Fix the String representation of non-scalar mass quantities

si-units/CHANGELOG.md

@@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Generalized indexing, such that, e.g., general multidimensional slicing operations are possible for quantities containing NumPy arrays. [#110](https://github.com/itt-ustutt/quantity/pull/110)
 
+### Fixed
+- Fixed the String representation of non-scalar mass quantities. [#111](https://github.com/itt-ustutt/quantity/pull/111)
+
 ## [0.11.3] - 2026-04-22
 ### Added
 - Added stub information for methods and constants. [#106](https://github.com/itt-ustutt/quantity/pull/106)

si-units/src/lib.rs

@@ -83,7 +83,14 @@ impl PySIObject {
                 .value
                 .call_method0(py, "__repr__")?
                 .extract::<String>(py)?;
-            Ok(format!("{} {}", value, self.unit))
+            let unit = if self.unit == _KILOGRAM {
+                // because the base unit already has a prefix, we cannot call
+                // unit.to_string() on it (it would return 'g').
+                "kg".into()
+            } else {
+                self.unit.to_string()
+            };
+            Ok(format!("{} {}", value, unit))
         }
     }