[issue41047] argparse: misbehavior when combining positionals and choices

[paul j3]

Change by paul j3 :


--
stage: patch review -> resolved
status: open -> closed

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue41047] argparse: misbehavior when combining positionals and choices

[paul j3]

paul j3  added the comment:

It is an old unresolved issue (which resurfaced a few months ago)

https://bugs.python.org/issue9625
argparse: Problem with defaults for variable nargs when using choices

--
resolution:  -> duplicate

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue41047] argparse: misbehavior when combining positionals and choices

[paul j3]

paul j3  added the comment:

The error is raised in ._get_values with this section:

# when nargs='*' on a positional, if there were no command-line
# args, use the default if it is anything other than None
elif (not arg_strings and action.nargs == ZERO_OR_MORE and
  not action.option_strings):
if action.default is not None:
value = action.default
else:
value = arg_strings
self._check_value(action, value)

An empty 'arg_strings' satisfies the nargs='*', so 'value' is set to that, and 
passed to _check_value.

We could avoid this by setting the default to one of the valid choices, e.g. 
'a'.  But then the attribute would be that string, not a list with that string.

I suspect this issue has come up before, since I don't this code has been 
changed in a long time.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue41047] argparse: misbehavior when combining positionals and choices

[James Corbett]

Change by James Corbett :


--
keywords: +patch
pull_requests: +20172
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/20997

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue41047] argparse: misbehavior when combining positionals and choices

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


--
nosy: +paul.j3, rhettinger

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue41047] argparse: misbehavior when combining positionals and choices

[James Corbett]

New submission from James Corbett :

The `argparse.ArgumentParser` sometimes rejects positional arguments with no 
arguments when `choices` is set and `nargs="*"`. 

When there are no arguments and `nargs` is `"*"`, the default value is chosen, 
or `[]` if there is no default value. This value is then checked against 
`choices` and an error message is printed if the value is not in `choices`. 
However, sometimes the value is intentionally not in `choices`, and this leads 
to problems. An example will explain this much better, and show that the issue 
only occurs with the particular combination of positionals, `nargs="*"`, and 
`choices`:

```
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument("foo", choices=["a", "b", "c"], nargs="*")
>>> parser.add_argument("--bar", choices=["d", "e", "f"], nargs="*")
>>> parser.add_argument('--baz', type=int, choices=range(5, 10), default="20")
>>> parser.parse_args("a --bar".split())
Namespace(foo=['a'], bar=[], baz=20)
>>> parser.parse_args(["a"])
Namespace(foo=['a'], bar=None, baz=20)
>>> parser.parse_args([])
usage: [-h] [--bar [{d,e,f} ...]] [--baz {5,6,7,8,9}] [{a,b,c} ...]
: error: argument foo: invalid choice: [] (choose from 'a', 'b', 'c')
```

In this case I could have got around the last error by adding `[]` to choices, 
but that pollutes the help and usage messages.

--
components: Library (Lib)
messages: 371915
nosy: jameshcorbett
priority: normal
severity: normal
status: open
title: argparse: misbehavior when combining positionals and choices
type: behavior
versions: Python 3.9

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com