Re: [Tutor] Changing a class into a subclass

[Ismael Garrido]

Alan Gauld wrote:
Absolutely, looks like you answered your own question... :-)
But if you want an OOP approach thre are some things to try.
First you can create a BuildingFactory class that has a
single instance (or indeed no instances because you could
use a static method... or get really fancy and create a
meta-class!). The factory class then takes the XML string
fragment and figures out which subclass of Building to
create and returns the required object. You can avoid
having to recode the factory class each time by using a
dictionary object and each subclass definition adds an entry
to the Factory class dictionary - possibly by calling a
class method of Building in the __init__() code
 

My current aproach is something like that. I subclassed 
Building(Building) :-)  and wrote there the "factory" which returns self 
of the created instance. But that didn't go with all my other classes 
that have, as a design rule, that they manage the loading and saving 
from/to XML. That meant that Building was different to the other 
classes, and instead of doing:
a = Ship()
a.fromXML(xml)
listOfA.append(a)

I had to do:
a = Building()
listOfBuildings.append(a.fromXML(xml))
When I wrote my "factory" I thought about the dictionary approach, but I 
didn't write it because I hoped I could find something like the 
__class__ thing. Which, btw, is pure magic :-)

But there's something that I couldn't understand. In the following code, 
my guess would be that "I'm back from the death" would never get 
printed... but it is... and twice! Why?

>>> class A:
   def pong(self):
       print self.__class__
       print "Ping!"
       self.times -=1
       if self.times >= 0:
           self.__class__ = B
           self.pong()
>>> class B:
   def pong(self):
       print self.__class__
       print "Pong!"
       self.times -=1
       self.__class__ = A
       self.pong()
       print "I'm back from the death"
      
>>> a = A()
>>> a.times = 3
>>> a.pong()
__main__.A
Ping!
__main__.B
Pong!
__main__.A
Ping!
__main__.B
Pong!
__main__.A
Ping!
I'm back from the death  ##Weird!
I'm back from the death
>>> a.__class__
<class __main__.A at 0x00C1AF00>

Thanks,
Ismael
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