Fix IfElse.grad when true or false condition contain ints. fix gh-4471

theano/ifelse.py

@@ -213,17 +213,21 @@ class IfElse(PureOp):
                              gpu=self.gpu,
                              name=nw_name_f)
 
-        if_true = ([ins[0]] + grads + [theano.tensor.zeros_like(t)
-                                       for t in ts])
-        if_false = ([ins[0]] + [theano.tensor.zeros_like(f)
-                                for f in fs] + grads)
+        # The grads can have a different type then the inputs.
+        # As all condition must have the same dtype, we must
+        dtype = grads[0].dtype
+        if_true = ([ins[0]] +
+                   grads +
+                   [theano.tensor.zeros_like(t, dtype=dtype) for t in ts])
+        if_false = ([ins[0]] +
+                    [theano.tensor.zeros_like(f, dtype=dtype) for f in fs] +
+                    grads)
 
         condition = ins[0]
         # condition does affect the elements of the output so it is connected.
         # For the sake of making the gradient convenient we assume that
         # condition + epsilon always triggers the same branch as condition
         condition_grad = condition.zeros_like().astype(theano.config.floatX)
-
         return ([condition_grad] +
                 if_true_op(*if_true, **dict(return_list=True)) +
                 if_false_op(*if_false, **dict(return_list=True)))

theano/tests/test_ifelse.py

@@ -482,6 +482,21 @@ class test_ifelse(unittest.TestCase, utt.TestOptimizationMixin):
         finally:
             theano.config.compute_test_value = backup
 
+    def test_grad_int_value(self):
+        w = theano.shared(numpy.random.rand(10))
+        b = theano.shared(numpy.random.rand())
+        params = [w, b]
+
+        x = tensor.vector()
+        y = tensor.scalar()
+
+        score = w.dot(x) + b
+        correct = (score * y > 0)
+
+        loss = ifelse(correct, 0, 1)
+        updates = [(param, param - 0.5 * tensor.grad(cost=loss, wrt=param))
+                   for param in params]
+
 
 if __name__ == '__main__':
     print(' Use nosetests to run these tests ')