allow nils along with struct - from_list/2 (#854)

lib/explorer/polars_backend/shared.ex

@@ -136,10 +136,16 @@ defmodule Explorer.PolarsBackend.Shared do
     columns = Map.new(fields, fn {k, _v} -> {k, []} end)
 
     columns =
-      Enum.reduce(list, columns, fn row, columns ->
-        Enum.reduce(row, columns, fn {field, value}, columns ->
-          Map.update!(columns, field, &[value | &1])
-        end)
+      Enum.reduce(list, columns, fn
+        nil, columns ->
+          Enum.reduce(fields, columns, fn {field, _}, columns ->
+            Map.update!(columns, field, &[nil | &1])
+          end)
+
+        row, columns ->
+          Enum.reduce(row, columns, fn {field, value}, columns ->
+            Map.update!(columns, field, &[value | &1])
+          end)
       end)
 
     series =

lib/explorer/shared.ex

@@ -387,13 +387,17 @@ defmodule Explorer.Shared do
   Downcasts lists of mixed numeric types (float and int) to float.
   """
   def cast_series(list, {:struct, dtypes}) when is_list(list) do
-    Enum.map(list, fn item ->
-      Enum.map(item, fn {field, inner_value} ->
-        column = to_string(field)
-        {^column, inner_dtype} = List.keyfind!(dtypes, column, 0)
-        [casted_value] = cast_series([inner_value], inner_dtype)
-        {column, casted_value}
-      end)
+    Enum.map(list, fn
+      nil ->
+        nil
+
+      item ->
+        Enum.map(item, fn {field, inner_value} ->
+          column = to_string(field)
+          {^column, inner_dtype} = List.keyfind!(dtypes, column, 0)
+          [casted_value] = cast_series([inner_value], inner_dtype)
+          {column, casted_value}
+        end)
     end)
   end
 

test/explorer/series/struct_test.exs

@@ -21,6 +21,17 @@ defmodule Explorer.Series.StructTest do
              ]
     end
 
+    test "allow nils" do
+      s = Series.from_list([nil, %{"a" => 1, "b" => 2}, nil])
+      assert s.dtype == {:struct, [{"a", {:s, 64}}, {"b", {:s, 64}}]}
+
+      assert Series.to_list(s) == [
+               %{"a" => nil, "b" => nil},
+               %{"a" => 1, "b" => 2},
+               %{"a" => nil, "b" => nil}
+             ]
+    end
+
     test "allows struct values" do
       s = Series.from_list([%{a: 1}, %{a: 3}, %{a: 5}])