[GitHub] [arrow] westonpace commented on a diff in pull request #14378: ARROW-17933: [C++] SparseCOOTensor raises error when created with zero elements

["westonpace (via GitHub)" <gi...@apache.org>]

westonpace commented on code in PR #14378:
URL: https://github.com/apache/arrow/pull/14378#discussion_r1172672469


##########
python/pyarrow/tests/test_sparse_tensor.py:
##########
@@ -434,6 +433,23 @@ def test_sparse_coo_tensor_scipy_roundtrip(dtype_str, arrow_type):
     assert sparse_tensor.has_canonical_format
     assert out_scipy_matrix.has_canonical_format
 
+    scipy_matrix = coo_matrix([[0, 0], [0, 0]])
+    sparse_tensor = pa.SparseCOOTensor.from_scipy(scipy_matrix,
+                                                  dim_names=dim_names)
+    out_scipy_matrix = sparse_tensor.to_scipy()

Review Comment:
   Can we verify the shape of `sparse_tensor` here?  It should be 2 x 2 correct?



##########
cpp/src/arrow/sparse_tensor.cc:
##########
@@ -134,6 +134,10 @@ inline Status CheckSparseCOOIndexValidity(const std::shared_ptr<DataType>& type,
 
   RETURN_NOT_OK(internal::CheckSparseIndexMaximumValue(type, shape));
 
+  // Indexes with no values are considered valid

Review Comment:
   I don't understand what is happening here (I don't understand much about the tensors so not surprising).  Why is it ok if one of the shape elements is zero?  I would expect an empty sparse matrix to still have a shape:
   
   ```
   >>> scipy.sparse.coo_matrix(numpy.zeros((2,4)), dtype=numpy.float32).shape
   (2, 4)
   ```



##########
python/pyarrow/tests/test_sparse_tensor.py:
##########
@@ -434,6 +434,21 @@ def test_sparse_coo_tensor_scipy_roundtrip(dtype_str, arrow_type):
     assert sparse_tensor.has_canonical_format
     assert out_scipy_matrix.has_canonical_format
 
+    scipy_matrix = coo_matrix([[0, 0], [0, 0]])

Review Comment:
   I believe @rok has the correct test here but I could be wrong.  The original issue is about a sparse matrix with a valid non-zero shape but no elements:
   
   ```
   >>> scipy.sparse.coo_matrix(numpy.zeros((2,4)), dtype=numpy.float32)
   <2x4 sparse matrix of type '<class 'numpy.float32'>'
   	with 0 stored elements in COOrdinate format>
   >>> numpy.zeros((2,4))
   array([[0., 0., 0., 0.],
          [0., 0., 0., 0.]])
   >>> numpy.zeros((2,4)).shape
   (2, 4)
   ```



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