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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 clear(self, key):
 # type: (Hashable) -> DataLoader """ Clears the value at `key` from the cache, if it exists. Returns itself for method chaining. """

        cache_key = self.get_cache_key(key)
        self._promise_cache.pop(cache_key, None)
        return self