Marcus Goldfish wrote:
> Hi,
>
> I ran across a strange problem using mulitple inheritance that I hope
> someone can explain. Basically, I am implementing a Publisher pattern,
> and when I try to add threading, the order I list Base classes matters!
> Here is the short sample code-- all help explaining this is appreciated!
>
> Thanks,
> Marcus
>
> ---
> import threading
>
> class Publisher(object):
> def __init__(self): self.listeners = {}
> def register(self, id, object):
> self.listeners[id] = self.listeners.get(id, object)
>
>
> # This *FAILS* with AttributeError: 'FancyPublisher'
> # object has no attribute 'listeners'
> class FancyPublisher(threading.Thread, Publisher):
> def __init__(self):
> super(FancyPublisher, self).__init__()
>
> F = FancyPublisher()
> F.register('me', None)
>
>
> # However, this succeeds!?
> class FancyPublisher(Publisher, threading.Thread):
> def __init__(self):
> super(FancyPublisher, self).__init__()
>
> F = FancyPublisher()
> F.register('me', None)
Wow, the third (sort of) threading question in a week!
The problem is that neither Publisher nor threading.Thread calls
super(...).__init__() in *its* __init__() method, so you are only
initializing one of your base classes. I don't think your second
FancyPublisher would work when you actually tried to run it as a thread.
One solution is to just call both base class __init__() methods directly:
class FancyPublisher(Publisher, threading.Thread):
def __init__(self):
FancyPublisher.__init__(self)
threading.Thread.__init__(self)
I think it would also work if Publisher.__init__ called
super(Publisher, self).__init__()
and Publisher is the first base class.
This essay highlights some pitfalls with super() including the one you
found:
http://fuhm.net/super-harmful/
though it provoked a strong response on python-dev:
http://mail.python.org/pipermail/python-dev/2005-January/050656.html
Kent
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