argparse missing optparse capabilities?

Ian Kelly ian.g.kelly at
Thu Jan 5 13:46:02 EST 2012

On Thu, Jan 5, 2012 at 11:14 AM, Ian Kelly <ian.g.kelly at> wrote:
> On Thu, Jan 5, 2012 at 1:05 AM, rurpy at <rurpy at> wrote:
>> I have optparse code that parses a command line containing
>> intermixed positional and optional arguments, where the optional
>> arguments set the context for the following positional arguments.
>> For example,
>> arg1 -c33 arg2 arg3 -c44 arg4
>> 'arg1' is processed in a default context, 'args2' and 'arg3' in
>> context '33', and 'arg4' in context '44'.
>> I am trying to do the same using argparse but it appears to be
>> not doable in a documented way.
>> Here is the working optparse code (which took 30 minutes to write
>> using just the optparse docs):
>>  import optparse
>>  def append_with_pos (option, opt_str, value, parser):
>>        if getattr (parser.values, option.dest, None) is None:
>>            setattr (parser.values, option.dest, [])
>>        getattr (parser.values, option.dest).append ((value, len
>> (parser.largs)))
>>  def opt_parse():
>>        p = optparse.OptionParser()
>>        p.add_option ("-c", type=int,
>>            action='callback', callback=append_with_pos)
>>        opts, args = p.parse_args()
>>        return args, opts
>>  if __name__ == '__main__':
>>        args, opts = opt_parse()
>>        print args, opts
>> Output from the command line above:
>>  ['arg1', 'arg2', 'arg3', 'arg4'] {'c': [(33, 1), (44, 3)]}
>> The -c values are stored as (value, arglist_position) tuples.
>> Here is an attempt to convert to argparse using the guidelines
>> in the argparse docs:
>>  import argparse
>>  class AppendWithPos (argparse.Action):
>>    def __call__ (self, parser, namespace, values,
>> option_string=None):
>>        if getattr (namespace, self.dest, None) is None:
>>            setattr (namespace, self.dest, [])
>>        getattr (namespace, self.dest).extend ((values, len
>> (parser.largs)))
>>  def arg_parse():
>>        p = argparse.ArgumentParser (description='description')
>>        p.add_argument ('src', nargs='*')
>>        p.add_argument ('-c', type=int, action=AppendWithPos)
>>        opts = p.parse_args()
>>        return opts
>>  if __name__ == '__main__':
>>        opts = arg_parse()
>>        print opts
>> This fails with,
>>  AttributeError: 'ArgumentParser' object has no attribute 'largs'
>> and of course, the argparse.parser is not documented beyond how
>> to instantiate it.  Even were that not a problem, argparse complains
>> about "unrecognised arguments" for any positional arguments that
>> occur after an optional one.  I've been farting with this code for
>> a day now.
>> Any suggestions on how I can convince argparse to do what optparse
>> does easily will be very welcome.  (I tried parse_known_args() but
>> that breaks help and requires me to detect truly unknown arguments.)
>> (Python 2.7.1 if it matters and apologies if Google mangles
>> the formatting of this post.)
> You have the namespace object in your custom action.  Instead of
> "len(parser.largs)", couldn't you just do "len(namespace.src)"?

Sorry, I missed the second part of that.  You seem to be right, as far
as I can tell from tinkering with it, all the positional arguments
have to be in a single group.  If you have some positional arguments
followed by an option followed by more positional arguments, and any
of the arguments have a loose nargs quantifier ('?' or '*' or '+'),
then you get an error.

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