return與exit的區別

xit()是一個函數
,結束一個進程，它將刪除進程使用的內存空間，同時把錯誤信息返回父進程，在父進程中wait系統調用將接受到此返回信息。


return返回函數值，是關鍵字

在main函數中我們通常使用return (0);這樣的方式返回一個值。

  但這是限定在非void情況下的也就是void main()這樣的形式。

  exit()通常是用在子程序中用來終結程序用的，使用後程序自動結束跳會操作系統。

  但在如果把exit用在main內的時候無論main是否定義成void返回的值都是有效的，並且exit不需要考慮類型，exit(1)等價於return (1)

exit(0);   //正常退出  
非0即是非正常退出
數字0,1,-1會被寫入環境變量ERRORLEVEL，其它程序可以由此判斷程序結束狀態。  
一般0爲正常推出，其它數字爲異常，其對應的錯誤可以自己指定。

 

Python main() functions

I've written a few main() functions in my time. They usually have a structure roughly like this:

"""Module docstring.

This serves as a long usage message.
"""
import sys
import getopt

def main():
    # parse command line options
    try:
        opts, args = getopt.getopt(sys.argv[1:], "h", ["help"])
    except getopt.error, msg:
        print msg
        print "for help use --help"
        sys.exit(2)
    # process options
    for o, a in opts:
        if o in ("-h", "--help"):
            print __doc__
            sys.exit(0)
    # process arguments
    for arg in args:
        process(arg) # process() is defined elsewhere

if __name__ == "__main__":
    main()

I'm sure many people write similar main() functions. I've got a few suggestions that make main() a little more flexible, especially as option parsing becomes more complex.

First, we change main() to take an optional 'argv' argument, which allows us to call it from the interactive Python prompt:

def main(argv=None):
    if argv is None:
        argv = sys.argv
    # etc., replacing sys.argv with argv in the getopt() call.

Note that we fill in the default for argv dynamically. This is more flexible than writing

def main(argv=sys.argv):
    # etc.

because sys.argv might have been changed by the time the call is made; the default argument is calculated at the time the main() function is defined, for all times.

Now the sys.exit() calls are annoying: when main() calls sys.exit(), your interactive Python interpreter will exit! The remedy is to let main()'s return value specify the exit status. Thus, the code at the very end becomes

if __name__ == "__main__":
    sys.exit(main())

and the calls to sys.exit(n) inside main() all become return n.

Another refinement is to define a Usage() exception, which we catch in an except clause at the end of main():

import sys
import getopt

class Usage(Exception):
    def __init__(self, msg):
        self.msg = msg

def main(argv=None):
    if argv is None:
        argv = sys.argv
    try:
        try:
            opts, args = getopt.getopt(argv[1:], "h", ["help"])
        except getopt.error, msg:
             raise Usage(msg)
        # more code, unchanged
    except Usage, err:
        print >>sys.stderr, err.msg
        print >>sys.stderr, "for help use --help"
        return 2

if __name__ == "__main__":
    sys.exit(main())

This gives the main() function a single exit point, which is preferable over multiple return 2 statements. This also makes it easier to refactor the argument parsing: raise Usage works just fine from inside a helper function, but return 2 would require careful passing on of the return values.

You might think that taking this to the extreme would move the try/except clause out of the main() function, into the code at the end of the module (if __name__ == "__main__": .... But that would mean that when you call main() interactively, you'd get a traceback for command line syntax errors, which isn't very helpful.

However, another generalization can be helpful: define another exception, perhaps called Error, which is treated just like Usage but returns 1. This can then be used for expected errors like failure to open necessary files, which are not command line syntax errors, but yet expected, and where again a traceback doesn't feel very friendly.

What's your favorite convention for writing main()?