I would prefer to disable the popups and messages to stderr by default in Python directly. If some users want to see them, we can add a -X option to enable them. Example: -X enable_mscrt_checks. What do you think? Do you recall who complain that Python hides MSCRT warnings? And why he/she complained?

I don't want these warnings, especially popups blocking my program, on my own applications. Python already raises OSError on errors.

The problem in tests is also that some tests use subprocesses and regrtest only configures the test runner, not subprocesses. For example, I had to modify a subprocess started by the test. Otherwise, the test *hangs* :-(

Popups are annoying because it's not possible to ignore them all by a single click. I have to click N times for N errors, with N>100 it's just a nightmare...

For me, it's a regression compared to older Python versions. I don't remember to have to click on so much popups or have stderr flooded by warnings when working on Windows, when I worked on Python 3.4 or older.