python之else : after a try/except clause有什么用

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when is it necessary to add an else clause to a try..except in Python?
Python try-else

for arg in sys.argv[1:]: 
    try: 
        f = open(arg, 'r') 
    except IOError: 
        print 'cannot open', arg 
    else: 
        print arg, 'has', len(f.readlines()), 'lines' 
        f.close() 

这个else子句有什么用，什么时候执行？