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On 6 December 2017 at 14:50, Nick Coghlan <ncoghlan@gmail.com> wrote:
\> On 6 December 2017 at 14:34, Nick Coghlan <ncoghlan@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/6e93c8d2e6ad698834578d4077b92a8fc84a70f5

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, , '\_\_main\_\_', 0)

('ignore', None, , None, 0)

('ignore', None, , None, 0)

('ignore', None, , None, 0)

('ignore', None, , None, 0)

('ignore', None, , None, 0)

$ ./python -X dev -c "import warnings; print('\\n'.join(map(str,
warnings.filters)))"

('ignore', None, , None, 0)

('default', None, , None, 0)

('default', None, , None, 0)

$ ./python -Wd -c "import warnings; print('\\n'.join(map(str,
warnings.filters)))"

('default', None, , None, 0)

('default', None, , '\_\_main\_\_', 0)

('ignore', None, , None, 0)

('ignore', None, , None, 0)

('ignore', None, , None, 0)

('ignore', None, , None, 0)

('ignore', None, , 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@gmail.com | Brisbane, Australia