Lists are weird when they are instance members

python newbie mesteve_b at
Sat Jan 3 23:20:59 CET 2004

 okay, I'm trying to figure out why my books: Quick Python, Python in a
Nutshell, Python Cookbook and Learning Python don't say anything about the
weird behavior of a list when you have one as an object instance member.

for instance (my first pun of 2004), if I have,

 global_filegroup_array = []  # array of filegroup objects

 class FileGroup:
       a = 0
       mylist = []      # here it is

       def  put_stuff_in_my_list(self, anyfile):
              self.mylist.append( get just a single string from file)   #
pls excuse the psuedo

       def __init__(self):

def main():                         #     do ten times:  instantiate the
above object, and add to the global array
     for i in xrange(10):
          filegroup = FileGroup()

   # print the list contents
    print global_filegroup_array[0].mylist

------------ end of file

  Output is:    [u'.string1', u'.string2', u'.string3' ]      # only
u'string1' should show up

   No matter which index I use into the global array, I always get ALL of
the strings
   showing up. I should only get  u'string1' showing up,   and u'string2'
   I used  "[1].mylist"

How I resolved it, is by slipping in

             self.mylist = []



 in   __init__(self)

Why should I have to empty out the list when it's a member of a newly
instantiated object?


p.s.    ( I'm actually not doing exactly the thing with the
             files,  but instead iterating through a xml file with dom,
            debugging it showed the behavior )

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