Ha, i think i got it:

pdiff := proc( f,x )
        description "Take a partial with respect to any expression";
        thaw( value(subs( x=freeze(x),Diff(f,x) ) ) );
end proc;
 

thanks

Thanks,  is there a way to program this into one command or procedure somehow?  I would like to use this regularly.

What about:

frontend(diff,[cos(theta(t)),theta(t)]);

                                              0

this should yield -sin(theta(t));

i guess frontend also freezes the cos.

Is there a way around this?

I'm doing analytical mechanics and i am constantly taking partials with respect to coordinates which depend on time.  How do I keep time dependence, in the coordinates while still taking these partials?