Get or instantiate a SparkContext and register it as a singleton object.

        :param conf: SparkConf (optional)

def getOrCreate(cls, conf=None):
        """
        Get or instantiate a SparkContext and register it as a singleton object.

        :param conf: SparkConf (optional)
        """
        with SparkContext._lock:
            if SparkContext._active_spark_context is None:
                SparkContext(conf=conf or SparkConf())
            return SparkContext._active_spark_context