The __setslice__ method

__setslice__(self, i, j, sequence)

Called to implement assignment to self[i:j]. Same notes for i and j as for __getslice__().

This method is deprecated. If no __setslice__() is found, or for extended slicing of the form self[i:j:k], a slice object is created, and passed to __setitem__(), instead of __setslice__() being called.