Re: [Tutor] Changing a class into a subclass
single instance (or indeed no instances because you could use a static method... or get really fancy and create a meta-class!). or make it a static method of Building Yes that's actually what I meant, but reading it back it sounds like I meant static Factory method Ho hum. or get really simple and use a module-level function... Yeah., but I did say if he wanted to be OOPish. class Shack(Building): def __init__(self,p1,p2...): Building.__init__(self,...) Building.register(self,'Shack', Shack) ...rest of init... I think you want to register Shack before you ever create one. Yeah, you are right. My way you register it every time you create an instance. A single registration call right after the class definition is probably better. or you could probably get tricky and do it in a metaclass... I would do in Smalltalk but meta classes in Python are somewhere that I haven't visited yet! :-) Alan G. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Changing a class into a subclass
On Mar 24, 2005, at 18:07, Ismael Garrido wrote: Hello. I have a program that saves/loads to/from XML. I have a main class Building, and a subclass House(Building). When I save the code I instruct each object to save itself to XML (using ElementTree), so House adds itself to the XML tree. My problem is when I load the XML. Before having subclasses what I did was to send the XML object (again in ElementTree) to the object and let itself load. Now, with subclasses, if I send the House XML to Building I get a Building instance, when I need a House instance. You can't make an object transform into an object of a different class. You need to identify what class the object will be an instance of before loading it. Which, if you're using XML tags like Building type=House or House, shouldn't be very difficult to do. -- Max maxnoel_fr at yahoo dot fr -- ICQ #85274019 Look at you hacker... A pathetic creature of meat and bone, panting and sweating as you run through my corridors... How can you challenge a perfect, immortal machine? ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Changing a class into a subclass
At 10:30 AM 3/24/2005, Max Noel wrote: On Mar 24, 2005, at 18:07, Ismael Garrido wrote: Hello. I have a program that saves/loads to/from XML. I have a main class Building, and a subclass House(Building). When I save the code I instruct each object to save itself to XML (using ElementTree), so House adds itself to the XML tree. My problem is when I load the XML. Before having subclasses what I did was to send the XML object (again in ElementTree) to the object and let itself load. Now, with subclasses, if I send the House XML to Building I get a Building instance, when I need a House instance. You can't make an object transform into an object of a different class Are you referring to a Python object? If so you may assign a class to the object's __class__ attribute. class A:pass class B:pass b = b() print b.__class__ # displays class __main__.B at 0x011BB9C0 b.__class__ = A print b.__class__ # displays class __main__.A at 0x011BBA20 [snip] Bob Gailer mailto:[EMAIL PROTECTED] 510 558 3275 home 720 938 2625 cell ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Changing a class into a subclass
let itself load. Now, with subclasses, if I send the House XML to Building I get a Building instance, when I need a House instance. You can't make an object transform into an object of a different class. You need to identify what class the object will be an instance of before loading it. Which, if you're using XML tags like Building type=House or House, shouldn't be very difficult to do. 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 class Shack(Building): def __init__(self,p1,p2...): Building.__init__(self,...) Building.register(self,'Shack', Shack) ...rest of init... Now the factory code can go name = self.getClassName(xmlString) obj = self.classList[name]() and instantiate a Shack object. But unless you expect to create an awful lot of subclasses, or just want to be purist in approach its probably overkill! Alan G. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Changing a class into a subclass
Are you referring to a Python object? If so you may assign a class to the object's __class__ attribute. class A:pass class B:pass b = b() print b.__class__ # displays class __main__.B at 0x011BB9C0 b.__class__ = A print b.__class__ # displays class __main__.A at 0x011BBA20 Interesting Bob, but presumably that doesn't change the internal state so the OP would need to run an initialise method using the XML? Or no, I suppose if he did this inside the class's init method, as the first step, it would then initialise as usual? I feel a spell at the coming on... The problem is you still need to identify the class itself (not the class name) before you can do this, but I guess you might get that from either the builtins dictionary or the Building module if it has all the Building subclasses in? But it certainly does allow you to 'convert' from one class to another, much like the C++ cast operator. Alan g. Learning something new every day. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Changing a class into a subclass
Alan Gauld wrote:
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!).
or make it a static method of Building or get really simple and use a
module-level function...
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
class Shack(Building):
def __init__(self,p1,p2...):
Building.__init__(self,...)
Building.register(self,'Shack', Shack)
...rest of init...
I think you want to register Shack before you ever create one. You could do
it after Shack is defined:
class Shack(Building):
def __init__(self,p1,p2...):
Building.__init__(self,...)
...rest of init...
Building.register('Shack', Shack)
or you could probably get tricky and do it in a metaclass...but I'm not that
tricky.
Kent
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Re: [Tutor] Changing a class into a subclass
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 ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Changing a class into a subclass
Ismael Garrido wrote: 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? It's printed every time B.pong() is called, just as Pong is. You print each one twice - no mystery! Kent 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 ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
