Saves pipeline as a Python source code file.
        
        Arguments:
        
          - filename(``path``) Path to save the pipeline source code.

def save(self, filename):
        """
        Saves pipeline as a Python source code file.
        
        Arguments:
        
          - filename(``path``) Path to save the pipeline source code.
          
        """
        handle = open(filename, 'wb')
        handle.write(P_LAY % self._code())
        handle.close()