Python main() functions by Guido van van Rossum

http://www.artima.com/weblogs/viewpost.jsp?thread=4829

All Things Pythonic

Python main() functions

by Guido van van Rossum

May 15, 2003

Summary

For Python programmers, I've got some suggestions on how to write a main() function that's easy to invoke in other contexts, e.g. from the interactive Python prompt when you feel like experimenting.

I've written a few main() functions in my time. They usually have astructure 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 fewsuggestions that make main() a little more flexible, especially asoption parsing becomes more complex.
First, we change main() to take an optional 'argv' argument, whichallows 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 moreflexible 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 isdefined, for all times.
Now the

sys.exit()

calls are annoying: when main()calls

sys.exit()

, your interactive Python interpreterwill exit! The remedy is to let main()'s return value specify the exitstatus. 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 catchin 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 ispreferable over multiple

return 2

statements. This alsomakes it easier to refactor the argument parsing:

raiseUsage

works just fine from inside a helper function, but

return 2

would require careful passing on of the returnvalues.
You might think that taking this to the extreme would move thetry/except clause out of the main() function, into the code at the endof the module (

if __name__ == "__main__": ...

. But thatwould mean that when you call main() interactively, you'd
get atraceback for command line syntax errors, which isn't very helpful.

However, another generalization can be helpful: defineanother exception, perhaps called

Error

, which is treatedjust like

Usage

but returns 1. This can then be used forexpected errors like failure to open necessary files, which are notcommand line syntax errors, but yet expected, and where again atraceback doesn't feel
very friendly.

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

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About the Blogger

Guido van Rossum is the creator of Python, one of the majorprogramming languages on and off the web. The Python community refers to him as the BDFL (Benevolent Dictator For Life), a title straightfrom a Monty Python skit. He moved from the Netherlands to the USA in1995, where he met his wife. Until July 2003 they lived in thenorthern Virginia suburbs of Washington, DC with their son Orlijn, whowas born in 2001. They then moved to Silicon Valley where Guido now works for Google(spending 50% of his time on Python!).

This weblog entry is Copyright © 2003 Guido van van Rossum. All rights reserved.

呵呵，关于正文后面段落的部分倒不是很理解