'Constructor
@param content_types_file: a file like object of [Content_Types].xml'

def __init__(self, content_types_file):

    self.overrides = {}
    context = etree.iterparse(content_types_file, tag=('{%s}Override' % ns.CONTENT_TYPES))
    for (dummy, override) in context:
        key = override.get('ContentType')
        if self.overrides.has_key(key):
            self.overrides[key].append(override.get('PartName'))
        else:
            self.overrides[key] = [override.get('PartName')]
    return
