[Zope-dev] The Application object

[Jeff K. Hoffman]

On Mon, 17 Jul 2000, Chris McDonough wrote:

> Jeff, what exactly do you need to do with setstate?  can you provide an
> example?

I am simply trying to "upgrade" a persistent instance of a Python class.
For instance:

  class MyClass:

      def __init__(self, id, title=''):
          self.myAttr1 = 'Value 1'

      def getMyAttr1(self):
          return self.myAttr1

Imagine that I use the class, as defined above, for a while, then
decide I need to modify it as follows:

  class MyClass:
 
      def __init__(self, id, title=''):
          self.myAttr1 = 'Value 1'
          self.myAttr2 = 'Value 2'

      def getMyAttr1(self):
          return self.myAttr1

      def getMyAttr2(self):
          return self.myAttr2

I already have a few instances of MyClass in the ZODB, and they only have
myAttr1; myAttr2 is not defined on existing instances, since they will not
be constructed via the new __init__. Therefore, getMyAttr2() will throw an
exception.

The simple solution is to also define __setstate__:

  def __setstate__(self, state):
      Persistent.__setstate__(self, state)
      if not hasattr(self, 'myAttr2'):
          self.myAttr2 = 'Value 2'

Now, when the old instances of my object are de-ghosted from the ZODB,
__setstate__ notices that they don't have a myAttr2, and sets it to the
value normally given by the constructor.

Unfortunately, in my case, 'Value 2' is an instance of a ZClass.
Therefore, I need to get at Control_Panel.

Hope this helps. Now I'm going to try Shane's wacky __of__ idea. :-)

--Jeff

---
Jeff K. Hoffman                               704.849.0731 x108
Chief Technology Officer                      mailto:jeff.hoffman@goingv.com
Going Virtual, L.L.C.                         http://www.goingv.com/