gistfile1.py

class A(object):
    def __init__(self, i):
        self.i = i
    def __radd__(self, other):
        return self.i

a = [A(42)]

# Test with built-in sum()
print(sum(a))
# prints 42

def sum(s, start=None):
    it = iter(s)
    try:
        n= next(it)
    except:
        return 0
    if start is None:
        start = type(n)()
    return n + __builtins__.sum(it, start)

# Test with proposed new version
print(sum(a))
# results in
# TypeError: __init__() takes exactly 2 arguments (1 given)