Don't implement TensorVariable.__len__ to give better error message as this cause other bug.

theano/tensor/basic.py

@@ -1222,12 +1222,16 @@ class _tensor_py_operators:
     shape = property(lambda self: shape(self))
     size = property(lambda self: prod(self.shape))
 
-    def __len__(self):
-        # We can't implement __len__ as Python requests that this
-        # function returns an integer >=0
-        raise TypeError("Theano Variables can't work with len(Theano "
-                        "Variable) due to Python restriction. You can use "
-                        "TheanoVariable.shape[0] instead.")
+    # We can't implement __len__ to provide a better error message.
+    # Otherwise TensorVariable[:-1] don't work as Python 2.5.1 call
+    # __len__ before calling __getitem__. It also don't catch the raised
+    # Exception!
+#     def __len__(self):
+#         # We can't implement __len__ as Python requests that this
+#         # function returns an integer >=0
+#         raise Exception("Theano Variables can't work with len(Theano "
+#                         "Variable) due to Python restriction. You can use "
+#                         "TheanoVariable.shape[0] instead.")
 
     def reshape(self, shape, ndim=None):
         """Return a reshaped view/copy of this variable.