re-enable get_scalar_constant_value() on Second when only the value is constant.

theano/tensor/basic.py

@@ -567,6 +567,12 @@ def get_scalar_constant_value(v):
         # mess with the stabilization optimization and be too slow.
         # We put all the scalar Ops used by get_canonical_form_slice()
         # to allow it to determine the broadcast pattern correctly.
+        if ((isinstance(v.owner.op, Elemwise) and
+             isinstance(v.owner.op.scalar_op, scal.Second)) or
+            isinstance(v.owner.op, scal.Second)):
+            # We don't need both input to be constant for second
+            shape, val = v.owner.inputs
+            return get_scalar_constant_value(val)
         elemwises = (scal.Cast, scal.Switch,
                      scal.NEQ, scal.EQ,
                      scal.LT, scal.GT, scal.LE, scal.GE,

theano/tensor/tests/test_basic.py

@@ -5943,6 +5943,13 @@ class T_get_scalar_constant_value(unittest.TestCase):
         s = tensor.second(c, .4)
         assert get_scalar_constant_value(s) == .4
 
+    def test_second(self):
+        #Second should apply when the value is constant but not the shape
+        c = theano.tensor.constant(numpy.random.rand())
+        shp = theano.tensor.vector()
+        s = theano.tensor.second(shp, c)
+        assert get_scalar_constant_value(s) == c.data
+
 
 class T_as_tensor_variable(unittest.TestCase):
     """