Lists are weird when they are instance members

python newbie mesteve_b at hotmail.com
Sun Jan 4 00:29:57 CET 2004


thanks for the helpful replies.
 I guess I was just in confusion as to why I was able to leave alone the
string variables in class FileGroup, such
as sourceDir and destinDir, and nothing unpredictable happened with those,
but with the list variable, I had to treat differently.
 But after I again read closely, Python In a Nutshell, I'm sure I will
understand.


"python newbie" <mesteve_b at hotmail.com> wrote in message
news:ffHJb.5475$HL3.4277 at newssvr29.news.prodigy.com...
> hey,
>  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,
>
> test.py
> ----------------
>
>  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):
>             put_stuff_in_my_list(anyfile)
>
> def main():                         #     do ten times:  instantiate the
> above object, and add to the global array
>      for i in xrange(10):
>           filegroup = FileGroup()
>           global_filegroup_array.append(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'
> if
>    I used  "[1].mylist"
>
>
> How I resolved it, is by slipping in
>
>              self.mylist = []
>
>    before
>
>       put_stuff_in_my_list(anyfile)
>
>  in   __init__(self)
>
> Why should I have to empty out the list when it's a member of a newly
> instantiated object?
>
> thanks
>
> 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 )
>
>
>
>
>
>
>
>





More information about the Python-list mailing list