Transformations:

foldbykey() same as fold, with zero value with the data type to be returned.

groupbykey() grouping all data of same key.

*** rdd.reduceByKey(func) produces the same RDD as rdd.groupBy Key().mapValues(value => value.reduce(func)) but is more
efficient as it avoids the step of creating a list of values for each key.

sortbykey()

Custom sort order in Python, sorting integers as if strings

rdd.sortByKey(ascending=True, numPartitions=None, keyfunc = lambda x: str(x))