Allow slicing of tensor variables to return symbolic shape refs

theano/tensor/basic.py

@@ -5,6 +5,7 @@ import warnings
 
 import numpy
 from six.moves import xrange
+import numbers
 
 import theano
 from theano.compat import izip
@@ -3922,15 +3923,22 @@ def get_vector_length(v):
         return len(v.owner.inputs)
     if v.owner and isinstance(v.owner.op, Shape):
         return v.owner.inputs[0].type.ndim
-    # If we take this slice: var[:0], we know it will have 0 elements.
+    # If we take a slice, we know how many elements it will result in
     if ((v.owner and
          isinstance(v.owner.op, theano.tensor.subtensor.Subtensor) and
-         isinstance(v.owner.op.idx_list[0], slice) and
-         v.owner.op.idx_list[0].start in [None, 0])):
-        stop = theano.tensor.subtensor.get_idx_list(
-            v.owner.inputs, v.owner.op.idx_list)[0].stop
-        if extract_constant(stop) == 0:
-            return 0
+         isinstance(v.owner.op.idx_list[0], slice))):
+        start = extract_constant(theano.tensor.subtensor.get_idx_list(
+            v.owner.inputs, v.owner.op.idx_list)[0].start)
+        stop = extract_constant(theano.tensor.subtensor.get_idx_list(
+            v.owner.inputs, v.owner.op.idx_list)[0].stop)
+        if start is None:
+            start = 0
+        if stop is None:
+            stop = 0
+        if ((isinstance(stop, numbers.Integral) and
+             isinstance(start, numbers.Integral))):
+            return stop - start
+
     raise ValueError("length not known")