def append_executable(self, executable):
        """Append san executable os command to the list to be called.

        Argument:
          executable (str): os callable 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)