def fetch_access_token(self, oauth_request):
        """Processes an access_token request and returns the
        access token on success.
        """
        version = self._get_version(oauth_request)
        consumer = self._get_consumer(oauth_request)
        try:
            verifier = self._get_verifier(oauth_request)
        except OAuthError:
            verifier = None
        # Get the request token.
        token = self._get_token(oauth_request, 'request')
        self._check_signature(oauth_request, consumer, token)
        new_token = self.data_store.fetch_access_token(consumer, token, verifier)
        return new_token
Processes an access_token request and returns the
        access token on success.
        
twitter/oauth.py