Added negative axis support.

theano/tensor/basic.py

@@ -3984,7 +3984,7 @@ def shape_padaxis(t, axis):
 def stack(*tensors, **kwargs):
     """Insert the arguments as slices into a tensor of 1 rank greater.
 
-    The size in dimension 0 of the result will be equal to the number
+    The size in dimension `axis` of the result will be equal to the number
     of tensors passed.
 
     Note: The interface stack(*tensors) is deprecated, you should use
@@ -4028,6 +4028,13 @@ def stack(*tensors, **kwargs):
         raise Exception('tensors is empty. You should at least provide one'
                         ' tensor to theano.tensor.stack(tensors, axis).')
 
+    ndim = tensors[0].ndim + 1
+    if not -ndim <= axis < ndim:
+        msg = 'axis {0} is out of bounds [-{1}, {1})'.format(axis, ndim)
+        raise IndexError(msg)
+    if axis < 0:
+        axis += ndim
+
     # If all tensors are scalars of the same type, call make_vector.
     # It makes the graph simpler, by not adding DimShuffles and Rebroadcasts
 

theano/tensor/tests/test_basic.py

@@ -3443,6 +3443,7 @@ class T_Join_and_Split(unittest.TestCase):
 
     def test_stack_new_interface(self):
         """Test the new numpy-like interface: stack(tensors, axis=0)."""
+        # Testing against old interface
         warnings.simplefilter('always', DeprecationWarning)
         a = tensor.imatrix('a')
         b = tensor.imatrix('b')
@@ -3452,12 +3453,28 @@ class T_Join_and_Split(unittest.TestCase):
         v1, v2 = f([[1, 2]], [[3, 4]])
         self.assertTrue(v1.shape == v2.shape)
         self.assertTrue(numpy.all(v1 == v2))
+        # Testing axis parameter
         s3 = stack([a, b], 1)
         f = function([a, b], s3, mode=self.mode)
         v3 = f([[1, 2]], [[3, 4]])
         v4 = numpy.array([[[1, 2], [3, 4]]])
         self.assertTrue(v3.shape == v4.shape)
         self.assertTrue(numpy.all(v3 == v4))
+        # Testing negative axis
+        s = stack([a, b], axis=-1)
+        f = function([a, b], s, mode=self.mode)
+        v1 = [[1, 2, 3], [4, 5, 6]]
+        v2 = [[7, 8, 9], [10, 11, 12]]
+        v = numpy.zeros((2, 3, 2))
+        v[:,:,0] = v1
+        v[:,:,1] = v2 
+        out = f(v1, v2) 
+        self.assertTrue(v.shape == out.shape)
+        self.assertTrue(numpy.all(v == out))
+        # Testing out-of-bounds axis
+        self.assertRaises(IndexError, stack, [a, b], 4)
+        self.assertRaises(IndexError, stack, [a, b], -4)
+        # Testing depreciation warning
         with warnings.catch_warnings(record=True) as w:
             s = stack(a, b)
             assert len(w) == 1