code: def output(self, filename):
          
          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('}')
docs: Output the graph in filename
              Args:
                  filename(string)