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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 _session_check(self):
 """Attempt to authenticate the user through a session file. This process is done to avoid having to authenticate the user every single time. It uses a session file that is saved when a valid session is captured and then reused. Because sessions can expire, we need to test the session prior to calling the user authenticated. Right now that is done with a test string found in an unauthenticated session. This approach is not an ideal method, but it works. """

        if not os.path.exists(SESSION_FILE):
            self._log.debug("Session file does not exist")
            return False
        with open(SESSION_FILE, 'rb') as f:
            cookies = requests.utils.cookiejar_from_dict(pickle.load(f))
            self._session.cookies = cookies
            self._log.debug("Loaded cookies from session file")
        response = self._session.get(url=self.TEST_URL, headers=self.HEADERS)
        if self.TEST_KEY in str(response.content):
            self._log.debug("Session file appears invalid")
            return False
        self._is_authenticated = True
        self._process_state()
        return True