The PEP 587 adds new functions taking command line arguments:

Py_PreInitializeFromArgs(config, argc, argv)
Py_PreInitializeFromWideArgs(config, argc, argv)
Py_InitializeFromArgs(config, argc, argv)
Py_InitializeFromWideArgs(config, argc, argv)

I propose to add _Py_RunMain() (currently private, until PEP 587 is accepted) to be able to implement something similar to Py_Main() but with way more configuration options:

Example to create an "isolated Python" in a few line of C:
---
#include <Python.h>

int main(int argc, char *argv[])
{
    _PyCoreConfig config = _PyCoreConfig_INIT;
    config.isolated = 1;

    _PyInitError err = _Py_InitializeFromArgs(&config, argc, argv);
    if (_Py_INIT_FAILED(err)) {
        _Py_ExitInitError(err);
    }

    return _Py_RunMain();
}
---

It's like the regular "python3" program, except that it's always isolated:
---
$ my-isolated-python
Python 3.8.0a3+ (heads/run_main-dirty:d167362ecc, Mar 29 2019, 13:03:30) 
>>> 1+1
2
>>> import sys
>>> sys.flags.isolated
1
---

Using _Py_RunMain(), it becomes trivial to implement something like PEP 432's "system python" idea ;-)

I'm working on a PR to implement it.

New changeset 2f54908afc5665937d763510b4430f10cf764641 by Victor Stinner in branch 'master':
bpo-36471: Add _Py_RunMain() (GH-12618)
https://github.com/python/cpython/commit/2f54908afc5665937d763510b4430f10cf764641

The PEP 587 will decide if this function is made public or not.