ENH: Make 0-length dim handling of tensordot consistent with dot/einsum

3 files changed, 22 insertions, 5 deletions

diff --git a/doc/release/1.14.0-notes.rst b/doc/release/1.14.0-notes.rst
index aa4c69f9d..e819bfc2c 100644
--- a/doc/release/1.14.0-notes.rst
+++ b/doc/release/1.14.0-notes.rst
@@ -28,6 +28,12 @@ Build System Changes
 Compatibility notes
 ===================
 
+``np.tensordot`` now returns zero array when contracting over 0-length dimension
+--------------------------------------------------------------------------------
+Previously ``np.tensordot`` raised a ValueError when contracting over 0-length
+dimension. Now it returns a zero array, which is consistent with the behaviour
+of ``np.dot`` and ``np.einsum``.
+
 
 C API changes
 =============
diff --git a/numpy/core/numeric.py b/numpy/core/numeric.py
index 013c8a92a..f8ca49429 100644
--- a/numpy/core/numeric.py
+++ b/numpy/core/numeric.py
@@ -22,9 +22,9 @@ if sys.version_info[0] < 3:
     from .multiarray import newbuffer, getbuffer
 
 from . import umath
-from .umath import (invert, sin, UFUNC_BUFSIZE_DEFAULT, ERR_IGNORE,
-                    ERR_WARN, ERR_RAISE, ERR_CALL, ERR_PRINT, ERR_LOG,
-                    ERR_DEFAULT, PINF, NAN)
+from .umath import (multiply, invert, sin, UFUNC_BUFSIZE_DEFAULT,
+                    ERR_IGNORE, ERR_WARN, ERR_RAISE, ERR_CALL, ERR_PRINT,
+                    ERR_LOG, ERR_DEFAULT, PINF, NAN)
 from . import numerictypes
 from .numerictypes import longlong, intc, int_, float_, complex_, bool_
 from ._internal import TooHardError, AxisError
@@ -1326,7 +1326,7 @@ def tensordot(a, b, axes=2):
     N2 = 1
     for axis in axes_a:
         N2 *= as_[axis]
-    newshape_a = (-1, N2)
+    newshape_a = (multiply.reduce([as_[ax] for ax in notin]), N2)
     olda = [as_[axis] for axis in notin]
 
     notin = [k for k in range(ndb) if k not in axes_b]
@@ -1334,7 +1334,7 @@ def tensordot(a, b, axes=2):
     N2 = 1
     for axis in axes_b:
         N2 *= bs[axis]
-    newshape_b = (N2, -1)
+    newshape_b = (N2, multiply.reduce([bs[ax] for ax in notin]))
     oldb = [bs[axis] for axis in notin]
 
     at = a.transpose(newaxes_a).reshape(newshape_a)
diff --git a/numpy/core/tests/test_numeric.py b/numpy/core/tests/test_numeric.py
index 0f87ffdf2..a5149d4f7 100644
--- a/numpy/core/tests/test_numeric.py
+++ b/numpy/core/tests/test_numeric.py
@@ -2664,5 +2664,16 @@ class TestKeepdims(TestCase):
         assert_raises(TypeError, np.sum, x, keepdims=True)
 
 
+class TestTensordot(TestCase):
+
+    def test_zero_dimension(self):
+        # Test resolution to issue #5663
+        a = np.ndarray((3,0))
+        b = np.ndarray((0,4))
+        td = np.tensordot(a, b, (1, 0))
+        assert_array_equal(td, np.dot(a, b))
+        assert_array_equal(td, np.einsum('ij,jk', a, b))
+
+
 if __name__ == "__main__":
     run_module_suite()