Rechunk original frame

crates/polars-core/src/frame/row/transpose.rs

@@ -87,10 +87,13 @@ impl DataFrame {
 
     /// Transpose a DataFrame. This is a very expensive operation.
     pub fn transpose(
-        &self,
+        &mut self,
         keep_names_as: Option<&str>,
         new_col_names: Option<Either<String, Vec<String>>>,
     ) -> PolarsResult<DataFrame> {
+        // We must iterate columns as [`AnyValue`], so we must be contiguous.
+        self.as_single_chunk_par();
+
         let mut df = Cow::Borrowed(self); // Can't use self because we might drop a name column
         let names_out = match new_col_names {
             None => (0..self.height()).map(|i| format!("column_{i}")).collect(),
@@ -263,7 +266,7 @@ mod test {
 
     #[test]
     fn test_transpose() -> PolarsResult<()> {
-        let df = df![
+        let mut df = df![
             "a" => [1, 2, 3],
             "b" => [10, 20, 30],
         ]?;
@@ -277,7 +280,7 @@ mod test {
         ]?;
         assert!(out.equals_missing(&expected));
 
-        let df = df![
+        let mut df = df![
             "a" => [Some(1), None, Some(3)],
             "b" => [Some(10), Some(20), None],
         ]?;
@@ -290,7 +293,7 @@ mod test {
         ]?;
         assert!(out.equals_missing(&expected));
 
-        let df = df![
+        let mut df = df![
             "a" => ["a", "b", "c"],
             "b" => [Some(10), Some(20), None],
         ]?;

py-polars/src/dataframe.rs

@@ -1371,7 +1371,11 @@ impl PyDataFrame {
     }
 
     #[pyo3(signature = (keep_names_as, column_names))]
-    pub fn transpose(&self, keep_names_as: Option<&str>, column_names: &PyAny) -> PyResult<Self> {
+    pub fn transpose(
+        &mut self,
+        keep_names_as: Option<&str>,
+        column_names: &PyAny,
+    ) -> PyResult<Self> {
         let new_col_names = if let Ok(name) = column_names.extract::<Vec<String>>() {
             Some(Either::Right(name))
         } else if let Ok(name) = column_names.extract::<String>() {