Get variable of polynomial ring

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I have a univariate polynomial ring and a polynomial in this ring, e.g.

R.<x> = PolynomialRing(ZZ);
p = 3*x^2 + 7*x - 5;

My question is: If I call a function with such a polynomial how can I figure out what the original variable was? For instance, say I have the following:

def f(q) :
   return q*x;
print f(p)

Of course that is a rather stupid example but I hope the idea is clear. How can I swap x with something more general like f.parent().variable() or is there a better way to do such things?

Thank you very much for your help!