cast null to categorical (#4254)

polars/polars-lazy/src/physical_plan/expressions/cast.rs

@@ -18,6 +18,7 @@ impl CastExpr {
         // in a ternary or binary operation, we then do type coercion to matching supertype.
         // here we create a null array for the types we cannot cast to from a booleanarray
 
+        // todo! check if the expression is really null
         if input.bool().is_ok() && input.null_count() == input.len() {
             match &self.data_type {
                 DataType::List(inner) => {
@@ -56,6 +57,12 @@ impl CastExpr {
                         .unwrap()
                         .into_series());
                 }
+                #[cfg(feature = "dtype-categorical")]
+                DataType::Categorical(_) => {
+                    return Ok(
+                        CategoricalChunked::full_null(input.name(), input.len()).into_series()
+                    )
+                }
                 _ => {}
             }
         }

py-polars/tests/test_categorical.py

@@ -194,3 +194,9 @@ def test_categorical_error_on_local_cmp() -> None:
         ),
     ):
         df_cat.filter(pl.col("a_cat") == pl.col("b_cat"))
+
+
+def test_cast_null_to_categorical() -> None:
+    assert pl.DataFrame().with_columns(
+        [pl.lit(None).cast(pl.Categorical).alias("nullable_enum")]
+    ).dtypes == [pl.Categorical]