I finally resolved it with a little research. Apparently the error had to do something with the __slots__ attribute's conflicts which is used when allocating heap memory for the new type.Here are the changes I made :
from traits.api import HasTraits
from PyQt4 import QtCore, QtGui, uic
class Main_Excel_Class_Meta(type(HasTraits), type(QtGui.QMainWindow)):
pass
class HasTraits(QtGui.QMainWindow):
pass
class Main_Excel_Class(HasTraits):
__metaclass__ = Main_Excel_Class_Meta
For a better understanding of the working I had suggest you check this post