df = spark.sparkContext.parallelize([
    ("a", None), ("a", 1), ("a", -1), ("b", 3), ("b", 1)
]).toDF(["k", "v"])
w = Window().partitionBy("k").orderBy('k','v')

df.select(F.col("k"), F.last("v",True).over(w).alias('v')).show()

+---+----+
|  k|   v|
+---+----+
|  b|   1|
|  b|   3|
|  a|null|
|  a|  -1|
|  a|   1|
+---+----+

+---+----+
|  k|   v|
+---+----+
|  b|   3|
|  b|   3|
|  a|   1|
|  a|   1|
|  a|   1|
+---+----+

df.orderBy('k','v').show()
    +---+----+
    |  k|   v|
    +---+----+
    |  a|null|
    |  a|  -1|
    |  a|   1|
    |  b|   1|
    |  b|   3|
    +---+----+

df.orderBy('k','v').groupBy('k').agg(F.first('v')).show()