Evaluation map and ideal from polynomial ring K[X] -> K

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How can I define a localized evaluation map $\phi_{a_1, \dots, a_n} : K[x_1, \dots, x_n] \rightarrow K$ by $\phi_{a_1, \dots, a_n}(p(x_1, \dots, x_n)) = p(a_1, \dots, a_n)$?

K = GF(47)
R.<x, y> = K[]
H = R.hom_eval(K(10), K(20))
assert H((x - 10)*(y - 20)) == 0

And also how can I construct the ideal of this map?