'Retrieve Credential from datastore.
Returns:
oauth2client.Credentials'

@db.non_transactional(allow_existing=True)
def locked_get(self):

    credentials = None
    if self._cache:
        json = self._cache.get(self._key_name)
        if json:
            credentials = client.Credentials.new_from_json(json)
    if (credentials is None):
        entity = self._get_entity()
        if (entity is not None):
            credentials = getattr(entity, self._property_name)
            if self._cache:
                self._cache.set(self._key_name, credentials.to_json())
    if (credentials and hasattr(credentials, 'set_store')):
        credentials.set_store(self)
    return credentials
