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The problem comes from the fact that funkce is a symbolic expression and so it is f. Since f is not a symbolic callable function, you can not use the notation f(x). You have to replace x by the corresponding value, writing either f.subs(x=value) or f(x=value). So a simple solution is to replace

ys = [f(x) for x in xs]

and

yz = [f(t) for t in xz]

by

ys = [f(x=x) for x in xs]

and

yz = [f(x=t) for t in xz]

Hope this helps.