Substitute a variable in a Sympy matrix without having the symbol

Question

Some function in my code returns a sympy.Matrix, but not the symbols that were used to create the matrix. I now want to substitute some of its value using its subs() method, but I do not have the symbols. Is there a way other than creating symbols with the same label again and using those?

Some piece of code to recreate the behavior would be the following:

import sympy as sp
x = sp.Symbol('x', real=True)
m = sp.Matrix([[0, x], [0, 0]])
x = 'something_else'
print(m.subs(x, 10))

Answer 1

If you are allowed to modify the code of your function, you could returned the symbols in the order of your liking:

def func():
    x, y, z = symbols("x, y, z")
    return x, y, z, Matrix([[x, y], [z, 0]])

Otherwise you could extract the symbols from the free_symbol attribute of a Matrix:

from sympy import *
x, y, z = symbols("x, y, z")
M = Matrix([[x, y], [z, 0]])
list(M.free_symbols)
# out: [x, z, y]