making a class return None from __init__

[Chad Netzer]

On Friday 04 October 2002 13:46, Max M wrote:
> Rajarshi Guha wrote:
> > (where v will fail) I always get <__main__.Graph instance at
> > 0xXXXXXX> Is there any way to make the constructor return a None
> > object?
>
> def noner():
>      return

ie.  Create a factory function that returns the initialized class when 
everything goes well, or None upon failure:

class MyClass:
    def __init__( self, arg1, arg2, arg3 ):
        return

def MyClassFactory( arg1, arg2, arg3 ):
    if valid_args( arg1, arg2, arg3 )
        return MyClass( arg1, arg2, arg3 )
    else:
        return None

Note - this is arguably bad style, UNLESS it is clear that you are 
invoking a factory rather than a class constructor.  Typically, you use 
a factory to have flexibility in choosing which class is actually 
instantiated, NOT because you need to test error conditions.

Better to use exceptions in the __init__, and abort the class 
initialization if you cannot use the class.  Returning either a class 
or None, just doesn't feel right, especially if the user is assuming 
only one specific class to be created.

-- 

Chad Netzer
cnetzer at mail.arc.nasa.gov