[Twisted-Python] "Old style" classes in Twisted and inheritance
I hope this is the correct place for this question. I was trying to create a little customized task that sends a (ping) message to the clients every X seconds. Because I needed some extra bookkeeping, I implemented it as a class instance with a __call__ method, that is passed to the LoopingCall. However, I also wanted to automatically start the task on init, so I thought - it makes a lot of sense to simply Inherit it, call its constructor and pass self as the parameter. It was then I realized that the classes in twisted are old-style, when my super() call didn't work. Is there any practicular reason for that? Am I wrong to try inherit the Looping Call ? Basically my code looks like : class SillyPinger(object): def __init__(self, factory): self.factory = factory self.pingTargets = {} pingTask = task.LoopingCall(self) pingTask.start(5.0) def __call__(self): # do stuff pass while I'd like to be doing: class SillyPinger(task.LoopingCall): def __init__(self, factory): super(SillyPinger, self).__init__(self) self.factory = factory self.pingTargets = {} self.start(5.0) # get value from .ini here Should I just switch to calling the LoopingCall constructor in the old way? I'm not sure I'd like to do that, even if only for consistency reasons.
On Tue, 13 Nov 2007 15:07:21 +0100, Atilla <theatilla@gmail.com> wrote:
I hope this is the correct place for this question.
I was trying to create a little customized task that sends a (ping) message to the clients every X seconds. Because I needed some extra bookkeeping, I implemented it as a class instance with a __call__ method, that is passed to the LoopingCall.
__call__ doesn't have much benefit over a method which a useful name, for what it's worth. This isn't really related to your question or problem, though.
However, I also wanted to automatically start the task on init, so I thought - it makes a lot of sense to simply Inherit it, call its constructor and pass self as the parameter. It was then I realized that the classes in twisted are old-style, when my super() call didn't work.
Inheritance makes sense for sharing code. It doesn't make sense for the case you're describing. You could just as easily have your class create a LoopingCall in its __init__ and start it, rather than making your class _be_ a LoopingCall. More generally, some classes in Twisted are classic because they predate Python's new-style classes. We can't simply change them all to be new- style now since there are various incompatibilities between classic and new-style classes. Speaking of super() specifically, it isn't even correct to use super() even if you have new-style classes. super() is for cooperative multiple inheritance: all of the classes in the inheritance hierarchy have to agree to use it. Even if LoopingCall were new-style, it would also have to claim that using super() with its __init__ method was supported (actually, required). And even then there would be no benefit until you also had multiple inheritance.
Is there any practicular reason for that? Am I wrong to try inherit the Looping Call ? Basically my code looks like :
class SillyPinger(object): def __init__(self, factory): self.factory = factory self.pingTargets = {}
pingTask = task.LoopingCall(self) pingTask.start(5.0)
def __call__(self): # do stuff pass
The above is good. Do that.
while I'd like to be doing:
class SillyPinger(task.LoopingCall): def __init__(self, factory): super(SillyPinger, self).__init__(self)
self.factory = factory self.pingTargets = {}
self.start(5.0) # get value from .ini here
This will always be more fragile, even if you ignore the fact that super() doesn't work here. Jean-Paul
I ran into this too, but have had no trouble 'converting' to new style classes by just adding "object" to the superclass list. Anyone have reasons why we can't do that? -Andy Fundinger On Nov 13, 2007 9:07 AM, Atilla <theatilla@gmail.com> wrote:
I hope this is the correct place for this question.
I was trying to create a little customized task that sends a (ping) message to the clients every X seconds. Because I needed some extra bookkeeping, I implemented it as a class instance with a __call__ method, that is passed to the LoopingCall.
However, I also wanted to automatically start the task on init, so I thought - it makes a lot of sense to simply Inherit it, call its constructor and pass self as the parameter. It was then I realized that the classes in twisted are old-style, when my super() call didn't work.
Is there any practicular reason for that? Am I wrong to try inherit the Looping Call ? Basically my code looks like :
class SillyPinger(object): def __init__(self, factory): self.factory = factory self.pingTargets = {}
pingTask = task.LoopingCall(self) pingTask.start(5.0)
def __call__(self): # do stuff pass
while I'd like to be doing:
class SillyPinger(task.LoopingCall): def __init__(self, factory): super(SillyPinger, self).__init__(self)
self.factory = factory self.pingTargets = {}
self.start(5.0) # get value from .ini here
Should I just switch to calling the LoopingCall constructor in the old way? I'm not sure I'd like to do that, even if only for consistency reasons.
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On 13/11/2007, Andy Fundinger <Andy@newworldelectric.com> wrote:
I ran into this too, but have had no trouble 'converting' to new style classes by just adding "object" to the superclass list. Anyone have reasons why we can't do that?
-Andy Fundinger
I actually had problems with that, python complained when I tried to inherit "object" and LoopingCall together and then use super(), which doesn't make sense for Old style objects. Thanks for the responses. I was getting the feeling that I'm just trying to abuse language features with no real benefit. I'll stick to the composition I have above, or even clean it up a little bit to be more clear.
On Nov 13, 2007 9:07 AM, Atilla <theatilla@gmail.com> wrote:
Is there any practicular reason for that? Am I wrong to try inherit the Looping Call ?
Yes. You're not really getting any benefit from inheriting from it. I suggest you use composition like in your first example. -- Christopher Armstrong International Man of Twistery http://radix.twistedmatrix.com/ http://twistedmatrix.com/ http://canonical.com/
participants (4)
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Andy Fundinger -
Atilla -
Christopher Armstrong -
Jean-Paul Calderone