Add tests and don't apply in other cases

theano/tensor/basic.py

@@ -4259,6 +4259,8 @@ def get_vector_length(v):
             start = 0
         elif isinstance(start, types) and start < 0:
             start += ndim
+            if start < 0:
+                start = 0
         if stop is None:
             stop = ndim
         elif isinstance(stop, types):
@@ -4271,7 +4273,9 @@ def get_vector_length(v):
 
         if (isinstance(stop, types) and
                 isinstance(start, types) and
-                start >= 0 and stop >= 0):
+                isinstance(step, types) and
+                start >= 0 and stop >= 0 and
+                step > 0 and stop >= start):
             return (stop - start - 1) // step + 1
     if isinstance(v, Variable):
         msg = theano.printing.debugprint(v, file='str')

theano/tensor/tests/test_basic.py

@@ -3352,12 +3352,14 @@ class T_GetVectorLength(unittest.TestCase):
         assert len(list(x.shape[2:4])) == 2
         assert len(list(x.shape[2:])) == 2
         assert len(list(x.shape[1:4])) == 3
+        assert len(list(x.shape[2:2])) == 0
         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
+        assert len(list(x.shape[-6:4])) == 4
         # test neg stop
         assert len(list(x.shape[1:-2])) == 1
         assert len(list(x.shape[1:-1])) == 2