code: def subtractByKey(self, other, numPartitions=None):
          
          def filter_func(pair):
              key, (val1, val2) = pair
              return val1 and not val2
          return self.cogroup(other, numPartitions).filter(filter_func).flatMapValues(lambda x: x[0])
docs: Return each (key, value) pair in C{self} that has no pair with matching
          key in C{other}.
  
          >>> x = sc.parallelize([("a", 1), ("b", 4), ("b", 5), ("a", 2)])
          >>> y = sc.parallelize([("a", 3), ("c", None)])
          >>> sorted(x.subtractByKey(y).collect())
          [('b', 4), ('b', 5)]