"""
Ex. 7.12 from "A primer on..."
Implement a class for a sum. The function defining
the terms of the sum should be an argument to 
the constructor. The sum should have a __call__
method which evaluated the sum for a given x.
"""


class Sum:
    def __init__(self, term, M, N):
        self.term = term
        self.M = M
        self.N = N
    
    def __call__(self, x):
        s = 0
        for k in range(self.M, self.N+1):
            s += self.term(k,x)
        return s
    

def term(k, x):
    return (-x)**k

S = Sum(term, M=0, N=3)
x = 0.5
print(S(x))       
print(S.term(k=4, x=x))  # (-0.5)**4

"""
Terminal> python Sum.py 
0.625
0.0625
"""