How to Assign value to a symbolic function?

edit

I am not able to use subs. But yes I would like something as:

P=function('P')
x,y=var('x y')

def S(x):
    return sum(P(x,y),y,0,1)

P(0,1)=100
P(0,0)=100

S(0)

Why can't you use subs? This works:

sage: S(0).subs({P(0,1):100,P(0,0):100})
200

I want to be able to assign a specific value to P(2,0) and to P(2,1) For instance I would like to be able to set the values: P(2,0)=100 and P(2,1)=100