gh-106368: Argument clinic tests: improve failure message when tests in ClinicExternalTests fail

[AlexWaygood]

Currently, if I apply this diff to Tools/clinic/clinic.py...

--- a/Tools/clinic/clinic.py
+++ b/Tools/clinic/clinic.py
@@ -4702,6 +4702,7 @@ def state_modulename_name(self, line: str | None) -> None:
                 fields = [x.strip() for x in existing.split('.')]
                 function_name = fields.pop()
                 module, cls = self.clinic._module_and_class(fields)
+                1/0

                 for existing_function in (cls or module).functions:
                     if existing_function.name == function_name:

...then the argument clinic tests fail for me locally (good!), but the failure message is terrible (bad!):

======================================================================
FAIL: test_external (test.test_clinic.ClinicExternalTest.test_external)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_clinic.py", line 1410, in test_external
    out = self.expect_success("-f", "-o", TESTFN, source)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\test\test_clinic.py", line 1397, in expect_success
    return self._do_test(*args)
           ^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\test\test_clinic.py", line 1389, in _do_test
    self.fail("".join(proc.stderr))
AssertionError

----------------------------------------------------------------------

This change makes the error message much more helpful!

======================================================================
FAIL: test_external (test.test_clinic.ClinicExternalTest.test_external)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_clinic.py", line 1411, in test_external
    out = self.expect_success("-f", "-o", TESTFN, source)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\test\test_clinic.py", line 1398, in expect_success
    return self._do_test(*args)
           ^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\test\test_clinic.py", line 1390, in _do_test
    self.fail("".join(itertools.chain(proc.stdout, proc.stderr)))
AssertionError: Error in file "C:\Users\alexw\coding\cpython\Lib\test\clinic.test.c" on line 117:
Exception raised during parsing:
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Tools\clinic\clinic.py", line 2244, in parse
    parser.parse(block)
  File "C:\Users\alexw\coding\cpython\Tools\clinic\clinic.py", line 4608, in parse
    self.state(line)
  File "C:\Users\alexw\coding\cpython\Tools\clinic\clinic.py", line 4666, in state_dsl_start
    self.next(self.state_modulename_name, line)
  File "C:\Users\alexw\coding\cpython\Tools\clinic\clinic.py", line 4648, in next
    self.state(line)
  File "C:\Users\alexw\coding\cpython\Tools\clinic\clinic.py", line 4705, in state_modulename_name
    1/0
    ~^~
ZeroDivisionError: division by zero


----------------------------------------------------------------------

(@erlend-aasland do you see the same thing locally? Am I seeing something different because I'm using a Windows box?)

• Issue: Increase Argument Clinic test coverage #106368

[erlend-aasland]

Thanks! (BTW, I love itertools!)

[miss-islington]

Thanks @AlexWaygood for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-bot]

GH-107365 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

GH-107366 is a backport of this pull request to the 3.11 branch.