Parent initialization don't working

I'll say first that I'm using xadmin, although it doesn't matter for this problem.

Python 2.7.3 with Django 1.9.5

The problem is in this piece of code (take it as an example):

class ParentAdmin(object):
   def __init__(self):
     # Boring stuff
     self.list_filter = ["name"]
     # Calling super()

class SonAdmin(ParentAdmin):

    # Boring stuff

If I set up list_filter in SonAdmin, it shows the filter and all goes well. But, if I set up it instead in ParentAdmin, it just doesn't show. And its weird because it works when setting list_display the same way. No errors throwed, and no other part of the code is the problem. I think that Django overrides it in some moment, but that wouldn't make sense. Any ideas?

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