# How do I exec a function of N variables.

The python code: f(x,y)=sin(x) + cos(y) Can be both differentiated and evaluated.

like this:
dfx=diff(f,x);
dfy=diff(f,y)
then evaluated like this:
f(pi/6,pi/9) -> cos(1/9*pi) + 1/2
dfx(pi/6,pi/9) -> 1/2*sqrt(3)

I'm wanting to do something like: exec("f(t1,t2)=cos(t1)+sin(t2)") so that I can write an @interact function to define the equations of N variables interactively. When I try the above exec I receive the error: "SyntaxError: can't assign to function call"

I can almost do it, but I'm getting into a horrible inelegant tangle of of eval statements in the process!

Is there a clean way to do it with sage?