group, send an email to > sage-devel+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/sage-devel > URL: http://www.sagemath.org > -- To post to this group, send an email to sage-devel@googlegroups.com To unsubscribe from this group, send an email to sage-devel+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-devel URL: http://www.sagemath.org
