Python operations fallback to the reverse operation when they are not defined, so e.g., x + y does x.__add__(y) and if that returns NotImplemented, it does y.__radd__(x). For the comparison operators, it does the flipped operator, so this would be (0).__le__(x + 3), which returns NotImplemented, so it then calls (x + 3).__ge__(0), which works. The only downside here is that you end up getting inequality in the reversed order from what you wanted.
Aaron Meurer On Sun, Mar 22, 2015 at 4:31 AM, Joachim Durchholz <j...@durchholz.org> wrote: > Am 21.03.2015 um 20:05 schrieb Aaron Meurer: >> >> https://github.com/sympy/sympy/pull/9181 > > > I see, S(0) is not necessary after all. > > I stand corrected, but how does this work? > > It is doing > >> 0 <= x + 3 > > > which evaluates to > >> 0 <= Add(Symbol: x, Integer: 3) > > and then calls the __le__ of Add. > However, in the Python docs, it says > >> a <= b is equivalent to a.__le__(b) > > so I'd have expected it to go into > > 0.__le__(Add(...)) > > and try a numeric comparison. > > Does SymPy install its own __le__ on int? (Technically possible but I'd be > shocked.) > Is Python's int.__le__ checking the other operator's type and calling into > it? (If yes, is there a reference to that behaviour, or is it a CPython > implementation detail?) > > I'm confused... > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sympy+unsubscr...@googlegroups.com. > To post to this group, send email to sympy@googlegroups.com. > Visit this group at http://groups.google.com/group/sympy. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sympy/550E8BE8.5050104%40durchholz.org. > > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sympy+unsubscr...@googlegroups.com. To post to this group, send email to sympy@googlegroups.com. Visit this group at http://groups.google.com/group/sympy. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/CAKgW%3D6LNZdoB%3DCqeDz%2BQ0V9%2BG9ztBmS7pQ-P5SY%2Bi_qz1Jwh1A%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
