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The support.args_from_interpreter_flags() function recreates Python command line arguments from sys.flags, but it omits -I (sys.flags.isolated).
Because of that, "./python -I -m test ..." behaves differently than "./python -I -m test -j0 ...":
https://bugs.python.org/issue28655#msg326477 

Thanks Victor for the details. Can this be classified as an easy issue? I guess the fix will be as below : 
1. Add an entry for '-I' at https://github.com/python/cpython/blob/4b430e5f6954ef4b248e95bfb4087635dcdefc6d/Lib/subprocess.py#L260
2. Add a test for '-I' at https://github.com/python/cpython/blob/4b430e5f6954ef4b248e95bfb4087635dcdefc6d/Lib/test/test_support.py#L472. 
The only thing here is that '-I' returns '-s -E -I' unlike other options where args can be used for comparison logic in check_options. check_options should be changed so that this can take given args and the expected args for comparison to accommodate -I. Maybe there is a better way?
Off topic : I don't know why '-I' is not documented as sys.flags.isolated at https://docs.python.org/3.7/library/sys.html#sys.flags . Maybe I will open up a separate issue for this?

In the C code, sys.flags.isolated clearly documented as linked to the -I option:
static PyStructSequence_Field flags_fields[] = {
 {"debug", "-d"},
 {"inspect", "-i"},
 {"interactive", "-i"},
 {"optimize", "-O or -OO"},
 {"dont_write_bytecode", "-B"},
 {"no_user_site", "-s"},
 {"no_site", "-S"},
 {"ignore_environment", "-E"},
 {"verbose", "-v"},
 /* {"unbuffered", "-u"}, */
 /* {"skip_first", "-x"}, */
 {"bytes_warning", "-b"},
 {"quiet", "-q"},
 {"hash_randomization", "-R"},
 {"isolated", "-I"},
 {"dev_mode", "-X dev"},
 {"utf8_mode", "-X utf8"},
 {0}
};
> The only thing here is that '-I' returns '-s -E -I' unlike other options where args can be used for comparison logic in check_options.
I expect to get:
$ python3 -I -c 'import subprocess; print(subprocess._args_from_interpreter_flags())'
['-I']
instead of:
['-s', '-E']
-I is different from -s -E: it also avoids to add the script directory or an empty string to sys.path.

Thanks Victor for the details. 
> In the C code, sys.flags.isolated clearly documented as linked to the -I option:
With respect to documentation I was talking about '-I' not being documented in the table at https://docs.python.org/3.7/library/sys.html#sys.flags though it's present in the C code and in sys.flags.isolated.
> -I is different from -s -E: it also avoids to add the script directory or an empty string to sys.path.
'-I' also implies '-s -E' and hence adding isolated to args_from_interpreter_flags will also return ['-s', '-E', '-I'] as output and hence I suggested modifying the comparison logic.
# Since '-I' implies '-s' and '-E' those flags are also set returning '-s -E -I'
./python.exe --help | rg '\-I'
-I : isolate Python from the user's environment (implies -E and -s)
./python.exe -I -c 'import sys; print(sys.flags)'
sys.flags(debug=0, inspect=0, interactive=0, optimize=0, dont_write_bytecode=0, no_user_site=1, no_site=0, ignore_environment=1, verbose=0, bytes_warning=0, quiet=0, hash_randomization=1, isolated=1, dev_mode=False, utf8_mode=0)
# patching args_from_interpreter_flags to support '-I' would return below
./python.exe -I -c 'import subprocess; print(subprocess._args_from_interpreter_flags())'
['-s', '-E', '-I']
Thanks

> ./python.exe -I -c 'import subprocess; print(subprocess._args_from_interpreter_flags())'
> ['-s', '-E', '-I']
This looks wrong, I would prefer to only get ['-I'].

> With respect to documentation I was talking about '-I' not being documented in the table at https://docs.python.org/3.7/library/sys.html#sys.flags though it's present in the C code and in sys.flags.isolated.
Thanks for bringing this up Karthikeyan, however, could there be another reason why -I would be left out. Also, have you filed an issue for this?
Also, Victor and Karthikeyan, since this issue has been categorized as an easy issue, I would like to fix this if none of you have started working on this.

> Thanks for bringing this up Karthikeyan, however, could there be another reason why -I would be left out. Also, have you filed an issue for this?
I couldn't see any related issue for this though the table was changed in 3.7.0
> Also, Victor and Karthikeyan, since this issue has been categorized as an easy issue, I would like to fix this if none of you have started working on this.
I am not working on this. Feel free to pick it up.

Thank you Karthikeyan, I'm going to take care of both of these issues.

Linking this[1] here in case someone else stumbles upon this thread. I've created an issue and a PR for the documentation issue regarding the absence of -I flag from the sys.flags table which came into picture from the discussions in this thread.
[1]: https://bugs.python.org/issue34901 

> I expect to get: ['-I'] instead of ['-s', '-E', '-I']
From what I understand, this can be done in one of two ways. First, we could edit https://github.com/python/cpython/blob/ad73a9cf97770023665a1bb1c6390a3c99478139/Modules/main.c#L430 and not incrementing -s and -E. But I believe this would have consequences that I'm unable to think of right now, so I'd like some inputs on this. 
Secondly, we could handle this condition in `_args_from_interpreted_flags()` itself but that might be looked upon as bad practice.

> From what I understand, this can be done in one of two ways. First, we could edit https://github.com/python/cpython/blob/ad73a9cf97770023665a1bb1c6390a3c99478139/Modules/main.c#L430 and not incrementing -s and -E. But I believe this would have consequences that I'm unable to think of right now, so I'd like some inputs on this. 
As in the docs for -I it implies -s and -E so removing the increment is not a good solution in my opinion and will break code. 
I don't know how this can be handled since -I sets -s and -E implicitly and _args_from_interpreted_flags just looks for the set flag. This could also get a little complex if we remove -s and -E based on -I since one might pass -I and -s. Maybe we can do an intersection of the command line arguments passes and the set bits in _args_from_interpreted_flags so that only -I remains? Victor prefers -I only and maybe has an approach to solve this?

You're right Karthikeyan, although I personally think that returning ['-s', '-E', '-I'] should be a plausible solution here since it has been stated explicitly that it implies '-s' and '-E' but I'm still waiting for what Victor has to say on this. 
> The only thing here is that '-I' returns '-s -E -I' unlike other options where args can be used for comparison logic in check_options.
Karthikeyan, do you happen to have a use case where this might come into action?

>> The only thing here is that '-I' returns '-s -E -I' unlike other options where args can be used for comparison logic in check_options.
> Karthikeyan, do you happen to have a use case where this might come into action?
I don't have a use case in mind. The comment was that returning '-s -E -I' would need the helper function used in the test to be changed.
Thanks

Sorry for bumping this thread but Victor, could you please share your inputs on this if you have the time for it, thanks.

I tried to explain how to fix the bug, but nobody came up with a working change 2 months, so I wrote the PR myself. It's an important security issue, since the function is used by multiprocessing and distutils modules to spawn new child processes.

New changeset 9de363271519e0616f4a7b59427057c4810d3acc by Victor Stinner in branch 'master':
bpo-34812: subprocess._args_from_interpreter_flags(): add isolated (GH-10675)
https://github.com/python/cpython/commit/9de363271519e0616f4a7b59427057c4810d3acc

New changeset 01e579949ab546cd4cdd0d6d18e3ef41ce94f46e by Miss Islington (bot) in branch '3.7':
bpo-34812: subprocess._args_from_interpreter_flags(): add isolated (GH-10675)
https://github.com/python/cpython/commit/01e579949ab546cd4cdd0d6d18e3ef41ce94f46e

New changeset cc0e0a2214d6515cf6ba4c7b164902a87e321b45 by Victor Stinner in branch '3.6':
bpo-34812: subprocess._args_from_interpreter_flags(): add isolated (GH-10675) (GH-10688)
https://github.com/python/cpython/commit/cc0e0a2214d6515cf6ba4c7b164902a87e321b45

Ok, the bug is now fixed in Python 3.6, 3.7 and master branches ;-)