ctypes variable length structures

Ever since I read Dave Beazley's post on binary I/O handling (http://dabeaz.blogspot.com/2009/08/python-binary-io-handling.html) I've wanted to create a Python library for a certain wire protocol. However, I can't find the best solution for variable length structures. Here's what I want to do:

import ctypes as c

class Point(c.Structure):
    _fields_ = [
        ('x',c.c_double),
        ('y',c.c_double),
        ('z',c.c_double)
        ]

class Points(c.Structure):
    _fields_ = [
        ('num_points', c.c_uint32),
        ('points', Point*num_points) # num_points not yet defined!
        ]

The class Points won't work since num_points isn't defined yet. I could redefine the _fields_ variable later once num_points is known, but since it's a class variable it would effect all of the other Points instances.

What is a pythonic solution to this problem?