Subs object still remains after calling doit method in sympy

Question

I am trying to calculate general composite function derivative using sympy. In my specific case script is the following:

from sympy import *
t=symbols(\'t\')
p=F         


        

Answer 1

Indeed, repeated applications of doit() in this case result in flip-flopping between two forms of the expression: half the time the first addend has Subs, half the time it's the second.

But you can deal with the issue as follows:

for b in a.atoms(Subs):
  a = a.xreplace({b: b.doit()})

This returns Derivative(p(t), t)**2*Derivative(x(p(t)), p(t), p(t)) + Derivative(x(p(t)), p(t))*Derivative(p(t), t, t) as desired.

The trick is that atoms(Subs) is the set of all Subs objects in the expression, and doit is applied only to them, not to Derivative objects where it only messes things up. (Ideally, doit would not mess Derivative objects up in the first place...)