"""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 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
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