Entering partial derivatives?

[parisse]

This is not possible, because sometimes you want to eval the argument first (like here) and sometimes you want to eval it after the substitution/assumption is done (for example solve(x^2=4)|x>1). That's precisely why you have 2 instructions for subtitution.
If you want to use |, you must compute the derivative first, for example
fx:=diff(f(x,y),x);
fx|[x=1,y=2]