'Loads oauth2 configuration in order of priority.
Priority:
1. Config passed to the constructor or init_app.
2. Config passed via the GOOGLE_OAUTH2_CLIENT_SECRETS_FILE app
config.
3. Config passed via the GOOGLE_OAUTH2_CLIENT_ID and
GOOGLE_OAUTH2_CLIENT_SECRET app config.
Raises:
ValueError if no config could be found.'

def _load_config(self, client_secrets_file, client_id, client_secret):

    if (client_id and client_secret):
        (self.client_id, self.client_secret) = (client_id, client_secret)
        return
    if client_secrets_file:
        self._load_client_secrets(client_secrets_file)
        return
    if ('GOOGLE_OAUTH2_CLIENT_SECRETS_FILE' in self.app.config):
        self._load_client_secrets(self.app.config['GOOGLE_OAUTH2_CLIENT_SECRETS_FILE'])
        return
    try:
        (self.client_id, self.client_secret) = (self.app.config['GOOGLE_OAUTH2_CLIENT_ID'], self.app.config['GOOGLE_OAUTH2_CLIENT_SECRET'])
    except KeyError:
        raise ValueError('OAuth2    configuration    could    not    be    found.    Either    specify    the    client_secrets_file    or    client_id    and    client_secret    or    set    the    app    configuration    variables    GOOGLE_OAUTH2_CLIENT_SECRETS_FILE    or    GOOGLE_OAUTH2_CLIENT_ID    and    GOOGLE_OAUTH2_CLIENT_SECRET.')
