How to use the pydruid.utils.postaggregator.Field function in pydruid

self.assertEqual(postagg.name, "postagg_name")
        self.assertEqual(postagg.post_aggregator["probability"], "0.5")
        # Quantiles
        conf = {
            "type": "quantiles",
            "name": "postagg_name",
            "probabilities": "0.4,0.5,0.6",
        }
        postagg = get_post_agg(conf)
        self.assertTrue(isinstance(postagg, postaggs.Quantiles))
        self.assertEqual(postagg.name, "postagg_name")
        self.assertEqual(postagg.post_aggregator["probabilities"], "0.4,0.5,0.6")
        # FieldAccess
        conf = {"type": "fieldAccess", "name": "field_name"}
        postagg = get_post_agg(conf)
        self.assertTrue(isinstance(postagg, postaggs.Field))
        self.assertEqual(postagg.name, "field_name")
        # constant
        conf = {"type": "constant", "value": 1234, "name": "postagg_name"}
        postagg = get_post_agg(conf)
        self.assertTrue(isinstance(postagg, postaggs.Const))
        self.assertEqual(postagg.name, "postagg_name")
        self.assertEqual(postagg.post_aggregator["value"], 1234)
        # hyperUniqueCardinality
        conf = {"type": "hyperUniqueCardinality", "name": "unique_name"}
        postagg = get_post_agg(conf)
        self.assertTrue(isinstance(postagg, postaggs.HyperUniqueCardinality))
        self.assertEqual(postagg.name, "unique_name")
        # arithmetic
        conf = {
            "type": "arithmetic",
            "fn": "+",

"""
        For a metric specified as `postagg` returns the
        kind of post aggregation for pydruid.
        """
        if mconf.get("type") == "javascript":
            return JavascriptPostAggregator(
                name=mconf.get("name", ""),
                field_names=mconf.get("fieldNames", []),
                function=mconf.get("function", ""),
            )
        elif mconf.get("type") == "quantile":
            return Quantile(mconf.get("name", ""), mconf.get("probability", ""))
        elif mconf.get("type") == "quantiles":
            return Quantiles(mconf.get("name", ""), mconf.get("probabilities", ""))
        elif mconf.get("type") == "fieldAccess":
            return Field(mconf.get("name"))
        elif mconf.get("type") == "constant":
            return Const(mconf.get("value"), output_name=mconf.get("name", ""))
        elif mconf.get("type") == "hyperUniqueCardinality":
            return HyperUniqueCardinality(mconf.get("name"))
        elif mconf.get("type") == "arithmetic":
            return Postaggregator(
                mconf.get("fn", "/"), mconf.get("fields", []), mconf.get("name", "")
            )
        else:
            return CustomPostAggregator(mconf.get("name", ""), mconf)

post_aggs[metric_name] = JavascriptPostAggregator(
                        name=conf.get('name', ''),
                        field_names=conf.get('fieldNames', []),
                        function=conf.get('function', ''))
                elif conf.get('type') == 'quantile':
                    post_aggs[metric_name] = Quantile(
                        conf.get('name', ''),
                        conf.get('probability', ''),
                    )
                elif conf.get('type') == 'quantiles':
                    post_aggs[metric_name] = Quantiles(
                        conf.get('name', ''),
                        conf.get('probabilities', ''),
                    )
                elif conf.get('type') == 'fieldAccess':
                    post_aggs[metric_name] = Field(conf.get('name'), '')
                elif conf.get('type') == 'constant':
                    post_aggs[metric_name] = Const(
                        conf.get('value'),
                        output_name=conf.get('name', '')
                    )
                elif conf.get('type') == 'hyperUniqueCardinality':
                    post_aggs[metric_name] = HyperUniqueCardinality(
                        conf.get('name'), ''
                    )
                else:
                    post_aggs[metric_name] = Postaggregator(
                        conf.get('fn', "/"),
                        conf.get('fields', []),
                        conf.get('name', ''))

        aggregations = OrderedDict()