gh-128509: Add PyUnstable_IsImmortal for finding immortal objects

[ZeroIntensity]

• I've reused the test from my other PR.
• The plan is to go for this, and then possibly go for gh-128509: Add sys._is_immortal for identifying immortal objects #128510 afterwards.

• Issue: Add an API for determining if an object is immortal #128509

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📚 Documentation preview 📚: https://cpython-previews--129182.org.readthedocs.build/en/129182/c-api/object.html#c.PyUnstable_IsImmortal

[vstinner]

LGTM.

cc @colesbury@serhiy-storchaka

[ZeroIntensity]

I think @encukou's review would also be helpful.

[serhiy-storchaka]

Suggested change

	returnPyBool_FromLong(PyUnstable_IsImmortal(op));
	NULLABLE(op)
	returnPyLong_FromLong(PyUnstable_IsImmortal(op));

This allows to test with NULL, and check if the result is not 0 or 1.

[ZeroIntensity]

I think it makes sense to crash with NULL.

[serhiy-storchaka]

Does _testcapi.is_immortal(NULL) crash? If yes, add a comment, otherwise add a test.

[encukou]

If yes, add a comment

And ideally an assert to the implementation (as “executable documentation”).

[encukou]

If yes, add a comment

And ideally an assert to the implementation (as “executable documentation”).

[serhiy-storchaka]

Did you test this?

[vstinner]

is_immortal(None) checks if None is immortal, it doesn't crash.

I suggest to remove this comment. I don't think that it's worth it to bother with testing NULL, PyUnstable_IsImmortal() now starts with assert(op != NULL);. And it's interesting to test if None is immortal or not.

[serhiy-storchaka]

I suggest to apply my suggestions to be able to test with NULL.

[ZeroIntensity]

That would break the test with None. I think as long as we have the assertion there, it's not worth explicitly testing it.

[vstinner]

classTestCAPI(unittest.TestCase):
deftest_immortal_builtins(self):
_testcapi.test_immortal_builtins()
deftest_immortal_small_ints(self):
_testcapi.test_immortal_small_ints()

There is a problem with existing tests: all methods called test_xxx are run automatically by test_capi.test_misc:

vstinner@mona$ ./python -m test test_capi -v -m test_immortal_small_ints
(...)
test_immortal_small_ints (test.test_capi.test_immortal.TestCAPI.test_immortal_small_ints) ... ok
test_immortal_small_ints (test.test_capi.test_misc.Test_testcapi.test_immortal_small_ints) ... ok
(...)

Tests are run twice. Can you fix these tests as part of your PR? Just use a different function name prefix, such as check_xxx.

[vstinner]

Oh, I didn't know that Modules/_testcapi/immortal.c exists. Maybe put the new function there?

[vstinner]

LGTM