Collapse into a numpy scalar in 0-d and 1-d reduction results

Author: FrancescAlted

src/blosc2/lazyexpr.py

@@ -2366,7 +2366,13 @@ def reduce_slices(  # noqa: C901
             dtype = np.float64
         out = convert_none_out(dtype, reduce_op, reduced_shape)
 
-    out = out[()] if reduced_shape == () else out  # undo dummy dim from inside convert_none_out
+    if reduced_shape == ():
+        # convert_none_out() may allocate shape (1,) as an internal buffer for scalar reductions.
+        # Collapse it to a numpy scalar while handling both 0-d and 1-d singleton arrays.
+        if isinstance(out, np.ndarray):
+            out = out[()] if out.ndim == 0 else out[0]
+        else:
+            out = out[()]
     final_mask = tuple(np.where(mask_slice)[0])
     if np.any(mask_slice):  # remove dummy dims
         out = np.squeeze(out, axis=final_mask)

tests/ndarray/test_dsl_kernels.py

@@ -214,6 +214,16 @@ def test_dsl_kernel_with_no_inputs_works_with_explicit_shape():
     np.testing.assert_equal(res, expected)
 
 
+def test_dsl_kernel_with_no_inputs_sum_returns_scalar():
+    shape = (10, 5)
+    expr = blosc2.lazyudf(kernel_index_ramp_no_inputs, (), dtype=np.float32, shape=shape)
+    result = expr.sum()
+
+    expected = np.arange(np.prod(shape), dtype=np.float32).reshape(shape).sum()
+    assert np.isscalar(result)
+    np.testing.assert_allclose(result, expected, rtol=0.0, atol=0.0)
+
+
 def test_dsl_kernel_with_no_inputs_requires_shape_or_out():
     with pytest.raises(ValueError, match="shape"):
         _ = blosc2.lazyudf(kernel_index_ramp_no_inputs, (), dtype=np.float32)