[issue43192] Argparse complains argument required when default is provided

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> Specifying a 'default' with a positional doesn't make much sense.

That was my thought as well.  Perhaps that is worth a note in the docs but 
there is likely no real need to change the behavior.

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nosy: +rhettinger

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[issue43192] Argparse complains argument required when default is provided

[paul j3]

paul j3  added the comment:

To a large degree the Action parameters operate independently.  That is, 
different parts of the code use the parameters for their own purposes. The 
composite behavior is a result of those individual actors, rather than some 
sort of overall coordinated plan.

First your Action is 'positional' (no flag string), and "store" type.  nargs is 
the default None.  The only extra is the default value (the default default is 
None).

_ActionsContainer._get_positional_kwargs processes such an argument, setting 
the 'required' parameter with:

# mark positional arguments as required if at least one is
# always required
if kwargs.get('nargs') not in [OPTIONAL, ZERO_OR_MORE]:
kwargs['required'] = True
if kwargs.get('nargs') == ZERO_OR_MORE and 'default' not in kwargs:
kwargs['required'] = True

At the start of parsing, all 'default' values are added to the 'args' Namespace.

If an Action is "seen", either via the appropriate flag string, or a positional 
value, it is added to the "seen_actions" list.

At the end of parsing, it iterates through all the '_actions'.  If it is not in 
the 'seen_actions' list, and is marked "required", it gets added to the 
'reqiuired_action' error list.

So even though your Action has a 'default' it is still required.  The default 
is, in effect, ignored.  The nargs value (default or not) has priority in 
setting the 'required' value.

'?/*' nargs require special handling.  This nargs requirement is satisfied by 
an empty list.  Such a positional will, in effect, always be seen. But if a 
explicit default is provided, that will over write the empty list. 

Handling defaults is inherently tricky.  But except for the special '?/*' case, 
specifying a 'default' with a positional doesn't make much sense.

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nosy: +paul.j3

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[issue43192] Argparse complains argument required when default is provided

[Clint Olsen]

Clint Olsen  added the comment:

I think your original suggestion makes sense: '?'

My intuition is that nargs helps argparse discern whether it's dealing with 
single or multiple values. That may not be what was intended.

There probably shouldn't be multiple ways of indicating an argument is 
optional. It seems that required= and nargs='?'|'*' combinations could lead to 
conflicting requirements.

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[issue43192] Argparse complains argument required when default is provided

[Eric V. Smith]

Eric V. Smith  added the comment:

Would you infer it to be ‘?’ or ‘*’?

I’m not completely opposed, but I haven’t thought it through or looked at the 
code.

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[issue43192] Argparse complains argument required when default is provided

[Clint Olsen]

Clint Olsen  added the comment:

Do you think it's unreasonable/unintuitive to infer nargs from the default 
value?

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[issue43192] Argparse complains argument required when default is provided

[Eric V. Smith]

Eric V. Smith  added the comment:

Providing a default value does not imply setting nargs, so you need to specify 
both. I think it's not clear what nargs would default to, if you wanted to 
change the behavior if default= is supplied.. In your example you probably want 
nargs='?', not '*'.

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nosy: +eric.smith

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[issue43192] Argparse complains argument required when default is provided

[Clint Olsen]

Clint Olsen  added the comment:

Sorry, I meant to say args.foo should be 'bar'.

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[issue43192] Argparse complains argument required when default is provided

[Clint Olsen]

New submission from Clint Olsen :

When I run the following program, I expect args.run to be 'foo' if no argument 
is specified on the command-line.

parser = argparse.ArgumentParser()

parser.add_argument('foo', default='bar')

args = parser.parse_args()

$ ./test
usage: test [-h] foo
test: error: the following arguments are required: foo

However if I specify nargs='*' this error goes away.

Maybe I'm missing something obvious, but this seems non-intuitive to me.

--
components: Library (Lib)
messages: 386770
nosy: Clint Olsen
priority: normal
severity: normal
status: open
title: Argparse complains argument required when default is provided
type: behavior
versions: Python 3.8

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