Don't subtitute compute_test_value error with not implemented!

theano/tensor/basic.py

@@ -1132,40 +1132,59 @@ class _tensor_py_operators:
     def __add__(self,other):
         try:
             return add(self,other)
-        except Exception, e:
+        # We should catch only NotImplementedError.
+        # As otherwise this will transfer error to the wrong error
+        # Theano flags compute_test_value need to be able to raise
+        # other type of error.
+        except NotImplementedError, e:
+            # We must return NotImplemented and not an
+            # NotImplementedError or raise an NotImplementedError.
+            # That way python will give a good error message like this
+            # `TypeError: unsupported operand type(s) for +:
+            # 'TensorVariable' and 'TensorVariable'`
             return NotImplemented
     def __sub__(self,other):
+        # See explanation in __add__ for the error catched
+        # adn the return value in that case
         try:
             return sub(self,other)
-        except Exception, e:
+        except NotImplementedError, e:
             return NotImplemented
     def __mul__(self,other):
+        # See explanation in __add__ for the error catched
+        # adn the return value in that case
         try:
             return mul(self,other)
-        except Exception, e:
+        except NotImplementedError, e:
             return NotImplemented
     def __div__(self,other):
+        # See explanation in __add__ for the error catched
+        # adn the return value in that case
         try:
             return div_proxy(self,other)
         except IntegerDivisionError:
             # This is to raise the exception that occurs when trying to divide
             # two integer arrays (currently forbidden).
             raise
-        except Exception, e:
+        except NotImplementedError, e:
             return NotImplemented
     def __pow__(self,other):
+        # See explanation in __add__ for the error catched
+        # adn the return value in that case
         try:
             return pow(self,other)
-        except Exception, e:
+        except NotImplementedError, e:
             return NotImplemented
     def __mod__(self,other):
+        # See explanation in __add__ for the error catched
+        # adn the return value in that case
         try:
             return mod_check(self, other)
         except ComplexError:
             # This is to raise the exception that occurs when trying to compute
             # x % y with either x or y a complex number.
             raise
-        except Exception, e:
+        except NotImplementedError, e:
             return NotImplemented
 
     def __truediv__(self,other): return true_div(self, other)