Fix empty tensordot.

sparse/coo/common.py

@@ -1,4 +1,5 @@
 from functools import reduce, wraps
+from itertools import chain
 import operator
 import warnings
 from collections.abc import Iterable
@@ -148,6 +149,13 @@ def tensordot(a, b, axes=2):
     newshape_b = (N2, -1)
     oldb = [bs[axis] for axis in notin]
 
+    if any(dim == 0 for dim in chain(newshape_a, newshape_b)):
+        res = asCOO(np.empty(olda + oldb), check=False)
+        if isinstance(a, np.ndarray) or isinstance(b, np.ndarray):
+            res = res.todense()
+
+        return res
+
     at = a.transpose(newaxes_a).reshape(newshape_a)
     bt = b.transpose(newaxes_b).reshape(newshape_b)
     res = _dot(at, bt)

tests/test_coo.py

@@ -324,6 +324,15 @@ def test_tensordot(a_shape, b_shape, axes):
     # assert isinstance(sparse.tensordot(a, sb, axes), COO)
 
 
+def test_tensordot_empty():
+    x1 = np.empty((0, 0, 0))
+    x2 = np.empty((0, 0, 0))
+    s1 = sparse.COO.from_numpy(x1)
+    s2 = sparse.COO.from_numpy(x2)
+
+    assert_eq(np.tensordot(x1, x2), sparse.tensordot(s1, s2))
+
+
 @pytest.mark.parametrize('a_shape, b_shape', [
     ((3, 1, 6, 5), (2, 1, 4, 5, 6)),
     ((2, 1, 4, 5, 6), (3, 1, 6, 5)),