[issue37510] argparse removing more "--" than it should

[paul j3]

paul j3  added the comment:

https://bugs.python.org/file29845/dbldash.patch

while written against an earlier version of `argparse`, does what you want.  I 
moved the '--' removal out of `_get_values` and into `consume_positionals`.

Note that a full patch should include test cases and documentation changes if 
any.  

Also now github pull requests are the preferred patching route.  But don't 
expect rapid response on this.  The issue has been around for a long time 
without causing too many complaints.  

Backward compatibility is always a concern when making changes to core 
functionality.  We don't want to mess with someone's working code. 
 Though I doubt if there are many users who count on multiple '--' removals, 
one way or other.  The argparse docs explicity calls this a 'pseudo-argument'.

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[issue37510] argparse removing more "--" than it should

[Jorge L. Martinez]

Jorge L. Martinez  added the comment:

Maybe I can find the time to make a patch this weekend (either today or 
tomorrow). I hope I'm not underestimating this somehow, but I don't think this 
would take too long. The only issue I can foresee is in disagreement of what 
the correct behavior should be, which is why I gave my opinion that a single 
call of parse_args() should only ever remove a single "--".

If I don't submit a patch by Monday (PDT), everyone should assume I decided not 
to tackle this.

By the way, does this issue tracking platform support submitting to the issue 
thread by email? Maybe, I'll try that.

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[issue37510] argparse removing more "--" than it should

[paul j3]

paul j3  added the comment:

I looked at this issue way back, in 2013:

https://bugs.python.org/issue13922

I probably shouldn't have tacked this on to a closed issue.

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[issue37510] argparse removing more "--" than it should

[Jorge L. Martinez]

Jorge L. Martinez  added the comment:

> to remove the "--" present in arg_strings

*to remove the first "--" present...

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[issue37510] argparse removing more "--" than it should

[Jorge L. Martinez]

Jorge L. Martinez  added the comment:

> There are earlier bug/issues about the '--'.

Yes, there are:

https://bugs.python.org/issue9571
https://bugs.python.org/issue3
https://bugs.python.org/issue14364

But this one seems separate. Though they're related, they don't seem like 
duplicates, so that's why I thought I'd make this one.

> Also look at the parser code itself.  Keep in mind that parsing is done in 
> two passes - once to identify flags versus arguments ('O/A') and then to 
> allocate strings to arguments. I don't recall when '--' is being handled, 
> possibly in both.

I'm not sure what your point is here. I did take a quick look at the code, 
yesterday. I think the identification part you mention is done right. It marks 
'O/A' as you mention. Then, when it sees "--", it marks it "-", and the rest of 
the arguments are marked as "A".

I didn't look at the start of the code of the second pass or how it is 
concretely linked to the first pass. However, I did see that in the example I 
gave, _get_values() in argparse.py gets called twice (apparently once per 
argument group as determined by nargs, I guess) to remove the "--" present in 
arg_strings. The first time, arg_strings is ["foo", "--"] and the second time 
it's ["bar", "--", "baz", "--", "zap"].

So, that's what happens, and where part of the fix should probably be. I don't 
think the removal of "--" should happen in a function that gets called multiple 
times. Though I didn't spend the time to see where the code should be 
positioned, I can only imagine the correct behavior would be to remove the 
argument marked as "-" by the first pass mentioned.

I didn't mention this yesterday, because I figured there wouldn't be much value 
in sharing incomplete research like this, as opposed to a patch. I didn't want 
to influence the work of whoever chose to invest time in this for a proper fix.

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[issue37510] argparse removing more "--" than it should

[paul j3]

paul j3  added the comment:

There are earlier bug/issues about the '--'.  

Also look at the parser code itself.  Keep in mind that parsing is done in two 
passes - once to identify flags versus arguments ('O/A') and then to allocate 
strings to arguments.  I don't recall when '--' is being handled, possibly in 
both.

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[issue37510] argparse removing more "--" than it should

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


--
nosy: +paul.j3

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[issue37510] argparse removing more "--" than it should

[Jorge L. Martinez]

Jorge L. Martinez  added the comment:

To be clear, my opinion is that a single call of parse_args() should only ever 
remove the first "--". Right now, it seems that it removes the first of each 
argument group, as determined by nargs, I guess.

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[issue37510] argparse removing more "--" than it should

[Eric V. Smith]

Change by Eric V. Smith :


--
nosy: +eric.smith

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[issue37510] argparse removing more "--" than it should

[Jorge L. Martinez]

Jorge L. Martinez  added the comment:

Sorry, I forgot to add details on my machine.

Python: 3.7.3
OS: Archlinux

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[issue37510] argparse removing more "--" than it should

[Jorge L. Martinez]

New submission from Jorge L. Martinez :

$ python -c '
import argparse
p = argparse.ArgumentParser()
p.add_argument("first_arg")
p.add_argument("args", nargs="*")
r = p.parse_args(["foo", "--", "bar", "--", "baz", "--", "zap"])
print(r.first_arg + " " + " ".join(r.args))
'

returns:

foo bar baz -- zap

when I think it should return:

foo bar -- baz -- zap

--
components: Library (Lib)
messages: 347378
nosy: jol
priority: normal
severity: normal
status: open
title: argparse removing more "--" than it should
type: behavior
versions: Python 3.7

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