def METHOD_NAME(self):
    """fetch OAUH access token
    cf. https://docs.wootric.com/api/#authentication
    """
    response = requests.post(
        wootric_url('oauth/token'),
        data={
            'client_id': self.client_id,
            'client_secret': self.client_secret,
            'grant_type': 'client_credentials',
        },
    ).json()
    return {
        'access_token': response['access_token'],
        'expiration-date': datetime.now() + timedelta(seconds=int(response['expires_in'])),
    }