return an opened tempfile pointer that can be used

        http://docs.python.org/2/library/tempfile.html

def _get_file(self):
        '''
        return an opened tempfile pointer that can be used

        http://docs.python.org/2/library/tempfile.html
        '''
        f = tempfile.NamedTemporaryFile(delete=False)
        self.tmp_files.add(f.name)
        return f