I see. Yeah, that's because of the optimization built in to Derivative.__new__ that does diff(expr, x) => 0 if x is not in expr.free_symbols. This should probably be factored out to just be a default _eval_derivative action in Expr, so that subclasses can override it. On the other hand, if your object is going to be something akin to x(t) or x'(t), then t should be in the free_symbols.
Aaron Meurer On Fri, Jun 3, 2011 at 8:53 PM, Gilbert gede <[email protected]> wrote: > Yeah, if you try diff(x,y), the method is not called. > Ronan, I'm not sure how to do what you're describing? How would I > call that? > > -Gilbert > > > On Jun 3, 7:32 pm, Ronan Lamy <[email protected]> wrote: >> Le vendredi 03 juin 2011 à 19:16 -0700, Gilbert gede a écrit : >> >> > 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. >> >> That seems to be a recent "optimisation" from commit 2361dd86. You >> should revert this to the old behaviour: call >> expr._eval_derivative(symbol) in all cases. >> >> >> >> >> >> >> >> > On Jun 3, 6:53 pm, Aaron Meurer <[email protected]> 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 <[email protected]> >> > > 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 <[email protected]> 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" <[email protected]> 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 [email protected]. >> > > >> > > To unsubscribe from this group, send email to >> > > >> > > [email protected]. >> > > >> > > 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 [email protected]. >> > > > To unsubscribe from this group, send email to >> > > > [email protected]. >> > > > 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 [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/sympy?hl=en. > > -- You received this message because you are subscribed to the Google Groups "sympy" group. 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