def distance(xxx_todo_changeme, y):
    (x, d) = xxx_todo_changeme
    return y, d + math.hypot(y[0] - x[0], y[1] - x[1])
print(reduce(distance, [(3, 4), (1, 8)], ((1, 2), 0.0))[1])
Python: reduce on tuple of tuples