Different behavior for argparse between 2.7.8 and 2.7.9 when adding the same arguments to the root and the sub commands

[hhuang]

BPO	24251
Nosy	@bitdancer, @remram44

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-03-28.16:55:45.965>
created_at = <Date 2015-05-20.22:35:14.462>
labels = ['invalid', 'type-bug', 'library']
title = 'Different behavior for argparse between 2.7.8 and 2.7.9 when adding the same arguments to the root and the sub commands'
updated_at = <Date 2017-03-28.16:55:45.964>
user = 'https://bugs.python.org/hhuang'

bugs.python.org fields:

activity = <Date 2017-03-28.16:55:45.964>
actor = 'paul.j3'
assignee = 'none'
closed = True
closed_date = <Date 2017-03-28.16:55:45.965>
closer = 'paul.j3'
components = ['Library (Lib)']
creation = <Date 2015-05-20.22:35:14.462>
creator = 'hhuang'
dependencies = []
files = []
hgrepos = []
issue_num = 24251
keywords = []
message_count = 3.0
messages = ['243700', '243701', '244767']
nosy_count = 6.0
nosy_names = ['bethard', 'r.david.murray', 'paul.j3', 'remram', 'hhuang', 'smparkes']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24251'
versions = ['Python 2.7']

[hhuang]

When I was trying to add the same argument to both the root command and the sub command and you put the arguments before the subcommand, In 2.7.8 the arguments would be parsed correctly. But in 2.7.9, they wouldn't but these arguments would show up in the help doc:(

For python2.7.8

~$ python2.7
Python 2.7.8 (default, Oct 29 2014, 13:45:48)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import argparse
>>>
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument("-s", dest="start")
_StoreAction(option_strings=['-s'], dest='start', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>>
>>> subparsers = parser.add_subparsers()
>>> subparser = subparsers.add_parser("command")
>>> subparser.add_argument("-s", dest="start")
_StoreAction(option_strings=['-s'], dest='start', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>>
>>> parser.parse_args(["-s", "1", "command"])
Namespace(start='1')
>>> parser.parse_args(["command", "-s", "1"])
Namespace(start='1')

For python 2.7.9

~$ python2.7
Python 2.7.9 (default, Apr 2 2015, 15:33:21)
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument("-s", dest="start")
_StoreAction(option_strings=['-s'], dest='start', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>>
>>> subparsers = parser.add_subparsers()
>>> subparser = subparsers.add_parser("command")
>>> subparser.add_argument("-s", dest="start")
_StoreAction(option_strings=['-s'], dest='start', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>>
>>> parser.parse_args(["-s", "1", "command"])
Namespace(start=None)
>>> parser.parse_args(["command", "-s", "1"])
Namespace(start='1')

[hhuang]

I think this patch cause the difference:
https://hg.python.org/cpython/rev/1a3143752db2

[paulj3]

And the corresponding bug issue

http://bugs.python.org/issue9351