The `-X dev` option is currently integrated directly with the warnings module, rather than working indirectly through `sys.warnoptions`.

This means that any third party code that currently checks sys.warnoptions will need to be updated to check sys.flags.dev_mode as well.

Rather than doing that, I propose that we instead change the way dev mode works to *literally* be equivalent to `-Wdefault`, and remove the direct integration with the warnings machinery.

(PR for that coming shortly)

I did it on purpose. -Wdefault adds the filter at the start, whereas -Xdev
adds the filter at the end to respect BytesWarning. See tests in
test_cmd_line and bpo-32089.

Note that this change will make "-X dev" effectively treat "-bb" and "-b" as equivalent, the same way "-Wd" already does. I figure that's OK, since it means the runtime behaviour will match the documentation (which says that "-X dev" implies "-Wd".

I was explicitly asked to not change the behaviour of BytesWarning with -X dev.

-X dev documentation doesn't menton -Wd/-Wdefault anymore:
https://docs.python.org/dev/using/cmdline.html#id5

"Warning filters: add a filter to display all warnings ("default" action), except of BytesWarning which still depends on the -b option, and use "always" action for ResourceWarning warnings. For example, display DeprecationWarning warnings."

Another implementation issue is subprocess._args_from_interpreter_flags() which was unable to properly "rebuild" the -X dev option from sys.warnoptions. I mean that it's now simpler and safer with sys.flags.dev_mode.

But I agree that the fact that -X dev doesn't touch sys.warnoptions is surprising and can cause issues. IMHO sys.warnoptions shouldn't exist in the first place. It looks like a hack to pass options from the C main() function to the warnings initilization code. But it's not like we can remove it right now :-)

It isn't good to have "-X dev" do magical things that can't already be replicated with other options.

If we want "-bb" to take precedence over the passed in "-W" settings (which I agree would be a reasonable change), we should make that change universally, not restrict it to "-X dev".

> If we want "-bb" to take precedence over the passed in "-W" settings

It's not only that. Without -b nor -bb, the default::Warning filter should still come after ignore::BytesWarning, since these warnings must be ignored by default.

I filed https://bugs.python.org/issue32231 to cover having -bb always override other warnings settings when it comes to handling BytesWarning.

Writing that issue made me realise another quirk with `-X dev` though: as a command line option, it should really take precedence over PYTHONWARNINGS, while remain subordinate to other explicit -W options.

I've adjusted the PR to implement that change, but haven't added a test case that ensures the behaviour persists.

First I proposed that -X dev behaves as -Wd -b, but Antoine Pitrou and Serhiy Storchaka asked to remove -b:

https://mail.python.org/pipermail/python-dev/2017-November/150517.html
https://mail.python.org/pipermail/python-dev/2017-November/150519.html
https://mail.python.org/pipermail/python-dev/2017-November/150520.html

No, we don't need to worry about BytesWarning if neither -b nor -bb are set, as the interpreter doesn't even emit the warning in that case.

The only thing the warnings machinery gets used for is to print the messages and/or raise the exceptions (so it would arguably be clearer to omit the filter entirely in the case where the flag isn't set at all).

In relation to the utility of sys.warnoptions: it also lets other modules (like the unittest module) inspect the warnings that have been specified on the command line.

If it wasn't for that use case, I wouldn't be concerned about the fact that `-X dev` doesn't currently set it.

> It isn't good to have "-X dev" do magical things that can't already be replicated with other options.

While it might be nice to be able to replicate -X dev "manually", I didn't try to implement that. The current implementation of this option is more complex than what I expected, especially for warnings.

> Writing that issue made me realise another quirk with `-X dev` though: as a command line option, it should really take precedence over PYTHONWARNINGS, while remain subordinate to other explicit -W options.

IMHO -X dev should change the default, but let the developer overrides options set by -X dev. For example, -X dev -bb works as expected: raise BytesWarning, and this is a test for that. There is also a test for -X dev -W error: -W error has the priority over -X dev, I did that on purpose.

When I started to implement -X dev, I didn't expect so many corner cases. The problem is that the code reading the "configuration" (command line arguments, environment variables, a few configuration files, etc.) was spreaded around CPython code. You know that very well, since you wrote the PEP 432 :-) Fix the implementation of -X dev was my first motivation to work on the implementation of the PEP 432: bpo-32030.

For PYTHONWARNINGS vs -X dev, I'm not sure. It's rare to use PYTHONWARNINGS. I put PYTHONWARNINGS and -W options at the same level. If someone uses PYTHONWARNINGS, warnings are likely already well understood. Overriding PYTHONWARNINGS with -X dev can be seen as a bug. Sometimes, you cannot set command line options, only environment variable. There is -X dev, but there is also PYTHONDEVMODE=1 ...

Right, I hit some of those corner cases myself before I realised that pymain_add_warnings_options was the right place to inject "default" into sys.warnoptions after your refactoring improvements. It provides a nice pivot point where the command line machinery can decide relative priority and turn things into a neatly ordered list.

That gives the following priority order in my PR:

- first we add the filters from PYTHONWARNINGS
- then we optionally add an extra "default" entry based on core_config->dev_mode (which may be from either PYTHONDEVMODE or from "-X dev")
- then we add the filters from "-W" command line options

The "last added, first applied" behaviour of "warnings.simplefilter" then means that later entries in that list take precedence over earlier entries.

If we go on to also implement https://bugs.python.org/issue32231, then we'd add a 4th step to inject a "default::BytesWarning" or "error::BytesWarning" filter at the end based on cmdline->bytes_warning.

I prefer to not talk about sys.warnoptions since, as you wrote, the order is reversed.

IMHO the priority should be (high priority > low priority):

  -W options > PYTHONWARNINGS > BytesWarning > -X dev > default filters

With:

* BytesWarning: "ignore" by default, "default" for -b, "error" for -bb.
* default filters: 4 filters added in release mode

[('ignore', None, <class 'DeprecationWarning'>, None, 0),
 ('ignore', None, <class 'PendingDeprecationWarning'>, None, 0),
 ('ignore', None, <class 'ImportWarning'>, None, 0),
 ('ignore', None, <class 'ResourceWarning'>, None, 0)]

  or 1 filter for pydebug build:

[('default', None, <class 'ResourceWarning'>, None, 0)]


Currently, only "-W options > PYTHONWARNINGS" goes into sys.warnoptions, whereas "BytesWarning > -X dev > default filters" is added by the warnings module ("init_filters()").

If we wanted to go that way, then I'd go back to my original code order for - putting the '-X dev' setting first in sys.warnoptions, so both PYTHONWARNINGS and -W options override it.

Then https://bugs.python.org/issue32231 would determine the relative precedence of the BytesWarning filter. (I still think the interaction between "-Wd" and "-bb" is weird, but we can argue about that on the other issue).

"-W options > PYTHONWARNINGS > BytesWarning > -X dev > default filters"

Hum, sorry, according to bpo-20361 (and bpo-32231 which has been marked as a duplicate of bpo-20361), -b and -bb options must have the priority over -W/PYTHONWARNINGS. So it gives something closer to:

-b and -bb > -W > PYTHONWARNINGS > ignore::BytesWarning > -X dev > default filters

where ignore::BytesWarning is only not added if -b or -bb is used.

New changeset 747f48e2e92390c44c72f52a1239959601cde157 by Victor Stinner in branch 'master':
bpo-32230: Set sys.warnoptions with -X dev (#4820)
https://github.com/python/cpython/commit/747f48e2e92390c44c72f52a1239959601cde157

We found an agreement with Nick :-) I applied Nick's PR. The issue is now fixed:

vstinner@apu$ ./python -X dev -c 'import sys; print(sys.warnoptions)'
['default']

This PR also fix bpo-20361: -b and -bb options now have the highest priority.

The documentation is maybe not perfect, but Nick stated that he will update the doc with the implementation of his PEP 565.