Related python-dev thread:
https://mail.python.org/pipermail/python-dev/2017-November/150608.html
"[Python-Dev] Show DeprecationWarning in debug mode?"

I propose to display Display DeprecationWarning, PendingDeprecationWarning and ImportWarning when Python is compiled in debug mode.

Attached PR implements proposed change.

Warnings filters in debug mode.

Reference (current code):

$ ./python -c 'import warnings, pprint; pprint.pprint(warnings.filters)'
[('ignore', None, <class 'DeprecationWarning'>, None, 0),
 ('ignore', None, <class 'PendingDeprecationWarning'>, None, 0),
 ('ignore', None, <class 'ImportWarning'>, None, 0),
 ('ignore', None, <class 'BytesWarning'>, None, 0),
 ('always', None, <class 'ResourceWarning'>, None, 0)]

With my PR 4474:

$ ./python -c 'import warnings, pprint; pprint.pprint(warnings.filters)'
[('ignore', None, <class 'BytesWarning'>, None, 0),
 ('always', None, <class 'ResourceWarning'>, None, 0)]


BytesWarning is not modified by my PR since this specific warning is controlled by the -b option. Example with my PR.

-b option, ignore => default:

$ ./python -b -c 'import warnings, pprint; pprint.pprint(warnings.filters)'
[('default', None, <class 'BytesWarning'>, None, 0),
 ('always', None, <class 'ResourceWarning'>, None, 0)]

-bb option, ignore => error:

$ ./python -bb -c 'import warnings, pprint; pprint.pprint(warnings.filters)'
[('error', None, <class 'BytesWarning'>, None, 0),
 ('always', None, <class 'ResourceWarning'>, None, 0)]


Note: While writing this comment, I also found a bug in the new developer mode (-X dev): bpo-32089.

New changeset 895862aa01793a26e74512befb0c66a1da2587d6 by Łukasz Langa (Victor Stinner) in branch 'master':
bpo-32088: Display Deprecation in debug mode (#4474)
https://github.com/python/cpython/commit/895862aa01793a26e74512befb0c66a1da2587d6

Merged. This is an important improvement to make sure 3.7's quality is the best possible.

Thanks! ✨ 🍰 ✨