preventing creation of instance

[Peter Hansen]

Mirko Koenig wrote:
> 
> I have a class with a 'static' list of names to control if every instance
> of the class has a different name.
> 
> class test:
>         names = []
>         def __init__( self, name ):
>                 ...
> 
> Is it possible to chech in init if the given name exists in names and
> then destroy itself. So that it will be not possible to create an
> instance of this class, if the given name is existing.
> 
> adam = test( "adam" ) # should work
> eve = test ( "adam" ) #should then not work. eve should be None.

I believe raising any exception in __init__ is the way to prevent
the creation of the object.  I don't think you can do this with
a technique that returns None, unless you are actually calling
a factory function which internally attempts to create the object
and catches the exception thrown, returning None if it happens.

-Peter