pyspark os.getenv('SPARK_YARN_STAGING_DIR') can not get job path

when  i submit a spark job , use spark-submit and set option --files file1, 

then in python code, i use 

//代码占位符
path = str(os.environ["SPARK_YARN_STAGING_DIR"])

return path is None, but this can success in java code

//代码占位符
spark.read().textFile(System.getenv("SPARK_YARN_STAGING_DIR") + "/README.md")

which cause this problem.