Pysparkで別の列に基づいた式の評価に基づいて、列の値を条件付きで置き換えるには？

import numpy as np

df = spark.createDataFrame(
    [(1, 1, None),
     (1, 2, float(5)),
     (1, 3, np.nan),
     (1, 4, None),
     (0, 5, float(10)),
     (1, 6, float('nan')),
     (0, 6, float('nan'))],
    ('session', "timestamp1", "id2"))

+-------+----------+----+
|session|timestamp1| id2|
+-------+----------+----+
|      1|         1|null|
|      1|         2| 5.0|
|      1|         3| NaN|
|      1|         4|null|
|      0|         5|10.0|
|      1|         6| NaN|
|      0|         6| NaN|
+-------+----------+----+

+-------+----------+----+
|session|timestamp1| id2|
+-------+----------+----+
|      1|         1|null|
|      1|         2| 5.0|
|      1|         3| NaN|
|      1|         4|null|
|      0|       999|10.0|
|      1|         6| NaN|
|      0|       999| NaN|
+-------+----------+----+