[Zope-dev] adding attributes to a python product

[Brian Lloyd]

> I have a Python Product that I'm developing. During the course of
> development, I want to add a new attribute. All new instances get this
> attribute, as it is defined with a default value in the constructor.
> 
> In addition, all instances that get edited via the web get the
> attribute, as the edit-processing method is defined to have a default
> value for this attribute.
> 
> Is there any way of interacting with the ZODB persistence machinery to
> add the default attribute to all instances as they are brought out of
> persistent storage -- so that I can just restart Zope, and have all of
> my instances updated as I use them ?
> 
> I can't find the right method or whatever in the ZODB on-line docs, or
> in the source.

Steve, 

The "best way" to do this is to define the new attribute (with 
its default value) as a class attribute. That way when you ask 
for the attribute on old instances you'll get the default value 
found in the class. If you change the attribute (through the web 
or whatever), the changed attribute will be saved *in the instance*. 
That way you don't even have to bother setting a default in the 
constructor (though it won't really hurt anything to do so).

For example:

# old class
class Spam:
  """A spam object"""

  def cook(self):
    return 'cooked!'

# new class - we want to add a 'color' attribute that defaults 
# to 'pink' and to be able to change the color, but we need old 
# instances to support this too. 
class Spam:
  """A spam object"""

  def cook(self):
    return 'cooked!'

  # class attribute - doing a getattr on 'color' will find
  # this for old instances.
  color='pink'

  def setColor(self, color='green'):
    # if this is called, the color attr will be saved in the 
    # *instance*. The class default will not change.
    self.color=color


Note that there is one caveat to this approach - if the attr 
that you want a default for is a _mutable_ Python object 
(like a dict or list), then you have to be careful not to 
modify the class attribute directly. In those cases you need 
to do a little more work:

class Spam:
  """A spam object"""

  # A dict of spam flavors.
  flavors={}

  def addFlavor(self, name, desc):
    # We have to be careful here. We can't just say
    # 'self.flavors[name]=desc', because if we are an old
    # instance then self.flavors will give us the 'flavors'
    # *class attribute*. We don't want to mutate that, what 
    # we really want to do is create flavors as an instance 
    # attribute if we are getting the default.

    # set dict to either the 'flavors' from the _instance_ or
    # a fresh new dictionary.
    dict=self.__dict__.get('flavors', {})

    # add the entry
    dict[name]=desc

    # this is important - the Zope persistence machinery can't
    # automatically know about changes to mutable builtin Python
    # types like dictionaries, so we have do a setattr on self 
    # to make sure the changes to the dictionary are saved!
    self.flavors=dict



Hope this helps!


Brian Lloyd        brian@digicool.com
Software Engineer  540.371.6909              
Digital Creations  http://www.digicool.com