Replace PyErr_WriteUnraisable() with PyErr_FormatUnraisable()

[vstinner]

Python 3.13 added PyErr_FormatUnraisable(format, ...) which allows logging a specific error message, whereas PyErr_WriteUnraisable() always logs the generic "Exception ignored in ..." message.

I propose to use replace PyErr_WriteUnraisable() with PyErr_FormatUnraisable() to log better (more helpful) error messages.

Linked PRs

• [WIP] gh-129354: Use PyErr_FormatUnraisable() function #129359
• gh-129354: Use PyErr_FormatUnraisable() function #129435
• gh-129354: Fix grammar in PyErr_FormatUnraisable() #129475
• gh-129354: Use PyErr_FormatUnraisable() function #129511
• gh-129354: Use PyErr_FormatUnraisable() function #129514
• gh-129354: Use PyErr_FormatUnraisable() function #129518
• gh-129354: Use PyErr_FormatUnraisable() function #129523
• gh-129354: Use PyErr_FormatUnraisable() function #129524
• gh-129354: Use PyErr_FormatUnraisable() function #129656
• gh-129354: Cleanup test_coroutines #129684

[vstinner]

I wrote a single big PR to show how the code can look like: PR gh-129359.

I propose to use replace PyErr_WriteUnraisable() with PyErr_FormatUnraisable() to log better (more helpful) error messages.

The drawback is that it changes sys.unraisablehook arguments: the object is no longer set. In my PR, I had to update multiple tests to check err_msg (string) instead.

[vstinner]

cc @serhiy-storchaka

[serhiy-storchaka]

The flaw of PyErr_WriteUnraisable() is that it prolonges the life of the destroying object by keeping a link to it in the object attribute. So changing it to PyErr_FormatUnraisable() can not only improve the output, but fix some obscure regressions introduced by using PyErr_WriteUnraisable().

[vstinner]

Right.

The flaw of PyErr_WriteUnraisable() is that it prolonges the life of the destroying object by keeping a link to it in the object attribute.

There is a red warning in the documentation about that: https://docs.python.org/dev/library/sys.html#sys.unraisablehook

The default hook doesn't store exc_value nor object.