feat(rust, python): allow all null cast (#5933)

pola-rs · Dec 29, 2022 · 04b96e6 · 04b96e6
1 parent 6db2311
commit 04b96e6
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Showing 2 changed files with 13 additions and 1 deletion.
diff --git a/polars/polars-core/src/series/mod.rs b/polars/polars-core/src/series/mod.rs
@@ -228,7 +228,12 @@ impl Series {
 
     /// Cast `[Series]` to another `[DataType]`
     pub fn cast(&self, dtype: &DataType) -> PolarsResult<Self> {
-        self.0.cast(dtype)
+        let len = self.len();
+        if self.null_count() == len {
+            Ok(Series::full_null(self.name(), len, dtype))
+        } else {
+            self.0.cast(dtype)
+        }
     }
 
     /// Compute the sum of all values in this Series.

diff --git a/py-polars/tests/unit/test_schema.py b/py-polars/tests/unit/test_schema.py
@@ -292,3 +292,10 @@ def test_lazy_rename() -> None:
     assert (
         df.lazy().rename({"y": "x", "x": "y"}).select(["x", "y"]).collect()
     ).to_dict(False) == {"x": [2], "y": [1]}
+
+
+def test_all_null_cast_5826() -> None:
+    df = pl.DataFrame(data=[pl.Series("a", [None], dtype=pl.Utf8)])
+    out = df.with_column(pl.col("a").cast(pl.Boolean))
+    assert out.dtypes == [pl.Boolean]
+    assert out.item() is None