On Tuesday, 5 March 2013 18:05:18 UTC-5, Bi Ge wrote: > > I wasn't trying to rewrite the eval function. My original plan is in > eval() when two symbolic values are not in order, > I will return KroneckerDelta(j, i), is this method more preferred? > > Yes, except you should use cls(j, i). This way, if somebody subclasses KroneckerDelta, your code will correctly return an instance of that subclass.
> If not, I'm trying to swap inputs inside of eval() if inputs are not in > order but having trouble with accessing cls._args > > Well, cls isn't an instance here (see Stefan's mail for what this means), so you can't do that. Cheers, Julien > Bi Ge > > On Tuesday, March 5, 2013 5:50:52 PM UTC-5, Julien Rioux wrote: >> >> What Stefan writes is good advice for designing classes in general but in >> this particular case, KroneckerDelta being a subclass of Function, we don't >> need to overwrite __new__ since the eval method is meant exactly for this. >> From sympy/core/function.py: >> >> def eval(cls, *args): >> """ >> Returns a canonical form of cls applied to arguments args. >> >> The eval() method is called when the class cls is about to be >> instantiated and it should return either some simplified instance >> (possible of some other class), or if the class cls should be >> unmodified, return None. >> >> [...] >> """ >> >> In the eval method of KroneckerDelta we can already see that the first >> few lines is an attempt to order the args: >> >> def eval(cls, i, j): >> # ... >> if (i > j) is True: >> return cls(j, i) >> >> The problem is that comparison with i > j only works for numerical >> values, not for symbolic value. >> >> Cheers, >> Julien >> > -- 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?hl=en. For more options, visit https://groups.google.com/groups/opt_out.
