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The site.getusersitepackages, site.addusersitepackages and co. functions all call one function which makes sure site.USER_SITE is set according to envvars and command-line options; under python -s, addusersitepackages will not add the user site dir to sys.path, but it will cause site.USER_SITE to be set (to False), so that’s why I can’t be sure that your patch does not change behavior. test_site has good coverage for IIRC (just not sure if it covers starting python -s/-S and then importing site, and python -s/-S then import site then call site.main).
I regret that all these site functions are public, now we can’t simplify them to have the setting of site.USER_SITE in only one place.