code: def sampleByKey(self, withReplacement, fractions, seed=None):
          
          for fraction in fractions.values():
              assert fraction >= 0.0, "Negative fraction value: %s" % fraction
          return self.mapPartitionsWithIndex(
              RDDStratifiedSampler(withReplacement, fractions, seed).func, True)
docs: Return a subset of this RDD sampled by key (via stratified sampling).
          Create a sample of this RDD using variable sampling rates for
          different keys as specified by fractions, a key to sampling rate map.
  
          >>> fractions = {"a": 0.2, "b": 0.1}
          >>> rdd = sc.parallelize(fractions.keys()).cartesian(sc.parallelize(range(0, 1000)))
          >>> sample = dict(rdd.sampleByKey(False, fractions, 2).groupByKey().collect())
          >>> 100 < len(sample["a"]) < 300 and 50 < len(sample["b"]) < 150
          True
          >>> max(sample["a"]) <= 999 and min(sample["a"]) >= 0
          True
          >>> max(sample["b"]) <= 999 and min(sample["b"]) >= 0
          True