Method _pad_or_backfill

pandas/core/generic.py:6884–6919

(
        self,
        method: Literal["ffill", "bfill", "pad", "backfill"],
        *,
        axis: None | Axis = None,
        inplace: bool = False,
        limit: None | int = None,
        limit_area: Literal["inside", "outside"] | None = None,
    )

    @final
    def _pad_or_backfill(
        self,
        method: Literal["ffill", "bfill", "pad", "backfill"],
        *,
        axis: None | Axis = None,
        inplace: bool = False,
        limit: None | int = None,
        limit_area: Literal["inside", "outside"] | None = None,
    ):
        if axis is None:
            axis = 0
        axis = self._get_axis_number(axis)
        method = clean_fill_method(method)

        if axis == 1:
            if not self._mgr.is_single_block and inplace:
                raise NotImplementedError
            # e.g. test_align_fill_method
            result = self.T._pad_or_backfill(
                method=method, limit=limit, limit_area=limit_area
            ).T

            return result

        new_mgr = self._mgr.pad_or_backfill(
            method=method,
            limit=limit,
            limit_area=limit_area,
            inplace=inplace,
        )
        result = self._constructor_from_mgr(new_mgr, axes=new_mgr.axes)
        if inplace:
            self._update_inplace(result)
            return self
        else:
            return result.__finalize__(self, method="fillna")

    @final
    def fillna(