<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _transform(self, df):
 """Private transform method of a Transformer. This serves as batch-prediction method for our purposes. """

        output_col = self.getOutputCol()
        label_col = self.getLabelCol()
        new_schema = copy.deepcopy(df.schema)
        new_schema.add(StructField(output_col, StringType(), True))

        rdd = df.rdd.coalesce(1)
        features = np.asarray(
            rdd.map(lambda x: from_vector(x.features)).collect())
        # Note that we collect, since executing this on the rdd would require model serialization once again
        model = model_from_yaml(self.get_keras_model_config())
        model.set_weights(self.weights.value)
        predictions = rdd.ctx.parallelize(
            model.predict_classes(features)).coalesce(1)
        predictions = predictions.map(lambda x: tuple(str(x)))

        results_rdd = rdd.zip(predictions).map(lambda x: x[0] + x[1])
        results_df = df.sql_ctx.createDataFrame(results_rdd, new_schema)
        results_df = results_df.withColumn(
            output_col, results_df[output_col].cast(DoubleType()))
        results_df = results_df.withColumn(
            label_col, results_df[label_col].cast(DoubleType()))

        return results_df