subprocess: move shell arguments to a separate keyword param

[techtonik]

BPO	13197
Nosy	@rhettinger, @vstinner
Superseder	bpo-7839: Popen should raise ValueError if pass a string when shell=False or a list when shell=True

^{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 2022-03-25.11:42:00.480>
created_at = <Date 2011-10-17.11:41:11.103>
labels = ['type-feature', 'library']
title = 'subprocess: move shell arguments to a separate keyword param'
updated_at = <Date 2022-03-25.11:42:00.479>
user = 'https://bugs.python.org/techtonik'

bugs.python.org fields:

activity = <Date 2022-03-25.11:42:00.479>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-25.11:42:00.480>
closer = 'iritkatriel'
components = ['Library (Lib)']
creation = <Date 2011-10-17.11:41:11.103>
creator = 'techtonik'
dependencies = []
files = []
hgrepos = []
issue_num = 13197
keywords = []
message_count = 6.0
messages = ['145669', '145670', '145675', '145676', '145877', '145887']
nosy_count = 6.0
nosy_names = ['rhettinger', 'vstinner', 'techtonik', 'Arfrever', 'cvrebert', 'rosslagerwall']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '7839'
type = 'enhancement'
url = 'https://bugs.python.org/issue13197'
versions = ['Python 3.3']

[techtonik]

subprocess.Popen(args, shell=shell) is implicitly inconsistent on Unix:

1. when shell=False, the args should be a list or you'll lose program options
2. when shell=True, the args should be a string or your program options will be passed to shell itself

I propose to make consistent behaviour - require args to be a list, and pass shell options in a separate shell_args keyword argument. If it not for python4, then perhaps it can be implemented in PopenShell() and PopenSystem() functions.

This will also require unification of behaviour across platforms.

[vstinner]

Did you read the issue bpo-7839?

[techtonik]

No. I won't be fixed in 2.7 anyway, and if it can not be fixed, it should be documented at least.

[techtonik]

Oh, sorry - my attention span is limited under time pressure. bpo-7839 is related, of course. There is a lot of comments, so I'll take a look at it later.

[rhettinger]

I don't think you can tighten the API at this point. We could however make Popen complain when options are going to be thrown away. Or it could be made more liberal about what is accepts (running shlex on string input or join on list input as needed).

[techtonik]

List not only needs to be joined, it should be quoted as well. But the biggest problem will be the change of behavior between Python versions which may confuse users even more.

Perhaps it's a good time to start embedding 'best practice' recipes in manual.