Question about None

Javier Collado javier.collado at gmail.com
Fri Jun 12 17:26:15 CEST 2009


Hello,

You're right, types.NoneType is not available in python 3.0, I wasn't
aware of that change. Thanks for pointing it out.

Best regards,
    Javier

2009/6/12 Jeff McNeil <jeff at jmcneil.net>:
> On Jun 12, 10:05 am, Paul LaFollette <paul.lafolle... at gmail.com>
> wrote:
>> 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 self.next 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"
>>     self.next = 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"
>>     self.next = 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
>
> 1. The problem is described clearly by that Exception. The 'NoneType'
> name isn't bound to any objects at that current scope.  I know that
> with 2.6, you can import 'types.NoneType' but I don't believe the 3.0
> types module includes NoneType.  Someone else will have to clarify
> that as I don't have 3.0 installed.
>
> 2. A less ugly alternative? You could use the accepted method of
> testing against None:
>
> if var is None:
>    do_stuff()
>
> The use of the 'is' operator checks whether objects are exactly the
> same (id(var) == id(None)) as opposed to 'isinstance' or '==.'
>
> You might also try defining descriptors in order to make your type
> checks slightly more transparent. That might be confusing to other
> users of your class, though. Not many people expect a TypeError from
> an attribute assignment.
>
> 3. None is an instance of NoneType. Why should isinstance return True
> when it's tested against other types?
>
> HTH,
>
> Jeff
> mcjeff.blogspot.com
> --
> http://mail.python.org/mailman/listinfo/python-list
>



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