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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 _set_attachments(self, value):
        '''
        A special set function to ensure
        we're setting with a list
        '''
        if value is None:
            setattr(self, '_PMMail__attachments', [])
        elif isinstance(value, list):
            setattr(self, '_PMMail__attachments', value)
        else:
            raise TypeError('Attachments must be a list')