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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 access_token(self):
 """ WeChat access token """

        access_token = self.session.get(self.access_token_key)
        if access_token:
            if not self.expires_at:
                # user provided access_token, just return it
                return access_token

            timestamp = time.time()
            if self.expires_at - timestamp > 60:
                return access_token

        self.fetch_access_token()
        return self.session.get(self.access_token_key)