Question about None

Paul LaFollette paul.lafollette at
Fri Jun 12 10:05:29 EDT 2009

Kind people,

Using Python 3.0 on a Gatesware machine (XP).
I am building a class in which I want to constrain the types that can
be stored in various instance variables.  For instance, I want to be
certain that self.loc contains an int.  This is straightforward (as
long as I maintain the discipline of changing loc through a method
rather than just twiddling it directly.

  def setLoc(lo):
    assert isinstance(lo, int), "loc must be an int"
    self.loc = lo

does the trick nicely.

However, I also want to constrain to be either an instance
of class Node, or None.  I would think that the following should work
but it doesn't.

  def setNext(nxt):
    assert isinstance(nxt, (Node, NoneType)), "next must be a Node" = nxt

since type(Node) responds with <class, 'NoneType'> but the assertion
above gives "name 'NoneType' is not defined" suggesting that NoneType
is some sort of quasi-class.

  def setNext(nxt):
    assert nxt==None or isinstance(nxt, Node), "next must be a Node" = nxt

works ok, but it's uglier than it ought to be.

So, I have three questions.

1) Why doesn't isinstance(nxt, (Node, NoneType)) work?
2) is their a less ugly alternative that what I am using?
3) (this is purely philosophical but I am curious)  Would it not be
more intuitive if
isinstance(None, <anything at all>) returned true?

Thank you for your kind attention.

More information about the Python-list mailing list