simplify NDBuffer.as_scalar (#3027)

* index with an empty tuple to get scalar

* changelog

* add as_scalar test

Author: d-v-b

changes/3027.misc.rst

@@ -0,0 +1 @@
+Simplified scalar indexing of size-1 arrays.

src/zarr/core/buffer/core.py

@@ -427,16 +427,7 @@ def as_scalar(self) -> ScalarType:
         """Returns the buffer as a scalar value"""
         if self._data.size != 1:
             raise ValueError("Buffer does not contain a single scalar value")
-        item = self.as_numpy_array().item()
-        scalar: ScalarType
-
-        if np.issubdtype(self.dtype, np.datetime64):
-            unit: str = np.datetime_data(self.dtype)[0]  # Extract the unit (e.g., 'Y', 'D', etc.)
-            scalar = np.datetime64(item, unit)
-        else:
-            scalar = self.dtype.type(item)  # Regular conversion for non-datetime types
-
-        return scalar
+        return cast(ScalarType, self.as_numpy_array()[()])
 
     @property
     def dtype(self) -> np.dtype[Any]:

tests/test_buffer.py

@@ -155,3 +155,9 @@ def test_numpy_buffer_prototype() -> None:
     assert isinstance(ndbuffer.as_ndarray_like(), np.ndarray)
     with pytest.raises(ValueError, match="Buffer does not contain a single scalar value"):
         ndbuffer.as_scalar()
+
+
+# TODO: the same test for other buffer classes
+def test_cpu_buffer_as_scalar() -> None:
+    buf = cpu.buffer_prototype.nd_buffer.create(shape=(), dtype="int64")
+    assert buf.as_scalar() == buf.as_ndarray_like()[()]  # type: ignore[index]