Class Len

dask/dataframe/dask_expr/_reductions.py:1048–1076

class Len(Reduction):
    reduction_chunk = staticmethod(len)
    reduction_aggregate = sum

    def _simplify_down(self):
        from dask.dataframe.dask_expr.io.io import IO

        # We introduce Index nodes sometimes.  We special case around them.
        if isinstance(self.frame, Index) and self.frame.frame._is_length_preserving:
            return Len(self.frame.frame)

        # Pass through Elemwises, unless we just introduced an Index
        if self.frame._is_length_preserving and not isinstance(self.frame, Index):
            child = max(self.frame.dependencies(), key=lambda expr: expr.npartitions)
            return Len(child)

        # Let the child handle it.  They often know best
        if isinstance(self.frame, IO):
            return self

        if isinstance(self.frame, Concat) and self.frame.operand("axis") == 0:
            return sum(Len(obj) for obj in self.frame.dependencies())

        # Drop all of the columns, just pass through the index
        if self.frame.ndim == 2 and len(self.frame.columns):
            return Len(self.frame.index)

    def _simplify_up(self, parent, dependents):
        return


class Size(Reduction):