fix(rust): Do not panic when casting from an empty Series to pl.Decimal

pola-rs · Feb 7, 2024 · 97abb20 · 97abb20
1 parent 6fc0395
commit 97abb20
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Showing 3 changed files with 11 additions and 7 deletions.
diff --git a/crates/polars-core/src/chunked_array/logical/decimal.rs b/crates/polars-core/src/chunked_array/logical/decimal.rs
@@ -83,9 +83,7 @@ impl LogicalType for DecimalChunked {
     fn cast(&self, dtype: &DataType) -> PolarsResult<Series> {
         let (precision_src, scale_src) = (self.precision(), self.scale());
         if let &DataType::Decimal(precision_dst, scale_dst) = dtype {
-            let scale_dst = scale_dst.ok_or_else(
-                || polars_err!(ComputeError: "cannot cast to Decimal with unknown scale"),
-            )?;
+            let scale_dst = scale_dst.unwrap_or(scale_src);
             // for now, let's just allow same-scale conversions
             // where precision is either the same or bigger or gets converted to `None`
             // (these are the easy cases requiring no checks and arithmetics which we can add later)
@@ -95,6 +93,7 @@ impl LogicalType for DecimalChunked {
                 _ => false,
             };
             if scale_src == scale_dst && is_widen {
+                let dtype = &DataType::Decimal(precision_dst, Some(scale_dst));
                 return self.0.cast(dtype); // no conversion or checks needed
             }
         }

diff --git a/crates/polars-core/src/series/ops/null.rs b/crates/polars-core/src/series/ops/null.rs
@@ -39,10 +39,7 @@ impl Series {
                 .into_series(),
             #[cfg(feature = "dtype-decimal")]
             DataType::Decimal(precision, scale) => Int128Chunked::full_null(name, size)
-                .into_decimal_unchecked(
-                    *precision,
-                    scale.unwrap_or_else(|| unreachable!("scale should be set")),
-                )
+                .into_decimal_unchecked(*precision, scale.unwrap_or(0))
                 .into_series(),
             #[cfg(feature = "dtype-struct")]
             DataType::Struct(fields) => {

diff --git a/py-polars/tests/unit/datatypes/test_decimal.py b/py-polars/tests/unit/datatypes/test_decimal.py
@@ -138,6 +138,14 @@ def test_decimal_cast() -> None:
     assert result.to_dict(as_series=False) == expected
 
 
+def test_decimal_cast_no_scale() -> None:
+    s = pl.Series().cast(pl.Decimal)
+    assert s.dtype == pl.Decimal(precision=None, scale=0)
+
+    s = pl.Series([D("10.0")]).cast(pl.Decimal)
+    assert s.dtype == pl.Decimal(precision=None, scale=1)
+
+
 def test_decimal_scale_precision_roundtrip(monkeypatch: Any) -> None:
     monkeypatch.setenv("POLARS_ACTIVATE_DECIMAL", "1")
     assert pl.from_arrow(pl.Series("dec", [D("10.0")]).to_arrow()).item() == D("10.0")