[Python-Dev] PEP 565: Show DeprecationWarning in __main__

[Victor Stinner]

Let's discuss -Xdev implementation issue at
https://bugs.python.org/issue32230

In short, -Xdev must add its warning at the end to respect BytesWarning,
whereas it's not possible with -W option :-(

Victor

Le 6 déc. 2017 09:15, "Nick Coghlan" <ncoghlan at gmail.com> a écrit :

On 6 December 2017 at 14:50, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 6 December 2017 at 14:34, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> That said, I go agree we could offer easier to use APIs to app
>> developers that just want to hide warnings from their users, so I've
>> filed https://bugs.python.org/issue32229 to propose a straightforward
>> "warnings.hide_warnings()" API that encapsulates things like checking
>> for a non-empty sys.warnoptions list.
>
> I've updated the "Limitations" section of the PEP to mention that
> separate proposal:
> https://github.com/python/peps/commit/6e93c8d2e6ad698834578d4077b92a
8fc84a70f5

Having rebased the PEP 565 patch atop the "-X dev" changes, I think
that if we don't change some of the details of how `-X dev` is
implemented, `warnings.hide_warnings` (or a comparable convenience
API) is going to be a requirement to help app developers effectively
manage their default warnings settings in 3.7+.

The problem is that devmode doesn't currently behave the same way
`-Wd` does when it comes to sys.warnoptions:

    $ ./python -Wd -c "import sys; print(sys.warnoptions);
print(sys.flags.dev_mode)"
    ['d']
    False
    $ ./python -X dev -c "import sys; print(sys.warnoptions);
print(sys.flags.dev_mode)"
    []
    True

As currently implemented, the warnings module actually checks
`sys.flags.dev_mode` directly during startup (or `sys._xoptions` in
the case of the pure Python fallback), and populates the warnings
filter differently depending on what it finds:

    $ ./python -c "import warnings; print('\n'.join(map(str,
warnings.filters)))"
    ('default', None, <class 'DeprecationWarning'>, '__main__', 0)
    ('ignore', None, <class 'DeprecationWarning'>, None, 0)
    ('ignore', None, <class 'PendingDeprecationWarning'>, None, 0)
    ('ignore', None, <class 'ImportWarning'>, None, 0)
    ('ignore', None, <class 'BytesWarning'>, None, 0)
    ('ignore', None, <class 'ResourceWarning'>, None, 0)

    $ ./python -X dev -c "import warnings; print('\n'.join(map(str,
warnings.filters)))"
    ('ignore', None, <class 'BytesWarning'>, None, 0)
    ('default', None, <class 'ResourceWarning'>, None, 0)
    ('default', None, <class 'Warning'>, None, 0)

    $ ./python -Wd -c "import warnings; print('\n'.join(map(str,
warnings.filters)))"
    ('default', None, <class 'Warning'>, None, 0)
    ('default', None, <class 'DeprecationWarning'>, '__main__', 0)
    ('ignore', None, <class 'DeprecationWarning'>, None, 0)
    ('ignore', None, <class 'PendingDeprecationWarning'>, None, 0)
    ('ignore', None, <class 'ImportWarning'>, None, 0)
    ('ignore', None, <class 'BytesWarning'>, None, 0)
    ('ignore', None, <class 'ResourceWarning'>, None, 0)

This means the app development snippet proposed in the PEP will no
longer do the right thing, since it will ignore the dev mode flag:

    if not sys.warnoptions:
        # This still runs for `-X dev`
        warnings.simplefilter("ignore")

My main suggested fix would be to adjust the way `-X dev` is
implemented to include `sys.warnoptions.append('default')` (and remove
the direct dev_mode query from the warnings module code).

However, another possible way to go would be to make the correct
Python 3.7+-only snippet look like this:

    import warnings
    warnings.hide_warnings()

And have the forward-compatible snippet look like this:

    import warnings:
    if hasattr(warnings, "hide_warnings"):
        # Accounts for `-W`, `-X dev`, and any other implementation
specific settings
        warnings.hide_warnings()
    else:
        # Only accounts for `-W`
        import sys
        if not sys.warnoptions:
            warnings.simplefilter("ignore")

(We can also do both, of course)

Cheers,
Nick.

--
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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