fix: Added input validation for explode operation in the array namespace

crates/polars-plan/src/dsl/array.rs

@@ -193,4 +193,9 @@ impl ArrayNameSpace {
             None,
         )
     }
+    /// Returns a column with a separate row for every array element.
+    pub fn explode(self) -> Expr {
+        self.0
+            .map_private(FunctionExpr::ArrayExpr(ArrayFunction::Explode))
+    }
 }

crates/polars-plan/src/dsl/function_expr/array.rs

@@ -30,6 +30,7 @@ pub enum ArrayFunction {
     #[cfg(feature = "array_count")]
     CountMatches,
     Shift,
+    Explode,
 }
 
 impl ArrayFunction {
@@ -56,6 +57,7 @@ impl ArrayFunction {
             #[cfg(feature = "array_count")]
             CountMatches => mapper.with_dtype(IDX_DTYPE),
             Shift => mapper.with_same_dtype(),
+            Explode => mapper.try_map_to_array_inner_dtype(),
         }
     }
 }
@@ -96,6 +98,7 @@ impl Display for ArrayFunction {
             #[cfg(feature = "array_count")]
             CountMatches => "count_matches",
             Shift => "shift",
+            Explode => "explode",
         };
         write!(f, "arr.{name}")
     }
@@ -129,6 +132,7 @@ impl From<ArrayFunction> for SpecialEq<Arc<dyn ColumnsUdf>> {
             #[cfg(feature = "array_count")]
             CountMatches => map_as_slice!(count_matches),
             Shift => map_as_slice!(shift),
+            Explode => unreachable!(),
         }
     }
 }

crates/polars-plan/src/dsl/function_expr/schema.rs

@@ -486,6 +486,16 @@ impl<'a> FieldsMapper<'a> {
         Ok(first)
     }
 
+    #[cfg(feature = "dtype-array")]
+    /// Map the dtype to the dtype of the array elements, with typo validation.
+    pub fn try_map_to_array_inner_dtype(&self) -> PolarsResult<Field> {
+        let dt = self.fields[0].dtype();
+        match dt {
+            DataType::Array(_, _) => self.map_to_list_and_array_inner_dtype(),
+            _ => polars_bail!(InvalidOperation: "expected Array type, got: {}", dt),
+        }
+    }
+
     /// Map the dtypes to the "supertype" of a list of lists.
     pub fn map_to_list_supertype(&self) -> PolarsResult<Field> {
         self.try_map_dtypes(|dts| {

crates/polars-plan/src/plans/optimizer/simplify_expr/simplify_functions.rs

@@ -7,6 +7,12 @@ pub(super) fn optimize_functions(
     expr_arena: &mut Arena<AExpr>,
 ) -> PolarsResult<Option<AExpr>> {
     let out = match function {
+        #[cfg(feature = "dtype-array")]
+        // arr.explode() -> explode()
+        FunctionExpr::ArrayExpr(ArrayFunction::Explode) => {
+            let input_node = input[0].node();
+            Some(AExpr::Explode(input_node))
+        },
         // is_null().any() -> null_count() > 0
         // is_not_null().any() ->  null_count() < len()
         // CORRECTNESS: we can ignore 'ignore_nulls' since is_null/is_not_null never produces NULLS

crates/polars-python/src/expr/array.rs

@@ -132,4 +132,8 @@ impl PyExpr {
     fn arr_shift(&self, n: PyExpr) -> Self {
         self.inner.clone().arr().shift(n.inner).into()
     }
+
+    fn arr_explode(&self) -> Self {
+        self.inner.clone().arr().explode().into()
+    }
 }

py-polars/polars/expr/array.py

@@ -605,7 +605,7 @@ def explode(self) -> Expr:
         │ 6   │
         └─────┘
         """
-        return wrap_expr(self._pyexpr.explode())
+        return wrap_expr(self._pyexpr.arr_explode())
 
     def contains(
         self, item: float | str | bool | int | date | datetime | time | IntoExprColumn

py-polars/polars/series/array.py

@@ -17,7 +17,7 @@
 
 @expr_dispatch
 class ArrayNameSpace:
-    """Namespace for list related methods."""
+    """Namespace for array related methods."""
 
     _accessor = "arr"
 

py-polars/tests/unit/operations/namespaces/array/test_array.py

@@ -6,7 +6,7 @@
 import pytest
 
 import polars as pl
-from polars.exceptions import ComputeError
+from polars.exceptions import ComputeError, InvalidOperationError
 from polars.testing import assert_frame_equal, assert_series_equal
 
 
@@ -449,3 +449,14 @@ def test_array_n_unique() -> None:
         {"n_unique": [2, 1, 1, None]}, schema={"n_unique": pl.UInt32}
     )
     assert_frame_equal(out, expected)
+
+
+def test_explode_19049() -> None:
+    df = pl.DataFrame({"a": [[1, 2, 3]]}, schema={"a": pl.Array(pl.Int64, 3)})
+    result_df = df.select(pl.col.a.arr.explode())
+    expected_df = pl.DataFrame({"a": [1, 2, 3]}, schema={"a": pl.Int64})
+    assert_frame_equal(result_df, expected_df)
+
+    df = pl.DataFrame({"a": [1, 2, 3]}, schema={"a": pl.Int64})
+    with pytest.raises(InvalidOperationError, match="expected Array type, got: i64"):
+        df.select(pl.col.a.arr.explode())