Method _insert_inaxis_grouper

pandas/core/groupby/groupby.py:1245–1283

(
        self, result: Series | DataFrame, qs: npt.NDArray[np.float64] | None = None
    )

    @final
    def _insert_inaxis_grouper(
        self, result: Series | DataFrame, qs: npt.NDArray[np.float64] | None = None
    ) -> DataFrame:
        if isinstance(result, Series):
            result = result.to_frame()

        n_groupings = len(self._grouper.groupings)

        if qs is not None:
            result.insert(
                0, f"level_{n_groupings}", np.tile(qs, len(result) // len(qs))
            )

        # zip in reverse so we can always insert at loc 0
        for level, (name, lev) in enumerate(
            zip(
                reversed(self._grouper.names),
                self._grouper.get_group_levels(),
                strict=True,
            )
        ):
            if name is None:
                # Behave the same as .reset_index() when a level is unnamed
                name = (
                    "index"
                    if n_groupings == 1 and qs is None
                    else f"level_{n_groupings - level - 1}"
                )

            # GH #28549
            # When using .apply(-), name will be in columns already
            if name not in result.columns:
                # if in_axis:
                if qs is None:
                    result.insert(0, name, lev)
                else:
                    result.insert(0, name, Index(np.repeat(lev, len(qs)), copy=False))

        return result

    @final
    def _wrap_aggregated_output(