code: def append_executable(self, executable):
          
          if isinstance(executable, str) and not isinstance(executable, unicode):
              executable = unicode(executable)
          if not isinstance(executable, unicode):
              raise TypeError("expected executable name as str, not {}".
                              format(executable.__class__.__name__))
          self._executables.append(executable)
docs: Append san executable os command to the list to be called.
  
          Argument:
            executable (str): os callable executable.