Re: classes and list as parameter, whats wrong?
Thanks to everyone! I think I understand the things behind this behaviour, now. And as the life goes: Today I found the description of my "problem" in the Python Tutorial! I think I should read it more often, more carefully:-) Best, Dirk -- http://mail.python.org/mailman/listinfo/python-list
Re: classes and list as parameter, whats wrong?
"Dirk Zimmermann" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > But still, it is not absolutely clear for me, what is going on. So, at > least just for my understanding: The parameter LL is created just once > for the whole class and not for the object (because I del the object > explicitly, which should destroy the object)? Default parameter values are created (once) for the function, not for the class or instance. Function default parameter expressions are evaluated once and only once when the function is defined. Code that you want evaluated with each call goes in the function body. Terry J. Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: classes and list as parameter, whats wrong?
Dirk Zimmermann wrote: > But still, it is not absolutely clear for me, what is going on. So, at > least just for my understanding: The parameter LL is created just once > for the whole class and not for the object (because I del the object > explicitly, which should destroy the object)? del does nothing but remove one binding early. As far as effect on the underlying object, del v and v = None have the same effect. : >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 The del in main is superfluous. For all but the last iteration, the objC = c.cClass() will dereference the previous objC, and the final trip through the loop ends up by exiting the function which will have a similar effect. An experiment which will show this: import sys q = r = object() print sys.getrefcount(q), del r print sys.getrefcount(q), r = q print sys.getrefcount(q), r = None print sys.getrefcount(q) Note that whenever you call sys.getrefcount, the argument to the function itself will increase the count by 1. This demonstrates that: print sys.getrefcount(object()) --Scott David Daniels [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
Re: classes and list as parameter, whats wrong?
Dirk Zimmermann wrote: > But still, it is not absolutely clear for me, what is going on. So, at > least just for my understanding: The parameter LL is created just once > for the whole class and not for the object Yes. And because a lists are mutable, you can alter that one instance of the list - and teh following instances of your class ill see the changes. This is really a common mistake. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: classes and list as parameter, whats wrong?
Thanks for your help. * James <[EMAIL PROTECTED]> [2005-08-26 11:42]: > No, the default paramter LL is only ever created once, not > reinitialised every time the constructor is called - this is quite a > common gotcha! But still, it is not absolutely clear for me, what is going on. So, at least just for my understanding: The parameter LL is created just once for the whole class and not for the object (because I del the object explicitly, which should destroy the object)? Thanks, Dirk -- http://mail.python.org/mailman/listinfo/python-list
Re: classes and list as parameter, whats wrong?
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 <[EMAIL PROTECTED]> 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 > > > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
classes and list as parameter, whats wrong?
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 -- http://mail.python.org/mailman/listinfo/python-list
