Class Projection

dask/dataframe/dask_expr/_expr.py:2173–2245

Column Selection

class Projection(Elemwise):
    """Column Selection"""

    _parameters = ["frame", "columns"]
    operation = operator.getitem

    @functools.cached_property
    def unique_partition_mapping_columns_from_shuffle(self):
        col_op = self.operand("columns")
        columns = set(col_op) if isinstance(col_op, list) else {col_op}
        return {
            c
            for c in self.frame.unique_partition_mapping_columns_from_shuffle
            if c in columns or isinstance(c, tuple) and set(c).issubset(columns)
        }

    @property
    def columns(self):
        cols = self.operand("columns")
        if isinstance(cols, list):
            return cols
        elif isinstance(cols, pd.Index):
            return list(cols)
        else:
            return [cols]

    @functools.cached_property
    def _meta(self):
        if is_dataframe_like(self.frame._meta):
            return super()._meta
        # if we are not a DataFrame and have a scalar, we reduce to a scalar
        if not isinstance(self.operand("columns"), (list, slice)) and not hasattr(
            self.operand("columns"), "dtype"
        ):
            return meta_nonempty(self.frame._meta).iloc[0]
        # Avoid column selection for Series/Index
        return self.frame._meta

    def _node_label_args(self):
        return [self.frame, self.operand("columns")]

    def __str__(self):
        base = str(self.frame)
        if " " in base:
            base = f"({base})"
        return f"{base}[{self.operand('columns')!r}]"

    def _divisions(self):
        if self.ndim == 0:
            return (None, None)
        return super()._divisions()

    def _simplify_down(self):
        if (
            str(self.frame.columns) == str(self.columns)
            and self._meta.ndim == self.frame._meta.ndim
        ):
            # TODO: we should get more precise around Expr.columns types