R_op for print and a test for it

theano/printing.py

@@ -140,6 +140,9 @@ class Print(Op):
     def grad(self, input, output_gradients):
         return output_gradients
 
+    def R_op(self, inputs, eval_points):
+        return eval_points
+
     def __eq__(self, other):
         return (type(self) == type(other) and self.message == other.message
                 and self.attrs == other.attrs)

theano/tensor/tests/test_rop.py

@@ -216,6 +216,10 @@ class test_RopLop(RopLop_checker):
         out = tensor.set_subtensor(self.x[:3], t)
         self.check_rop_lop(out, self.in_shape)
 
+    def test_print(self):
+        out = theano.printing.Print('x',attrs=('shape',))(self.x)
+        self.check_rop_lop(out, self.in_shape)
+
 
     def test_setsubtensor2(self):
         tv = numpy.asarray( self.rng.uniform(size=(10,)),