TypeError：“ NoneType”对象在Python中不可迭代

Question

错误TypeError: 'NoneType' object is not iterable是什么意思？

我在以下Python代码上得到它：

def write_file(data, filename): # creates file and writes list to it

  with open(filename, 'wb') as outfile:

    writer = csv.writer(outfile)

    for row in data: # ABOVE ERROR IS THROWN HERE

      writer.writerow(row)

Answer 1

表示datais 的值None。