Inheriting from a namedtuple base class

You can, but you have to override __new__ which is called implicitly before __init__:

class Z(X):
  def __new__(cls, a, b, c, d):
    self = super(Z, cls).__new__(cls, a, b, c)
    self.d = d
    return self

>>> z = Z(1, 2, 3, 4)
>>> z
Z(a=1, b=2, c=3)
>>> z.d
4

But d will be just an independent attribute!

>>> list(z)
[1, 2, 3]