Output the graph in filename
            Args:
                filename(string)

def output(self, filename):
        """
            Output the graph in filename
            Args:
                filename(string)
        """
        if filename == '':
            filename = 'contracts.dot'
        if not filename.endswith('.dot'):
            filename += ".dot"
        info = 'Inheritance Graph: ' + filename
        self.info(info)
        with open(filename, 'w', encoding='utf8') as f:
            f.write('digraph "" {\n')
            for c in self.contracts:
                f.write(self._summary(c))
            f.write('}')