classes and list as parameter, whats wrong?

[James]

No, the default paramter LL is only ever created once, not
reinitialised every time the constructor is called - this is quite a
common gotcha! You want to do something like:

 class cClass:
     """ Base class to handle playlists, i.e. the files, the name, etc. """
     def __init__(self, LL=None):
         if LL == None:
             LL = []
         self.list=LL
     def addFile(self,L):
         self.list.append(L)

James

On 8/26/05, Dirk Zimmermann <python-list at d1z1.de> wrote:
> Hi!
> 
> I have a problem in a program. And I don't understand what is going on.
> I can code something, that the "error" doesn't occur anymore. But I
> still don't know the reason and this is unsatisfactory: Did I understood
> something wrong or is there a bug? To make it short, I boiled down the
> program to the crucial part.  It looks like this:
> 
> ----------- START ------------
> #!/usr/bin/env python
> 
> class cClass:
>     """ Base class to handle playlists, i.e. the files, the name, etc. """
>     def __init__(self, LL=[]):
>         self.list=LL
>     def addFile(self,L):
>         self.list.append(L)
> 
> def main():
>     l1 = ['a','b','c']
>     lNames=['n1','n2','n3']
>     for name in lNames:
>         objC=cClass()
>         for each in l1:
>             objC.addFile(each)
>         print objC.list
>         del objC
> 
> if __name__ == "__main__":
>     main()
> ----------- END ----------------
> 
> If I start it, I get the following output:
> 
> ['a', 'b', 'c']
> ['a', 'b', 'c', 'a', 'b', 'c']
> ['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c']
> 
> 
> Why does this list grow? I thought, with every new instance the list LL
> is set to []?
> 
> How can bring light in this?
> 
> 
> Best,
>     Dirk
> 
> 
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>