feat(rust): Format null arrays in Series

crates/polars-core/src/chunked_array/ops/any_value.rs

@@ -4,6 +4,7 @@ use polars_utils::sync::SyncPtr;
 #[cfg(feature = "object")]
 use crate::chunked_array::object::extension::polars_extension::PolarsExtension;
 use crate::prelude::*;
+use crate::series::implementations::null::NullChunked;
 
 #[inline]
 #[allow(unused_variables)]
@@ -282,3 +283,14 @@ impl<T: PolarsObject> ChunkAnyValue for ObjectChunked<T> {
         }
     }
 }
+
+impl ChunkAnyValue for NullChunked {
+    #[inline]
+    unsafe fn get_any_value_unchecked(&self, _index: usize) -> AnyValue {
+        AnyValue::Null
+    }
+
+    fn get_any_value(&self, _index: usize) -> PolarsResult<AnyValue> {
+        Ok(AnyValue::Null)
+    }
+}

crates/polars-core/src/fmt.rs

@@ -313,7 +313,7 @@ impl Debug for Series {
                 "Series"
             ),
             DataType::Null => {
-                writeln!(f, "nullarray; shape: {}", self.len())
+                format_array!(f, self.null().unwrap(), "null", self.name(), "Series")
             },
             DataType::Binary => {
                 format_array!(f, self.binary().unwrap(), "binary", self.name(), "Series")

crates/polars-core/src/series/ops/downcast.rs

@@ -1,4 +1,5 @@
 use crate::prelude::*;
+use crate::series::implementations::null::NullChunked;
 
 macro_rules! unpack_chunked {
     ($series:expr, $expected:pat => $ca:ty, $name:expr) => {
@@ -152,4 +153,9 @@ impl Series {
         }
         unpack_chunked!(self, DataType::Struct(_) => StructChunked, "Struct")
     }
+
+    /// Unpack to [`ChunkedArray`] of dtype `[DataType::Null]`
+    pub fn null(&self) -> PolarsResult<&NullChunked> {
+        unpack_chunked!(self, DataType::Null => NullChunked, "Null")
+    }
 }

py-polars/tests/unit/test_constructors.py

@@ -607,6 +607,26 @@ def test_init_ndarray(monkeypatch: Any) -> None:
     assert df2.rows() == [(1.0, 4.0), (2.5, None), (None, 6.5)]
 
 
+def test_null_array_print_format() -> None:
+    pa_tbl_null = pa.table({"a": [None, None]})
+    df_null = pl.from_arrow(pa_tbl_null)
+    assert df_null.shape == (2, 1)
+    assert df_null.dtypes == [pl.Null]  # type: ignore[union-attr]
+    assert df_null.rows() == [(None,), (None,)]  # type: ignore[union-attr]
+
+    assert (
+        str(df_null) == "shape: (2, 1)\n"
+        "┌──────┐\n"
+        "│ a    │\n"
+        "│ ---  │\n"
+        "│ null │\n"
+        "╞══════╡\n"
+        "│ null │\n"
+        "│ null │\n"
+        "└──────┘"
+    )
+
+
 def test_init_arrow() -> None:
     # Handle unnamed column
     df = pl.DataFrame(pa.table({"a": [1, 2], None: [3, 4]}))