return a trace of parents and children of the obect

def format_all(self):
        """
        return a trace of parents and children of the obect
        """
        res = '\n--- Format all : ' + str(self.name) + ' -------------\n'
        res += ' parent = ' + str(self.parent) + '\n'
        res += self._get_all_children()    
        res += self._get_links()
            
        return res