Comment by rburing for Instead of "assigning" a value to $\partial g/\partial x \vert_{x=0}$ beforehand, it's easier to make the substitution afterward:sage: der_f.subs(x==0).subs(diff(g,x).subs(x==0) == 10) 12 Or in steps, mimicking the order you proposed:sage: what_i_know = diff(g,x).subs(x==0) == 10 sage: der_f.subs(x==0).subs(what_i_know) 12

Maybe it's possible by specifying a `derivative_func` in the definition of `g`, returning another [symbolic function](https://doc.sagemath.org/html/en/reference/calculus/sage/symbolic/function_factory.html#sage.symbolic.function_factory.function) with a custom `eval_func`, but even if it worked (I didn't manage) it would be very awkward and convoluted to define. I think it's better to be explicit about such substitutions anyway.