[Zope-dev] Product upgrades

[Tim Hicks]

<snip>

> Basically, if you want to have a new attribute, say, myNewAttribute,
> instead of doing::
> 
>   class SomeClass(...):
> 
>       def __init__(self,...):
>           ...
>           self.myNewAttribute = something...
> 
> you do::
> 
>   class SomeClass(...):
> 
>       _myNewAttribute = None
> 
>       def myNewAttribute(self):
> 
>           if self._myNewAttribute is None:
>               self._myNewAttribute = something
> 
>           return self._myNewAttribute
> 
> 
> Of course this means that you can only access it as
> 'self.myNewAttribute()' instead of 'self.myNewAttribute', because you
> never know when it will already be initialized. Actually, you can know
> when it is not initialized, because it will be None, but resist the
> temptation and let myNewAttribute() do it for you. That's what
> encapsulation is for.
> 
> You don't have to worry about updating old versions because, if the old
> instance doesn't have _myNewAttribute then it will be taken from the
> class. And the myNewAttribute() method will always be taken from the
> class anyway (even for old versions of your object).

Leo,

thanks very much.  That was just what I was after.

much appreciated

tim