Python:无法写入CSV文件

我有这段Python代码:

import csv

def analyse(csvFileToRead, csvFileToWrite):
    # open file to read
    openedCsvFileToRead = open(csvFileToRead)
    reader = csv.reader(openedCsvFileToRead)

    # open file to write
    openedCsvFileToWrite = open(csvFileToWrite)
    writer = csv.writer(openedCsvFileToWrite)

    for row in reader:
        date = row[8]
        if date[0] == "5":
            writer.writerow(row)

    # close file
    openedCsvFileToRead.close()
    openedCsvFileToWrite.close()

if __name__ == "__main__":
    analyse("mydata.csv", "mynewdata.csv")

当使用Python 3.4运行时,我收到以下错误消息:

Traceback (most recent call last):
  File "main.py", line 40, in 
    analyse("mydata.csv", "mynewdata.csv")
  File "main.py", line 25, in analyse
    writer.writerow(row)
io.UnsupportedOperation: not writable

我究竟做错了什么？我在Windows 7 64bit上.