关于python：不理解sys模块的这种用法

do not understand this use of sys module

下面是一个教程中的python字节代码：

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import sys

filename = 'poem.txt'

def readfile(filename):
#Print a file to standard output
f = file(filename)
while True:
line = f.readline()
if len(line) == 0:
break
print line,
f.close()

if len(sys.argv) < 2:
print 'No action specified'
sys.exit() //<--This is where the error is occurring

if sys.argv[1].startswith('--'):
option = sys.argv[1][2:] #fetches sys.argv[1] without first 2 char
if option == 'version':
print 'Version 1.2'

elif option == 'help':
print '''\
This program prints files to the standard output.
Any number of files can be specified.
Options include:
--version: Prints the version number
--help: Displays this help'''

else:
print 'Unknown option'
sys.exit()

else:
for filename in sys.argv[1:]:
readfile(filename)

运行此代码时，出现以下错误：

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Traceback (most recent call last):
File"C:/Python/sysmodulepr.py", line 17, in <module>
sys.exit()
SystemExit

我不明白为什么。请帮忙。