Re: optparser question

[Steven Bethard]

Michele Petrazzo wrote:
 I'm trying optparse and I see a strange (for me) behavior:
 
 def store_value(option, opt_str, value, parser):
 setattr(parser.values, option.dest, value)
 
 parser = optparse.OptionParser()
 parser.add_option(-f, --foo,
   action=callback, callback=store_value,
   type=int, dest=foo)
 
 args = [-f, 1]
 (options, args) = parser.parse_args(args)
 print options, args
 
 {'foo': 1} [] # with the type
 {'foo': None} ['1'] #without it
 
 If I not specify the type in add_options, the value aren't passed to the
 store_value (into value variable), but it's understood as args!
 If I specify it, it
 
 Is this normal?

I believe so.  The optparse module lists 'callback' as one of the 
TYPED_ACTIONS but not one of the ALWAYS_TYPED_ACTIONS, so it should only 
set nargs=1 if a type= argument was provided.  That means that callbacks 
will be assumed to take zero arguments until you pass an nargs= or a 
type= argument.

You can try using argparse_, which doesn't make these weird inferences, 
and generally assumes that your action will take a single argument 
unless you specify otherwise::

  import argparse
  class StoreValue(argparse.Action):
 ... def __call__(self, parser, namespace, value, opt_str=None):
 ... setattr(namespace, self.dest, value)
 ...
  parser = argparse.ArgumentParser()
  parser.add_argument('-f', '--foo', action=StoreValue)
  parser.parse_args('-f 1'.split())
 Namespace(foo='1')

  parser = argparse.ArgumentParser()
  parser.add_argument('-f', '--foo', action=StoreValue, type=int)
  parser.parse_args('-f 1'.split())
 Namespace(foo=1)

Not sure exactly what your callback was trying to do though -- it seems 
like you're just duplicating the 'store' action (which is the default 
anyway).

FWIW, if you want to write a custom action in argparse, you don't have 
to worry about the weird TYPED_ACTIONS kinds of things in optparse. 
Just specify in the action's constructor whatever defaults you need, e.g.::

  class MyStoreValue(argparse.Action):
 ... def __init__(self, nargs=2, type=int, **kwargs):
 ... superinit = super(MyStoreValue, self).__init__
 ... superinit(nargs=nargs, type=type, **kwargs)
 ... def __call__(self, parser, namespace, value, opt_str=None):
 ... setattr(namespace, self.dest, value)
 ...
  parser = argparse.ArgumentParser()
  parser.add_argument('-f', '--foo', action=MyStoreValue)
  parser.parse_args('-f 1 2'.split())
 Namespace(foo=[1, 2])

Of course, you can always specify nargs= and type= in the 
``add_argument()`` call too.

.. _argparse: http://argparse.python-hosting.com/

STeVe
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Re: optparser question

[Michele Petrazzo]

Steven Bethard wrote:
 You can try using argparse_, which doesn't make these weird 
 inferences, and generally assumes that your action will take a single
  argument unless you specify otherwise::
 

-cut-

 Not sure exactly what your callback was trying to do though -- it 
 seems like you're just duplicating the 'store' action (which is the 
 default anyway).

I made only a copy from the python docs for show the problem.

Ok, I have not understand the trickle for transform the
action=callback and provide a callback to a new action.

I believe, however, that the doc has to be more explicit about this
strange behavior, because a not so expert dev (like me :) ), can don't
understand at the first time, it.

 STeVe

Thanks,
Michele
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Re: optparser question

[Steven Bethard]

Michele Petrazzo wrote:
 Ok, I have not understand the trickle for transform the
 action=callback and provide a callback to a new action.

Yeah, you're not the only one. ;-)

 I believe, however, that the doc has to be more explicit about this
 strange behavior, because a not so expert dev (like me :) ), can don't
 understand at the first time, it.

Yeah, it's pretty complicated. I didn't really understand it until I 
wrote argparse_ to replace the optparse module. If you have a good idea 
for how to improve the documentation, I'm sure the maintainers would be 
grateful. You can post your suggestion here:

 http://sourceforge.net/tracker/?group_id=5470atid=105470

You don't need to know LaTeX or anything -- just a simple snippet of 
text and where in the docs you think it should go is all they need.


.. _argparse: http://argparse.python-hosting.com/

STeVe
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