[issue45656] argparse bug: prefix_chars argument to add_argument_group doesn't really work

[paul j3]

paul j3  added the comment:

prefix_chars is a parameter of the parent _ActionsContainer

class _ActionsContainer(object):

def __init__(self,
 description,
 prefix_chars,
 argument_default,
 conflict_handler):
super(_ActionsContainer, self).__init__()

self.description = description
self.argument_default = argument_default
self.prefix_chars = prefix_chars
self.conflict_handler = conflict_handler

add_argument is also a method in this class.  It uses its own prefix_chars to 
categorize arguments as optional/positional

chars = self.prefix_chars
if not args or len(args) == 1 and args[0][0] not in chars: 

see also _get_optional_kwargs method.

class _ArgumentGroup(_ActionsContainer):

inherits from _ActionsContainer, and usually gets the prefix_chars from its 
container (the parser)

update('prefix_chars', container.prefix_chars)

The parser is created with

class ArgumentParser(_AttributeHolder, _ActionsContainer):

and documents the use of prefix_chars.

In addition to passing prefix_chars to its Super, it uses

default_prefix = '-' if '-' in prefix_chars else prefix_chars[0]

to define the -h help argument.

Early in parsing, args strings are categorized as 'O' or 'A'.  That is done in, 
which uses:

def _parse_optional(self, arg_string):

# if it doesn't start with a prefix, it was meant to be positional
if not arg_string[0] in self.prefix_chars:
return None

def _get_option_tuples(self, option_string):

also uses the parser's own prefix_chars.

During parsing (several layers down)

def consume_optional(start_index):

# if the action is a single-dash option and takes no
# arguments, try to parse more single-dash options out
# of the tail of the option string
chars = self.prefix_chars
if arg_count == 0 and option_string[1] not in chars:

Here, the parser's own prefix_chars is used to handle the chained short-dash 
options.


---

In sum, while Argument_Group can have its own prefix_chars, that is only used 
for help formatting.  It's the parser's prefix_chars that is used when parsing. 
 The group's chars is ignored.

The _ArgumentGroup__init__  suggests something more than simple inheritance may 
have been intended for prefix_chars, but in practice all it affects is the 
help.  

I haven't seen this issue raised before, and since this group parameter is  not 
documented, I think it's safe to ignore it.  

An alternative is to have add_argument_group (or _ArgumentGroup__init__) 
explicitly reject the prefix_chars parameter.

--

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[issue45656] argparse bug: prefix_chars argument to add_argument_group doesn't really work

[paul j3]

paul j3  added the comment:

Use of 'prefix_chars' as a argument_group parameter is not documented, nor 
intended.  

If it does appear to work in the help formatting, it is probably the result of 
inheritance, since both ArgumentParser and Argument_group subclass a 
_Actions_Container class.  I'd have to examine the code to see what's 
happening.  Argument_groups are used only for help formatting, not for parsing.

In any case, I don't think anything needs to be changed or fixed.

--
nosy: +paul.j3

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[issue45656] argparse bug: prefix_chars argument to add_argument_group doesn't really work

[Samwyse]

New submission from Samwyse :

Using the prefix_chars argument to parser.add_argument_group causes usage 
information to print correctly, but the resulting parser doesn't recognize the 
options.  The included program reproduces the issue; it produces the following 
output on my system.

--- python version
3.6.8 (default, Sep 26 2019, 11:57:09)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-39)]
--- test using prefix_chars='-/' with ArgumentParser
--- show help info
usage: test-1 [-h] [-foo FOO] [/bar BAR]

optional arguments:
  -h, --help  show this help message and exit
  -foo FOO
  /bar BAR

--- try parsing something
Namespace(bar='b', foo='f')
--- test using prefix_chars='-/' with an argument group
--- show help info
usage: test-2 [-h] [-foo FOO] [/bar BAR]

optional arguments:
  -h, --help  show this help message and exit

other arguments:
  -foo FOO
  /bar BAR

--- try parsing something
usage: test-2 [-h] [-foo FOO] [/bar BAR]
test-2: error: unrecognized arguments: /bar b

--
components: Library (Lib)
files: argparser-bug.py
messages: 405217
nosy: samwyse
priority: normal
severity: normal
status: open
title: argparse bug: prefix_chars argument to add_argument_group doesn't really 
work
type: behavior
versions: Python 3.6
Added file: https://bugs.python.org/file50410/argparser-bug.py

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