improve coverage the hard way and the easy way

Author: d-v-b

src/zarr/core/dtype/npy/bool.py

@@ -246,7 +246,7 @@ def cast_scalar(self, data: object) -> np.bool_:
         """
         if self._check_scalar(data):
             return np.bool_(data)
-        msg = (
+        msg = (  # pragma: no cover
             f"Cannot convert object {data!r} with type {type(data)} to a scalar compatible with the "
             f"data type {self}."
         )

src/zarr/core/dtype/npy/string.py

@@ -385,7 +385,7 @@ def cast_scalar(self, data: object) -> np.str_:
 
             return self.to_native_dtype().type(str(data)[: self.length])
 
-        msg = (
+        msg = (  # pragma: no cover
             f"Cannot convert object {data!r} with type {type(data)} to a scalar compatible with the "
             f"data type {self}."
         )
@@ -711,11 +711,11 @@ def cast_scalar(self, data: object) -> str:
         """
         if self._check_scalar(data):
             return self._cast_scalar_unchecked(data)
-        msg = (
+        msg = (  # pragma: no cover
             f"Cannot convert object {data!r} with type {type(data)} to a scalar compatible with the "
             f"data type {self}."
         )
-        raise TypeError(msg)
+        raise TypeError(msg)  # pragma: no cover
 
 
 if _NUMPY_SUPPORTS_VLEN_STRING:

tests/test_dtype/test_npy/test_bytes.py

@@ -24,6 +24,7 @@ class TestNullTerminatedBytes(BaseTestZDType):
         "|S",
         "|U10",
         "|f8",
+        {"name": "|S4", "object_codec_id": "vlen-bytes"},
     )
     invalid_json_v3 = (
         {"name": "fixed_length_ascii", "configuration": {"length_bits": 0}},