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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _description_columns_json(self, cols=None):
 """ Prepares dict with col descriptions to be JSON serializable """

        ret = {}
        cols = cols or []
        d = {k: v for (k, v) in self.description_columns.items() if k in cols}
        for key, value in d.items():
            ret[key] = as_unicode(_(value).encode("UTF-8"))
        return ret