Le mer. 4 mars 2020 à 03:01, Andrew Bartlett <report@bugs.python.org> a écrit :
> (...) but that a valid build script that operates correctly on Python 3.8 will no longer, without warning to us, (...)

Displaying warnings by default or not is an old debate.

I added the "Python Development Mode" (-X dev) to help the situation:

https://docs.python.org/dev/library/devmode.html#devmode

I just added a very explicit "You should check for DeprecationWarning in your code" section in What's New In Python 3.9:

https://docs.python.org/dev/whatsnew/3.9.html#you-should-check-for-deprecationwarning-in-your-code

Moreover, DeprecationWarning are now (Python 3.7) displayed *by default* in the __main__ module:
https://www.python.org/dev/peps/pep-0565/