the function factor() works in this fashion:
sage: x, y = PolynomialRing(GF(17), 2, ['x','y']).gens()
sage: f = 9y^6 - 9x^2y^5 - 18x^3y^4 - 9x^5y^4 + 9x^6y^2 + 9x^7y^3\ ... + 18x^8y^2 - 9x^11
sage: f.factor()
(-9) * (x^5 - y^2) * (x^6 - 2x^3y^2 - x^2*y^3 + y^4)
Is there a possibility to factorize a bivariate polynomial in x,y with respect to a single variable only (e.g. y) and get the answer in the form of (y - f(x)) as factors?