Generating json file from xml file in python using xmltodict

I am trying to create json file from input xml file using xmltodict with following code

import io, xmltodict, json
infile = io.open(filename_xml, 'r')
outfile = io.open(filename_json, 'w')
o = xmltodict.parse( infile.read() )
json.dump( o , outfile )

      

        
        
        
      

    

the last line will get the following error

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/__init__.py", line 182, in dump
    fp.write(chunk)
TypeError: must be unicode, not str

      

        
        
        
      

    

I think I need to change the encoding. My initial xml file seems to be ascii. Any idea on how to make this work? Thanks to