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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 index(self, state, code=None, error=None):
 """ Receive a Exist response containing a verification code. Use the code to fetch the access_token. """

        error = None
        if code:
            try:
                auth_token = self.fetch_token(code, state)
            except MissingTokenError:
                error = self._fmt_failure(
                    'Missing access token parameter.</br>Please check that '
                    'you are using the correct client_secret')
            except MismatchingStateError:
                error = self._fmt_failure('CSRF Warning! Mismatching state')
        else:
            error = self._fmt_failure('Unknown error while authenticating')
        # Use a thread to shutdown cherrypy so we can return HTML first
        self._shutdown_cherrypy()
        return error if error else self.success_html % (auth_token)