x.py

class Number(object):
  def __init__(self, value):
    self.value = value

  def __add__(self, other):
    return Average(self.value, 1) + other

  def __radd__(self, other):
    return Average(self.value, 1) + other

class Average(object):
  def __init__(self, value, n):
    self.value = value
    self.n = n

  def __add__(self, other):
    if not isinstance(other, Average):
      return NotImplemented
    new_n = self.n + other.n
    new_value = self.n * self.value + other.n * other.value
    new_value /= new_n
    return Average(new_value, new_n)