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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)