Overload __setitem__ method to for item setter operator
Overload __setitem__
__setitem__(self, key, value) stores key-value pair.
So the value can later be retrieved with __getitem__.
This method is only for mutable objects.
For a sequence, if the key is a negative integer, it should be used to count from the end.
In other words, treat x[-n] the same as x[len(x)-n].