setting mouse position

Can someone help me with this:

I am trying to modify a wxGrid component after an update to a cell.
Here is the code:
(snip)
    def GetValue(self, row, col):
        return self.__rows[row][col]

    def SetValue(self, row, col, valstr):
        # Implement this when we want to modify data
        stmt = "Update " + self.tableName + " set " +
self._colNames[col] + " = '" + valstr + "' where " + self._colNames[0] +
'=' + str(self.__rows[row][0])
        #print stmt
        #print 'update:', self.__rows[row][col],':',valstr
        #Update the database!
        x=self.__db.executeSQL(stmt)
        #Update the grid
        #xx=self.__SetValue(self, row, col, valstr)
        self.__rows[row][col] = valstr
        return True
(snip)

When I modify the cell's value it updates the Database correctly but I
get the following message:
.. line 54, in SetValue
    self.__rows[row][col] = valstr
TypeError: object doesn't support item assignment