On Fri, Apr 25, 2014 at 5:23 AM, Clifford Wolf <[email protected]> wrote:
> On Friday, April 25, 2014 1:16:19 AM UTC+2, Aaron Meurer wrote:
>>
>> If we override _eval_derivative on MatrixElement can we make it do the
>> right thing?
>
>
> No. Because in the case that fails, MatrixElement._eval_derivative is never
> called. That's the whole point of the existing _diff_wrt interface:
> Derivative.__new__ will try to replace the instances of the MatrixElement we
> differentiate wrt in the expression so we can differentiate wrt an ordinary
> symbol.
>
> I've now written a patch and created a pull request that adds a
> _eval_derivative_wrt interface that can be used to address that issue.
>
> https://github.com/sympy/sympy/pull/7437
>
> I preserved the current behavior of the other classes with _diff_wrt=True,
> but I think the is room for improvement there as well. For example should
> the following code really return "x"?
Yes it should. See the docstring of Derivative.
Aaron Meurer
>
> from sympy import *
> x = Symbol('x')
> f = Function('f')
> diff(x*f(x), f(x))
>
> With the new interface Function._eval_derivative_wrt could detect that even
> after substituting f(x) in x*f(x), the resulting term x*xi_1 still contains
> members of f(x).free_symbols and thus make diff() return "Derivative(x*f(x),
> f(x))" instead.
>
> Please comment on the pull request if you disagree with the approach I
> chose.
>
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