IHMO this PR depends on PR #10238 to properly define datetime.h when compiling with Py_BUILD_CORE.

Is not intended to build it without Py_BUILD_CORE for testing the same C API that is available in external extensions?

Is not intended to build it without Py_BUILD_CORE for testing the same C API that is available in external extensions?

Honestly, the exact purpose of _testcapi is now unclear to me :-) This module became giant. Maybe we need a new module to the test the Py_BUILD_CORE API? _testcapi.get_coreconfig() converts PyInterpreterState.core_config to a Python function, from my point of view, this code is not part of the public Python C API.

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Other options:

• Copy/paste _Py_wstrlist_as_pylist() function from main.c to _testcapimodule.c: I would prefer to not duplicate code!
• Move get_coreconfig() from _testcapimodule.c to coreconfig.c: But this function is only used for an unit test (at least, now ;-)), so I would prefer to not always compile the function in Python binary (or in libpython), not waste memory/disk space.

I'm going to split _testcapi.c onto several parts and move them into the new Modules/_testcapi/ directory (similarly to Modules/_io/). I'm working on adding tests for C API, and this adds too much code in _testcapi.c, so it is practical to add separate files for helpers for testing different parts of C API.

If the only problem is with _Py_wstrlist_as_pylist(), the simplest temporary workaround is to duplicate its declaration in _testcapi.c.

Is not intended to build it without Py_BUILD_CORE for testing the same C API that is available in external extensions?

Right. I abandon this PR and wrote PR #10362 which is the proper fix: it also ensures that _testcapimodule.c is not compiled with Py_BUILD_CORE defined.