[issue31012] suggestion: allow termination argument in argparse to be specified as argument

[paul j3]

paul j3 added the comment:

The earlier issue for '--' as arguement

http://bugs.python.org/issue14364

With the patch I suggested there '-f--' and '--foo=--' work as arguments.  
'--foo --' is still interpreted as a positionals switch.

In optparse parsing, a flagged option gets all the remaining strings, and 
consume what it needs.  In the argparse parser, strings are classed as flag and 
argument and '--'.  The top level allocates strings to Actions.  '--' is 
handled at the top level.  This difference in parsing makes it impossible to 
complete replicate POSIX action.

See also http://bugs.python.org/issue13922

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[issue31012] suggestion: allow termination argument in argparse to be specified as argument

[Leon Avery]

Leon Avery added the comment:

The POSIX guideline suggests that -- should be available as an option argument. 
One should be able to say "command --opt --" in order to make '--' the value of 
the '--opt' option. 

> There are some known problems with this '--', specially when there are more 
> than one.

Yes. That is exactly the problem I have.

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[issue31012] suggestion: allow termination argument in argparse to be specified as argument

[paul j3]

paul j3 added the comment:

argparse roughly follows POSIX practice:

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html

Guideline 10:
The first -- argument that is not an option-argument should be 
accepted as a delimiter indicating the end of options. Any 
following arguments should be treated as operands, even if they 
begin with the '-' character.

I don't think this kind of thing is fixed, but there should be good reason to 
make an enhancement.  For many users argparse is already too complicated, and 
the docs are hard to understand.  Yet another parameter can get lost.

There are some known problems with this '--', specially when there are more 
than one.  It may also have problems when used with subparers, though I forget 
the details.

It wouldn't hard to substitute a CONST variable for this '--', and wouldn't 
require changes to the API.  

Another possibility is to change this '--' to 'parser.prefix_chars[0]*2'.  
Documenting that would be more work than implementing it.  And we'd have to add 
a unittest case.

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[issue31012] suggestion: allow termination argument in argparse to be specified as argument

[paul j3]

Changes by paul j3 :


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nosy: +paul.j3

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[issue31012] suggestion: allow termination argument in argparse to be specified as argument

[Steven D'Aprano]

Steven D'Aprano added the comment:

> This means that a user who wishes to use '--' in some other way is out of 
> luck. 

I think that's a feature, not a limitation.

Command line arguments should use broadly consistent conventions. I don't want 
one program to use '--' for "end of options" and another program to use it for 
"something else".

Suppose you want to use '--' for "foo", and '%%' (say) as the end of options 
terminator. Whatever foo is. Okay, just swap them -- keep the usual convention 
where '--' is the options terminator, and use '%%' for foo. That way your users 
won't be surprised and confused by your application working differently from 
the great bulk of commandline applications. (Users never read the manual.)

But maybe I'll change my mind if you can show that there's already a very 
common, and useful, alternative convention of '--' for foo, for some definition 
of foo.

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nosy: +steven.daprano

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[issue31012] suggestion: allow termination argument in argparse to be specified as argument

[Leon Avery]

New submission from Leon Avery:

In the argparse module, the argument '--' is interpreted as a signal that 
everything after it is a positional argument. '--' is literally written into 
the argparse code, in _parse_known_args. This means that a user who wishes to 
use '--' in some other way is out of luck. 

I suggest the addition of an argument to __init__ (or a method call, or 
something) that allows '--' to be replaced with some string of the user's 
choice.

--
components: Library (Lib)
messages: 298967
nosy: Leon Avery
priority: normal
severity: normal
status: open
title: suggestion: allow termination argument in argparse to be specified as 
argument
type: enhancement
versions: Python 3.6

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