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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 refresh(self, force=False):
 """Refreshes the `access_token` and sets the praw instance `reddit_client` with a valid one. :param force: Boolean. Refresh will be done only when last refresh was done before `EXPIRY_DURATION`, which is 3500 seconds. However passing `force` will overrides this and refresh operation will be done everytime. """

        if self._is_token_expired() or force:
            tokens = self._get_refresh_access()
            self.access_token = tokens['access_token']
            self.refresh_token = tokens['refresh_token']
            self._set_access_credentials()