Add support for neg start and stop.

theano/tensor/basic.py

@@ -4252,14 +4252,20 @@ def get_vector_length(v):
             v.owner.inputs, v.owner.op.idx_list)[0].stop)
         step = extract_constant(theano.tensor.subtensor.get_idx_list(
             v.owner.inputs, v.owner.op.idx_list)[0].step)
-        if start is None:
-            start = 0
+
         ndim = v.owner.inputs[0].owner.inputs[0].ndim
         types = (numbers.Integral, numpy.integer)
+        if start is None:
+            start = 0
+        elif isinstance(start, types) and start < 0:
+            start += ndim
         if stop is None:
             stop = ndim
-        elif isinstance(stop, types) and stop > ndim:
-            stop = ndim
+        elif isinstance(stop, types):
+            if stop > ndim:
+                stop = ndim
+            elif stop < 0:
+                stop += ndim
         if step is None:
             step = 1
 

theano/tensor/tests/test_basic.py

@@ -3354,7 +3354,13 @@ class T_GetVectorLength(unittest.TestCase):
         assert len(list(x.shape[1:4])) == 3
         assert len(list(x.shape[1:5])) == 3
         assert len(list(x.shape[1:10])) == 3
+        # Test step
         assert len(list(x.shape[1:10:2])) == 2
+        # Test neg start
+        assert len(list(x.shape[-1:4])) == 1
+        # test neg stop
+        assert len(list(x.shape[1:-2])) == 1
+        assert len(list(x.shape[1:-1])) == 2
 
 
 class T_Join_and_Split(unittest.TestCase):