Forget it, I figured it out, the important thing is to handle exception, so replace:

jdata = json.load (open (filename, 'r'))

by:

...
    try:
        jdata = json.load (open (filename, 'r'))
    except ValueError:
        f = gzip.open(filename, 'rb')
        file_content = f.read()
        decoder = json.JSONDecoder()
        jdata = decoder.decode(file_content.decode("utf-8"))
...

Anyway, I was mondering... Is there any way of handling symmetry while morphing in editmode in Blender? I mean, without creating new mesh (add modifier->mirror or mesh option-> x symmetry). It is possible in sculpt mode...