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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 setdefault(self, key, default):
 """If key is in the flat dictionary, return its value. If not, insert key with a value of default and return default. default defaults to ``None``. :param mixed key: The key name :param mixed default: The default value :rtype: mixed """

        if key not in self or not self.__getitem__(key):
            self.__setitem__(key, default)
        return self.__getitem__(key)