How to generate error when argument are not supplied and there is no explicit defults (in optparse)?

Sat Oct 15 17:22:41 EDT 2011

On 10/14/2011 03:29 PM, Peng Yu wrote:
> Hi,
>> The following code doesn't give me error, even I don't specify the
> value of filename from the command line arguments. filename gets
> 'None'. I checked the manual, but I don't see a way to let
> OptionParser fail if an argument's value (which has no default
> explicitly specified) is not specified. I may miss some thing in the
> manual. Could any expert let me know if there is a way to do so?
> Thanks!
>> #!/usr/bin/env python
>> from optparse import OptionParser
>> usage = 'usage: %prog [options] arg1 arg2'
> parser = OptionParser(usage=usage)
> parser.set_defaults(verbose=True)
> parser.add_option('-f', '--filename')
>> #(options, args) = parser.parse_args(['-f', 'file.txt'])
> (options, args) = parser.parse_args()
>> print options.filename

You can check it yourself.
I find I use a pretty standard pattern with optparse:
 def main (args, opts):
 ...
 def parse_cmdline ():
 p = OptionParser()
 p.add_option('-f', '--filename')
 options, args = parser.parse_args()
 if not options.filename:
	p.error ("-f option required")
 if len (args) != 2:
	p.error ("Expected exactly 2 arguments")
 # Other checks can obviously be done here too.
 return args, options
 if __name__ == '__main__':
 args, opts = parse_cmdline()
 main (args, opts)
While one can probably subclass OptionParser or use callbacks
to achieve the same end, I find the above approach simple and
easy to follow.
I also presume you know that you have can optparse produce a
usage message by adding 'help' arguments to the add_option()
calls?
And as was mentioned in another post, argparse in Python 2.7
(or in earlier Pythons by downloading/installing it yourself)
can do the checking you want.