Question about None

Javier Collado javier.collado at
Fri Jun 12 10:22:05 EDT 2009


This should work for you:

In [1]: import types

In [2]: isinstance(None, types.NoneType)
Out[2]: True

Best regards,

2009/6/12 Paul LaFollette <paul.lafollette at>:
> 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.
> Paul
> --

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