test_exc() helper in test_format.py doesn't fail if exception message unexpected

[skirpichev]

Bug report

Bug description:

Follow up of #119780. Quick fix is to add raise statement: 7f9b4d0 But it was suggested to do refactoring of this function instead.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

[vstinner]

Do you want to propose a fix?

[skirpichev]

First, I think that raise statement from mentioned PR was a right quick fix. Code seems quite old (9f8ad3f) and that part could be easily backported.

[serhiy-storchaka]

We should use standard TestCase methods assertRaises()/assertRaisesRegex() to handle expected errors and subTest() to add more context and allow multiple failures per test.

[skirpichev]

Should we keep old helper interfaces, i.e. test_exc()?

[vstinner]

test_exc() can be kept, right.

[serhiy-storchaka]

It should be kept if it is used.