You're talking about Symbol._eval_derivative? I tried that within my extended Symbol class. It returns 0 or 1 testing self == symbol. I tried making some changes to it, but I don't think I can use it. I think it doesn't even get called unless you do something like: t = timevaryingsymbols('t') Derivative(2+3*t,t) I think only when t is both part of (expr, symbols, ...) within Derivative's __new__ definition does t's _eval_derivative() method get called.
-Gilbert On Jun 3, 6:53 pm, Aaron Meurer <asmeu...@gmail.com> wrote: > Can you just get what you want by overriding _eval_derivative()? > > Aaron Meurer > > > > > > > > On Fri, Jun 3, 2011 at 7:48 PM, Gilbert gede <gilbertg...@gmail.com> wrote: > > I guess I should have asked this as well; is it considered bad to > > write diff() and replace the current Sympy diff() within my code? Or > > is that OK? > > > -Gilbert > > > On Jun 3, 6:38 pm, Gilbert gede <gilbertg...@gmail.com> wrote: > >> Yeah, I had read some of them. I had already thought of writing my > >> own Diff method or something and do substitution with it, but was > >> hoping to have the functionality I want work like standard SymPy > >> operations. That's what I've been trying to do with my PyDy classes; > >> make them work more like you would expect other SymPy objects to. > >> I have read through Derivative() and diff(), and couldn't really find > >> a way to make them do what I want (like I said about my symbol > >> extension no longer having its methods called once it is inside a > >> SymPy add or mul). I guess what I was hoping for was input on whether > >> I could make Derivative do what I want with my extended Symbol, as I > >> couldn't really see how. But if writing my own Diff method is the > >> only option, there's not much I can do then. > > >> Thanks, > >> -Gilbert > > >> On Jun 3, 5:53 pm, "Aaron S. Meurer" <asmeu...@gmail.com> wrote: > > >> > This has actually been discussed quite a bit before (a lot of people > >> > want to use Lagrangians). You can search the mailing list. From what > >> > I've seen, you will either have to write your own custom diff routine or > >> > do clever substitution of functions and derivatives with symbols. I > >> > don't think I've ever seen anyone suggest extending Symbol to hold a > >> > time derivative, which is essentially just a more formal way of doing > >> > the substation method. It might work. > > >> > Aaron Meurer > > >> > On Jun 3, 2011, at 6:05 PM, Gilbert Gede wrote: > > >> > > Hi, > >> > > I was trying to implement some functionality for PyDy for this year's > >> > > GSoC, and was looking for some advice. > >> > > In dynamics problems, you usually have time-varying quantities, like > >> > > generalized coordinates, speeds, and accelerations. Often, you want > >> > > to take the partial derivative of an expression with respect to the > >> > > time derivative of one of these quantities. This come up when using > >> > > Lagrange's Method (or Kane's Method). It's described to some degree > >> > > here: > >> > >http://en.wikipedia.org/wiki/Lagrangian_mechanics > >> > >https://gist.github.com/1005937 > >> > > In Lagrange's Method, you end up taking the partial derivative of the > >> > > energy with respect to the time derivative of a generalized > >> > > coordinate. I'm trying to figure out a way to make this work in > >> > > PyDy/SymPy. Derivative won't take in anything but a Symbol. > >> > > The only idea I have come up with is to extend Symbol and write my own > >> > > .diff() method for it which returns a new symbol representing the time > >> > > differentiation of the original extended Symbol. Once my new object > >> > > is inside a Mul or Add sympy object, then my .diff() method is no > >> > > longer called. > >> > > Can anyone give some insight into how I could get this desired > >> > > behavior, taking the derivative of an expression wrt a > >> > > time-differentiated symbol, to work in a way consistent with existing > >> > > SymPy behavior? Thanks. > > >> > > -Gilbert > > >> > > -- > >> > > You received this message because you are subscribed to the Google > >> > > Groups "sympy" group. > >> > > To post to this group, send email to sympy@googlegroups.com. > >> > > To unsubscribe from this group, send email to > >> > > sympy+unsubscr...@googlegroups.com. > >> > > For more options, visit this group > >> > > athttp://groups.google.com/group/sympy?hl=en. > > > -- > > You received this message because you are subscribed to the Google Groups > > "sympy" group. > > To post to this group, send email to sympy@googlegroups.com. > > To unsubscribe from this group, send email to > > sympy+unsubscr...@googlegroups.com. > > For more options, visit this group > > athttp://groups.google.com/group/sympy?hl=en. -- You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to sympy@googlegroups.com. To unsubscribe from this group, send email to sympy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.