How can they be set afterwords?

alex@alex-laptop:~/projects/pypy$ python3.1 
Python 3.1.2 (release31-maint, Sep 17 2010, 20:34:23) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class A(object):
...     pass
... 
>>> A.__dict__[32] = "heh"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'dict_proxy' object does not support item assignment
>>> setattr(A, 32, "heh")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: attribute name must be string, not 'int'