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 <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
> >
> > >> > > --
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