improve exception message for nargs=0 about argparse HOT 9 CLOSED

Providing a patch to fix this problem. Apparently support for nargs=0 was 
already in, 
but there was a conditional preventing it from actually being used. Tested and 
it works 
great!

If it's just a flag, why don't you specify action='store_true'? Otherwise, 
you're
getting the default action='store', which expects a value to store.

So maybe that means that nargs=0 needs to override the value of 'action' and 
force it 
to 'store_true', or throw an exception if it is anything other than 
'store_true'. In 
any case, I think it makes perfect sense for nargs to accept 0 as a value 
(Especially 
since the code already has support for it). Also, will specifying 
action="store_true" 
affect the help output? If not, that is another reason why I do not find your 
proposed workaround acceptable.

Thanks for the info, though! I can extend the patch to address your concerns if 
you'd 
like. Would you prefer it throw an exception or silently override the value of 
'action'?

'store_true' sets nargs=0 and so yes, it does display the way you want. What do 
you
want nargs=0 to mean for the plain 'store' action? What should it store?

Hmm after studying the code a bit more, I think I understand where I'm getting 
confused.

nargs=0 has many different meanings as far as actions go. Many actions set 
nargs to 
0, so it would be impossible to assume the user meant StoreTrueAction, or 
StoreFalseAction.

I think an appropriate fix for this would be to make the error message for 
nargs=0 a 
bit more helpful. For example:

pa.add_argument( '--my-option', nargs=0 )

This should spit out:

You specified nargs=0 for the action 'store', which is not valid. The following 
actions can be specified which will set nargs to 0 for you:
  store_const
  store_true
  store_false
  ...

This would be very much more helpful! It may seem a bit over-kill for such a 
narrow, 
insignificant issue, but it will help redirect people (like me) in the right 
direction when their intention was simply to make an option take no arguments.

Thanks for hearing me out!

Yeah, I think that's a reasonable request. It could at least say something like
"nargs for store action must be > 0; if you have nothing to store, you might 
consider
other actions such as store_true". That would be an improvement.

(I don't really want to list all the possible actions because that will 
probably get
out of sync, and I suspect that folks who try specifying nargs=0 probably wanted
action='store_true'.)

Correct, it could get out of sync.. but in my example (which was entirely an 
example 
and very over-engineered) you would use string formatting to input the valid 
action 
types, that way they never are out of sync. However, your version is perfectly 
acceptable and is indeed an improvement.

Thanks for taking care of this! argparse is great, I hope it gets accepted into 
the 
standard library (optparse sucks).

Error messages improved as of r47:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', nargs=0)
Traceback (most recent call last):
  ...
ValueError: nargs for store actions must be > 0; if you have nothing to store,
actions such as store true or store const may be more appropriate