code: def _defaultReducePartitions(self):
          
          if self.ctx._conf.contains("spark.default.parallelism"):
              return self.ctx.defaultParallelism
          else:
              return self.getNumPartitions()
docs: Returns the default number of partitions to use during reduce tasks (e.g., groupBy).
          If spark.default.parallelism is set, then we'll use the value from SparkContext
          defaultParallelism, otherwise we'll use the number of partitions in this RDD.
  
          This mirrors the behavior of the Scala Partitioner#defaultPartitioner, intended to reduce
          the likelihood of OOMs. Once PySpark adopts Partitioner-based APIs, this behavior will
          be inherent.