用 ReferenceFrame 向量替换 SymPy sp.Symbol

Question

I'm having trouble subbing Symbols for Vectors with the subs function in SymPy. Below is an example of what I'd like to do:

N = ReferenceFrame('N')
zhat, Q, p = sp.symbols('\\hat{\\mathbf{z}} Q p')
expr = 4*Q*zhat/p
old = zhat
new = N.z
expr.subs(old, new)

I believe it necessary, given the context of this problem, to state that I require the initial expression expr to be comprised entirely of Symbols and to have type Mul before the substitution.

Answer 1

I haven't found a solution for the exact above problem, but a very similar one:

N = CoordSys3D('N')
zhat, Q, p = sp.symbols('\\hat{\\mathbf{z}} Q p')
expr = 4*Q*zhat/p
old = zhat
new = N.k
expr.subs(old, new)

Using CoordSys3D instead of ReferenceFrame allows for substitution.