subclass of integers

[Bruno Desthuilliers]

Mark Morss a écrit :
> I would like to construct a class that includes both the integers and
> None.   I desire that if x and y are elements of this class, and both
> are integers, then arithmetic operations between them, such as x+y,
> return the same result as integer addition.  However if either x or y
> is None, these operations return None.
> 
> It's simple enough to construct a subclass of integers that behave in
> this way:
> 
> class Nint(int):
>     def __add__(self,other):
>         if (other != None):
>             return self+other
>         else:
>             return None
>     def __radd__(self,other):
>         if (other != None):
>             return other+self
>         else:
>             return None
>     #...and so forth
> 
> However I have not been able to figure out how to make it so that
> None, as well as an integer, could be an element of my class.  My
> preliminary impression is that I have to override int.__new__; but I
> am uncertain how to do that and have been unable to find anything on
> the web explaining that.  Indeed I haven't been able to find much
> about __new__ at all.  Overriding this method of built-in classes
> seems to be quite unusual.
> 
> I would very much appreciate anyone's help.

You'll find documentation in the FineManual(tm):
http://docs.python.org/ref/customization.html
http://www.python.org/download/releases/2.2.3/descrintro/#__new__

and a possible (even if imperfect IMHO) example in my answer to 
Zentrader in the thread.

Now would you be kind enough to satisfy my curiousity and explain your 
use case ?-)