Add some units tests for result_dtype

conftest.py

@@ -358,11 +358,16 @@ def manifest_array(array_v3_metadata):
     def _manifest_array(
         shape: tuple = (5, 2),
         chunks: tuple = (5, 2),
+        data_type: np.dtype = np.dtype("int32"),
         codecs: list[dict] | None = [ARRAYBYTES_CODEC, ZLIB_CODEC],
         dimension_names: Iterable[str] | None = None,
     ):
         metadata = array_v3_metadata(
-            shape=shape, chunks=chunks, codecs=codecs, dimension_names=dimension_names
+            shape=shape,
+            chunks=chunks,
+            data_type=data_type,
+            codecs=codecs,
+            dimension_names=dimension_names,
         )
         entries = _generate_chunk_entries(shape, chunks, _entry_from_chunk_key)
         chunkmanifest = ChunkManifest(entries=entries)

virtualizarr/manifests/array_api.py

@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Any, Callable, cast
+from typing import TYPE_CHECKING, Any, Callable, Union, cast
 
 import numpy as np
 
@@ -34,12 +34,22 @@ def decorator(func):
 
 
 @implements(np.result_type)
-def result_type(*arrays_and_dtypes) -> np.dtype:
+def result_type(*arrays_and_dtypes: Union["ManifestArray", np.dtype]) -> np.dtype:
     """Called by xarray to ensure all arguments to concat have the same dtype."""
-    first_dtype, *other_dtypes = (np.dtype(obj) for obj in arrays_and_dtypes)
+    from virtualizarr.manifests.array import ManifestArray
+
+    dtypes = (
+        obj.dtype if isinstance(obj, ManifestArray) else np.dtype(obj)
+        for obj in arrays_and_dtypes
+    )
+    first_dtype, *other_dtypes = dtypes
+    unique_dtypes = set(dtypes)
     for other_dtype in other_dtypes:
         if other_dtype != first_dtype:
-            raise ValueError("dtypes not all consistent")
+            raise ValueError(
+                f"Cannot combine arrays with inconsistent dtypes, but got {len(unique_dtypes)} distinct dtypes: {unique_dtypes}"
+            )
+
     return first_dtype
 
 

virtualizarr/tests/test_manifests/test_array.py

@@ -9,7 +9,7 @@
 from virtualizarr.manifests import ChunkManifest, ManifestArray
 
 
-class TestManifestArray:
+class TestInit:
     def test_manifest_array(self, array_v3_metadata):
         chunks_dict = {
             "0.0.0": {"path": "s3://bucket/foo.nc", "offset": 100, "length": 100},
@@ -50,6 +50,23 @@ def test_manifest_array_dict_v3_metadata(self, array_v3_metadata):
         assert marr.ndim == 3
 
 
+class TestResultType:
+    def test_idempotent(self, manifest_array):
+        marr1 = manifest_array(shape=(), chunks=(), data_type=np.dtype("int32"))
+        marr2 = manifest_array(shape=(), chunks=(), data_type=np.dtype("int32"))
+
+        assert np.result_type(marr1) == marr1.dtype
+        assert np.result_type(marr1, marr1.dtype) == marr1.dtype
+        assert np.result_type(marr1, marr2) == marr1.dtype
+
+    def test_raises(self, manifest_array):
+        marr1 = manifest_array(shape=(), chunks=(), data_type=np.dtype("int32"))
+        marr2 = manifest_array(shape=(), chunks=(), data_type=np.dtype("int64"))
+
+        with pytest.raises(ValueError, match="inconsistent"):
+            np.result_type(marr1, marr2)
+
+
 class TestEquals:
     def test_equals(self, array_v3_metadata):
         chunks_dict = {