problem with multiprocessing and defaultdict

[wiso]

I'm using a class to read some data from files:

import multiprocessing
from collections import defaultdict

def SingleContainer():
    return list()


class Container(defaultdict):
    """
    this class store odd line in self["odd"] and even line in self["even"].
    It is stupid, but it's only an example.
    """
    def __init__(self,file_name):
        if type(file_name) != str:
            raise AttributeError, "%s is not a string" % file_name
        defaultdict.__init__(self,SingleContainer)
        self.file_name = file_name
        self.readen_lines = 0
    def read(self):
        f = open(self.file_name)
        print "start reading file %s" % self.file_name
        for line in f:
            self.readen_lines += 1
            values = line.split()
            key = {0: "even", 1: "odd"}[self.readen_lines %2]
            self[key].append(values)
        print "readen %d lines from file %s" % (self.readen_lines, 
self.file_name)

"""
Now I want to read more than one file per times
"""

def do(file_name):
    container = Container(file_name)
    container.read()
    return container

if __name__ == "__main__":
    file_names = ["prova_200909.log", "prova_200910.log"]
    pool = multiprocessing.Pool(len(file_names))
    result = pool.map(do,file_names)
    pool.close()
    pool.join()
    print "Finish"
        


but I got:
start reading file prova_200909.log
start reading file prova_200910.log
readen 142 lines from file prova_200909.log
readen 160 lines from file prova_200910.log
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib64/python2.6/threading.py", line 522, in __bootstrap_inner
    self.run()
  File "/usr/lib64/python2.6/threading.py", line 477, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.6/multiprocessing/pool.py", line 259, in 
_handle_results
    task = get()
  File "main2.py", line 11, in __init__
    raise AttributeError, "%s is not a string" % file_name
AttributeError: (AttributeError('<function SingleContainer at 
0x7f08b253d938> is not a string',), <class '__main__.Container'>, (<function 
SingleContainer at 0x7f08b253d938>,))


the problem is when pool share objects, but why is it calling 
Container.__init__ with a Container parameter?