box% sage
┌────────────────────────────────────────────────────────────────────┐
│ SageMath version 9.1, Release Date: 2020-05-20                     │
│ Using Python 3.8.3. Type "help()" for help.                        │
└────────────────────────────────────────────────────────────────────┘
sage: R.<x,y> = PolynomialRing(QQ,order='lex')
sage: f = x^2-4*x+4; I = ideal(x^2-y,f); I.groebner_basis()
[x - 1/4*y - 1, y^2 - 8*y + 16]
sage: g = _[-1]
sage: g.subs(y=4)
0