Member variable in object spontaneously turns to None

[Norbert]

I am encountering a strange problem where some member variables,
initialized upon instantiation of the object they are contained in,
suddenly appear as having the value 'None' when I try to access them.
Other member variables in the affected object are unchanged.

class Foo:
  def __init__(self):
    self.bar = 0
    self.x = 0
    self.y = 'a'

matrix = []
for i in range(10):
  row = []
  for j in range(10):
    row.append(Foo()) 
  matrix.append(row)

print "%d" % matrix[0][0].bar
  0
print "%d" % matrix[0][0].x
  0
....
print "%d" % matrix[1][1].bar
  0
print "%d" % matrix[1][1].x
  None

I am perplexed. Any help would be appreciated. Thanks.