They do things like 

    err = PyFile_WriteString("TypeError: print_exception(): Exception expected for value, ", f);
    err += PyFile_WriteString(Py_TYPE(value)->tp_name, f);
    err += PyFile_WriteString(" found\n", f);

which means that PyFile_XXX functions are called after error. They should return (abort printing the exception) instead.


It gets even more interesting with PyFile_WriteObject calls - this function asserts that the exception is not set, so the code calls PyErr_Clear() before calling PyFile_WriteObject(). It should avoid making the call altogether.

It's more intricate than I initially thought - the assertion that no exception is set is in PyObject_Str rather than PyFile_WriteObject, and it should remain there because it ensures that exceptions are not accidentally cleared by an str() call.

In the traceback display code, there are places where the exception is cleared because it's being overridden (like when modulename is not found and is replace by "<unknown>") or places where the code makes a best-effort attempt to print something anyway. In these situations the error is cleared before calling PyFile_WriteObject. It is probably correct, but it's hard to follow the logic when making changes to the code, so it should be made more explicit.

New changeset d596acbd3b4f6716ed98895eb0b48e9830e0b320 by Irit Katriel in branch 'main':
bpo-45635: standardize error handling in traceback.c (GH-29905)
https://github.com/python/cpython/commit/d596acbd3b4f6716ed98895eb0b48e9830e0b320

New changeset f893bb2e01307c92ca19597f44874ecaffe7f06f by Irit Katriel in branch 'main':
bpo-45635: refactor print_exception() into smaller functions (GH-29981)
https://github.com/python/cpython/commit/f893bb2e01307c92ca19597f44874ecaffe7f06f

New changeset dc4a212bd305831cb4b187a2e0cc82666fcb15ca by Irit Katriel in branch 'main':
bpo-45635: continue refactor of print_exception() to standardize error handling (GH-29996)
https://github.com/python/cpython/commit/dc4a212bd305831cb4b187a2e0cc82666fcb15ca

New changeset 0fe104fce7da709edddb701baa2249e3275db1fd by Irit Katriel in branch 'main':
bpo-45635: refactor print_exception_recursive into smaller functions to standardize error handling (GH-30015)
https://github.com/python/cpython/commit/0fe104fce7da709edddb701baa2249e3275db1fd

New changeset 8d6155ee9d1b05946f951d0ba602b9f63810fe0f by Irit Katriel in branch 'main':
bpo-45635: Do not suppress errors in functions called from _PyErr_Display (GH-30073)
https://github.com/python/cpython/commit/8d6155ee9d1b05946f951d0ba602b9f63810fe0f