Method set_index

dask/dataframe/dask_expr/_collection.py:3342–3532

Set the DataFrame index (row labels) using an existing column. If ``sort=False``, this function operates exactly like ``pandas.set_index`` and sets the index on the DataFrame. If ``sort=True`` (default), this function also sorts the DataFrame by the new index. This can have

(
        self,
        other,
        drop=True,
        sorted=False,
        npartitions: int | None = None,
        divisions=None,
        sort: bool = True,
        shuffle_method=None,
        upsample: float = 1.0,
        partition_size: float = 128e6,
        append: bool = False,
        **options,
    )

        return new_collection(Eval(self, _expr=expr, expr_kwargs=kwargs))

    def set_index(
        self,
        other,
        drop=True,
        sorted=False,
        npartitions: int | None = None,
        divisions=None,
        sort: bool = True,
        shuffle_method=None,
        upsample: float = 1.0,
        partition_size: float = 128e6,
        append: bool = False,
        **options,
    ):
        """Set the DataFrame index (row labels) using an existing column.

        If ``sort=False``, this function operates exactly like ``pandas.set_index``
        and sets the index on the DataFrame. If ``sort=True`` (default),
        this function also sorts the DataFrame by the new index. This can have a
        significant impact on performance, because joins, groupbys, lookups, etc.
        are all much faster on that column. However, this performance increase
        comes with a cost, sorting a parallel dataset requires expensive shuffles.
        Often we ``set_index`` once directly after data ingest and filtering and
        then perform many cheap computations off of the sorted dataset.

        With ``sort=True``, this function is much more expensive. Under normal
        operation this function does an initial pass over the index column to
        compute approximate quantiles to serve as future divisions. It then passes
        over the data a second time, splitting up each input partition into several
        pieces and sharing those pieces to all of the output partitions now in
        sorted order.

        In some cases we can alleviate those costs, for example if your dataset is
        sorted already then we can avoid making many small pieces or if you know
        good values to split the new index column then we can avoid the initial
        pass over the data. For example if your new index is a datetime index and
        your data is already sorted by day then this entire operation can be done
        for free. You can control these options with the following parameters.

        Parameters
        ----------
        other: string or Dask Series
            Column to use as index.
        drop: boolean, default True
            Delete column to be used as the new index.
        sorted: bool, optional
            If the index column is already sorted in increasing order.
            Defaults to False
        npartitions: int, None, or 'auto'
            The ideal number of output partitions. If None, use the same as
            the input. If 'auto' then decide by memory use.
            Only used when ``divisions`` is not given. If ``divisions`` is given,
            the number of output partitions will be ``len(divisions) - 1``.
        divisions: list, optional
            The "dividing lines" used to split the new index into partitions.
            For ``divisions=[0, 10, 50, 100]``, there would be three output partitions,
            where the new index contained [0, 10), [10, 50), and [50, 100), respectively.
            See https://docs.dask.org/en/latest/dataframe-design.html#partitions.