我正在阅读Zed Shaw的Python书，现在我正试图通过调整它来进一步练习16：
from sys import argv
script, filename = argv
print "We're going to erase %r." % filename
print "If you don't want that, hit CTRL-C (^C)."
print "If you do want that, hit RETURN."
raw_input("?")
print "Opening the file..."
target = open(filename, 'r+')
print "Displaying the file contents:"
print target.read()
print "Truncating the file. Goodbye!"
target.truncate()
print "Now i'm going to ask you for three lines."
line1 = raw_input("line 1: ")
line2 = raw_input("line 2: ")
line3 = raw_input("line 3: ")
print "I'm going to write these to the file."
target.write(line1)
target.write("\n")
target.write(line2)
target.write("\n")
target.write(line3)
target.write("\n")
print "And finally, we close it."
target.close()
请注意，我使用'r +'而不是'w'，因为我想在练习中读写，但是发生了一些事情：target.truncate（）不起作用，文件中的信息仍然存在。为什么程序以这种方式进行？
感谢您的时间。
P.S。我来自哥伦比亚，英语不是我的母语，所以请原谅我的错误。