Fix product for lazy series

lib/explorer/backend/lazy_series.ex

@@ -774,7 +774,7 @@ defmodule Explorer.Backend.LazySeries do
        do: {:u, 32}
 
   defp dtype_for_agg_operation(op, series)
-       when op in [:first, :last, :sum, :min, :max],
+       when op in [:first, :last, :sum, :min, :max, :product],
        do: series.dtype
 
   defp dtype_for_agg_operation(op, _) when op in [:all?, :any?], do: :boolean

lib/explorer/polars_backend/series.ex

@@ -248,7 +248,7 @@ defmodule Explorer.PolarsBackend.Series do
     do: Shared.apply_series(series, :s_quantile, [quantile, "nearest"])
 
   @impl true
-  def product(series), do: Shared.apply_series(series, :s_product)
+  def product(series), do: first(Shared.apply_series(series, :s_product))
 
   @impl true
   def skew(series, bias?),

lib/explorer/series.ex

@@ -2773,7 +2773,7 @@ defmodule Explorer.Series do
   @doc type: :aggregation
   @spec product(series :: Series.t()) :: float() | non_finite() | nil
   def product(%Series{dtype: dtype} = series) when is_numeric_dtype(dtype),
-    do: at(apply_series(series, :product), 0)
+    do: apply_series(series, :product)
 
   def product(%Series{dtype: dtype}),
     do: dtype_error("product/1", dtype, @numeric_dtypes)

test/explorer/data_frame_test.exs

@@ -4167,6 +4167,21 @@ defmodule Explorer.DataFrameTest do
       assert b in [2, 3]
     end
 
+    test "product/1 without groups (see grouped_test for groups)" do
+      df = DF.new(petal_width: [1, 2, 2, 3, 3, 120])
+
+      df1 =
+        DF.summarise(df, petal_width_product: product(petal_width))
+
+      assert DF.dtypes(df1) == %{
+               "petal_width_product" => {:s, 64}
+             }
+
+      result = DF.to_columns(df1)
+
+      assert result["petal_width_product"] == [4320]
+    end
+
     test "argmax/1 and argmin/1" do
       df =
         DF.new(