Currently C API is not completely covered by tests. Tests for particular parts of C API are scattered through different files. There are files completely purposed for testing C API (like test_capi.py, test_getargs2.py), there are classes (usually having "CAPI" in the name) in different files for testing C API specific for unicode, marshal. Argument parsing tests are split between two files, test_capi.py, test_getargs2.py. I need to add new tests for new features, and I'm going to add new tests for existing C API. But first I'm going to reorganize tests. Add a new directory Lib/test/test_capi, and move all C API tests into it, grouped by function prefixes. test_getargs.py for testing PyArg_*(), test_unicode.py for testing PyUnicode_*(), etc. Tests that use the _testcapi module, but don't test specific C API, will left on place. The benefit is that it will be easier to run all C API tests at once, and only them, and it will be clearer what C API is covered by tests. The disadvantage is that you will need to run several files for testing marshal for example. What are your thoughts?