Overriding 'to boolean' operator in python?

I’m using a class that is inherited from list as a data structure:

class CItem( list ) :
  pass
oItem = CItem()
oItem.m_something = 10
oItem += [ 1, 2, 3 ]

All is perfect, but if I use my object of my class inside of an ‘if’, python evaluates it to False if underlying the list has no elements. Since my class is not just list, I really want it to evaluate False only if it’s None, and evaluate to True otherwise:

a = None
if a :
  print "this is not called, as expected"
a = CItem()
if a :
  print "and this is not called too, since CItem is empty list. How to fix it?"