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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 fetch(self, key: typing.Union[str, None], default_value=None):
 """ Retrieves the value of the specified variable from the cache :param key: The name of the variable for which the value should be returned :param default_value: The value to return if the variable does not exist in the cache :return: The value of the specified key if it exists in the cache or the default_Value if it does not """


        environ.abort_thread()

        if key is None:
            return self._shared_cache_data

        return self._shared_cache_data.get(key, default_value)