NameError：在pyspark累加器中未定义名称“ acc”

在pyspark中测试累加器，但出错了：

def test():
    conf = SparkConf().setAppName("test").setMaster("local[*]")
    sc = SparkContext(conf=conf).getOrCreate()
    rdds = sc.parallelize([Row(user="spark", item="book"), Row(user="spark", item="goods"),
                            Row(user="hadoop", item="book"), Row(user="python", item="duck")])

    acc = sc.accumulator(0)
    print("accumulator: {}".format(acc))

    def imap(row):
        global acc
        acc += 1
        return row

    rdds.map(imap).foreach(print)
    print(acc.value)

错误是：

...
return f(*args, **kwargs)
File "test_als1.py", line 205, in imap
acc += 1
NameError: name 'acc' is not defined

But I set the acc as global variable, how can I write the code?