Better error message and simplicy code,doc.

theano/configparser.py

@@ -105,7 +105,7 @@ class change_flags(object):
         for k in args:
             l = [v for v in theano.configparser._config_var_list
                  if v.fullname == k]
-            assert len(l) == 1
+            assert len(l) == 1, l
             confs[k] = l[0]
         self.confs = confs
         self.new_vals = args

theano/gof/fg.py

@@ -482,7 +482,8 @@ class FunctionGraph(utils.object2):
                     print_type=True,
                     used_ids=used_ids).getvalue()
                 raise toolbox.BadOptimization(
-                    r, new_r, None, None, reason, old, new)
+                    r, new_r, None, None, str(reason) +
+                    ". The type of the replacement must be the same.", old, new)
             new_r = new_r2
         if r not in self.variables:
             # this variable isn't in the graph... don't raise an

theano/gof/toolbox.py

@@ -41,7 +41,8 @@ class BadOptimization(Exception):
     Exception: some variable and its substitute take different runtime values.
 
     Note: If there is only 1 parameter and it is a string, we will use
-    it as the error message.
+    it as the error message. This is needed when we catch, extend and
+    reraise an error.
 
     """
 
@@ -103,6 +104,8 @@ class BadOptimization(Exception):
         self.reason = reason
         self.old_graph = old_graph
         self.new_graph = new_graph
+
+        # To allow extending the error message of an existing error.
         self.full_err = None
         if isinstance(old_r, str):
             assert (new_r is None and old_r_val is None and new_r_val is None and
@@ -110,8 +113,6 @@ class BadOptimization(Exception):
             self.full_err = old_r
 
     def __str__(self):
-        if getattr(self, 'full_err', None) is not None:
-            return self.full_err
         return self.str_diagnostic()
 
     def str_diagnostic(self):
@@ -120,6 +121,9 @@ class BadOptimization(Exception):
         of the exception.
 
         """
+        # We have a pre-made message
+        if getattr(self, 'full_err', None) is not None:
+            return self.full_err
         sio = StringIO()
         val_str_len_limit = 800
         print("BadOptimization Error", super(BadOptimization,