merged

theano/gradient.py

@@ -123,6 +123,8 @@ def grad_sources_inputs(sources, graph_inputs, warn_type=True):
 
 def unimplemented_grad(op, x_pos, x):
     """
+    DO NOT USE. Remove this function after all usage of it has been removed from theano.
+
     Return an un-computable symbolic variable of type `x.type`.
 
     If any function tries to compute this un-computable variable, an exception
@@ -130,5 +132,14 @@ def unimplemented_grad(op, x_pos, x):
     `x_pos`'th input of `op` has not been implemented.
 
     """
+
+    #raise Exception("""
+    #                    unimplemented_grad is not a safe function to use.
+    #                    It depends on catching errors at the run-time of a theano function.
+    #                    However, it could be removed by the optimization during the compilation
+    #                    of the theano function, for example, if it is multiplied by 0. This
+    #                    results in theano functions returning 0 for gradients that are actually
+    #                    undefined. """)
+
     msg = '%s.grad not implemented for input %i'%(op, x_pos)
     return Raise(msg=msg)(x)