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Re: [PATCH 3/4] qemu-iotests: let "check" spawn an arbitrary test comman


From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH 3/4] qemu-iotests: let "check" spawn an arbitrary test command
Date: Tue, 23 Mar 2021 20:11:16 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

23.03.2021 20:00, Paolo Bonzini wrote:
On 23/03/21 17:43, Vladimir Sementsov-Ogievskiy wrote:

Interesting that REMAINDER documentation disappeared from latest (3.9) python 
documentation https://docs.python.org/3.9/library/argparse.html , but exists 
here https://docs.python.org/3.8/library/argparse.html  (and no mark of 
deprecation)

Whoa.  https://bugs.python.org/issue17050 says:

---
Since this feature is buggy, and there isn't an easy fix, we should probably 
remove any mention of it from the docs.  We can still leave it as an 
undocumented legacy feature.

There is precedent for leaving `nargs` constants undocumented. 
`argparse.PARSER` ('+...') is used by the subparser mechanism, but is not 
documented.  https://bugs.python.org/issue16988
---

The problematic case appears to be when you have more than one positional 
argument, which is exactly the case with the 3.8 documented use of REMAINDER.  
Hence the decision to drop the documentation.

However, "check" works fine because the REMAINDER argument is the only 
positional argument:

     $ ./check 001 -d
     Test "tests/-d" is not found

Another possibility is to pre-process sys.argv like this:

     if '--' in sys.argv:
         cmd = True
         args = sys.argv[0:sys.argv.index('--')]
         posargs = sys.argv[len(args)+1:]
     else:
         cmd = False
         args = list(x for x in sys.argv if x.startswith('-'))
         posargs = list(x for x in sys.argv if not x.startswith('-'))

But getting the help message right etc. would be messy.

Paolo



Hmm:

If you have positional arguments that must begin with - and don’t look like 
negative numbers, you can insert the pseudo-argument '--' which tells 
parse_args() that everything after that is a positional argument:

So, as I understand argparse supports '--' feature out of the box. So, we can 
keep '*' as is, and it would parse all remaining positional arguments which are 
either tests or the command, and '--' will be automatically dropped. So, we 
only need to check existing of '--' in original sys.argv to chose our behavior.

--
Best regards,
Vladimir



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