Attached PR reworks _PyCoreConfig_Read() to leave _Py_path_config unchanged: *reading* the core configuration must not *modify* the path configuration.

The PR adds _PyCoreConfig.dll_path field.

The change is related to the PEP 432.

See also bpo-34008: "Do we support calling Py_Main() after Py_Initialize()?".

PR 8353 is the last piece of my long work on Py_Main() to separate code *reading* configuration and code *applying* a new configuration. My work on the new _PyPathConfig structure, and the PR should finish that work.

Extract of the commit message:

"A new _PyCoreConfig_SetPathConfig() function now recreates the path configuration from the core configuration. This function is now called very late in _Py_InitializeCore(), just before calling initimport()."

New changeset b1147e43daeb3c51a63056b489e8d868404d4e22 by Victor Stinner in branch 'master':
bpo-34170: Rework _PyCoreConfig_Read() to avoid side effect (GH-8353)
https://github.com/python/cpython/commit/b1147e43daeb3c51a63056b489e8d868404d4e22

New changeset f2626ce6d4136f13a506e34ca8631ff2eab85fd9 by Victor Stinner in branch 'master':
bpo-34170: _PyCoreConfig_Read() leaves Py_IsolatedFlag unchanged (GH-8361)
https://github.com/python/cpython/commit/f2626ce6d4136f13a506e34ca8631ff2eab85fd9

New changeset d19d8d5279f156bc8f6736b5f16f069879b9519b by Victor Stinner in branch 'master':
bpo-34170: Add _PyCoreConfig.isolated (GH-8417)
https://github.com/python/cpython/commit/d19d8d5279f156bc8f6736b5f16f069879b9519b

Reminder for myself: I have to write unit tests :-)
https://github.com/python/cpython/pull/8417#pullrequestreview-139835391

New changeset 1dc6e3906acb81163725e98378bf4d1bd1ce771a by Victor Stinner in branch 'master':
bpo-34170: Add _Py_InitializeFromConfig() (GH-8454)
https://github.com/python/cpython/commit/1dc6e3906acb81163725e98378bf4d1bd1ce771a

I added many fields to _PyCoreConfig which duplicate global configuration varibles: _PyConfigCore.isolated duplicates Py_IsolatedFlag.

I started to modify Python to read the core configuration rather than global configuration flags. The problem is that sometimes, global configuration flags are updated, but not their duplicated core configuration fields.

Example from Modules/main.c:

static void
pymain_repl(_PyMain *pymain, _PyCoreConfig *config, PyCompilerFlags *cf)
{
    /* Check this environment variable at the end, to give programs the
       opportunity to set it from Python. */
    if (!Py_InspectFlag && config_get_env_var(config, "PYTHONINSPECT")) {
        Py_InspectFlag = 1;
    }
    ...
}

Only Py_InspectFlag is not, not core_config.inspect.

New changeset d3b191992e0f3c1eb27cb68478b875d60e859976 by Victor Stinner in branch 'master':
bpo-34170: Cleanup pymain_run_python() (GH-8455)
https://github.com/python/cpython/commit/d3b191992e0f3c1eb27cb68478b875d60e859976

TODO list, open questions:

* Should isolated=1 always imply config->ignore_environment=1 and config->user_site_directory = 0?
* Should we rename debug to parser_debug? "debug" is a very generic term, whereas this flag is really specific to the parser
* Remove _Py_CheckHashBasedPycsMode: replace it with config._check_hash_pycs_mode?

I created bpo-34228 "Allow PYTHONTRACEMALLOC=0 and -X tracemalloc=0 to disable explicitly tracemalloc" that I need to write unit tests on Python initiallization.

New changeset ecf411c59e33d3760dbfebf6d5b4b205bcc29d5a by Victor Stinner in branch 'master':
bpo-34170: Enhance _PyCoreConfig_Read() (GH-8468)
https://github.com/python/cpython/commit/ecf411c59e33d3760dbfebf6d5b4b205bcc29d5a

New changeset d145775b451bbae549391eb49ed612fb3b03bde1 by Victor Stinner in branch 'master':
bpo-34170: Cleanup pymain_read_conf() (GH-8476)
https://github.com/python/cpython/commit/d145775b451bbae549391eb49ed612fb3b03bde1

New changeset 56b29b6d6fa3eb32bb1533ee3f21b1e7135648a0 by Victor Stinner in branch 'master':
bpo-34170, test_embed: write Py_Initialize() tests (GH-8484)
https://github.com/python/cpython/commit/56b29b6d6fa3eb32bb1533ee3f21b1e7135648a0

New changeset b75d7e243512afcfb2285e6471262478383e09db by Victor Stinner in branch 'master':
bpo-34170: Add _PyCoreConfig._frozen parameter (GH-8591)
https://github.com/python/cpython/commit/b75d7e243512afcfb2285e6471262478383e09db

New changeset a4d20b2e5ece2120f129cb4dda951a6c2461e92d by Victor Stinner in branch 'master':
bpo-34170: Py_Main() updates config when setting Py_InspectFlag (GH-8593)
https://github.com/python/cpython/commit/a4d20b2e5ece2120f129cb4dda951a6c2461e92d

New changeset 9851227382431a40a138fdff994278d9e7743c74 by Victor Stinner in branch 'master':
bpo-34170: Rename _PyCoreConfig.unbuffered_stdip (GH-8594)
https://github.com/python/cpython/commit/9851227382431a40a138fdff994278d9e7743c74

New changeset ea68d83933e6de6cabfb115ec1b8888301947369 by Victor Stinner in branch 'master':
bpo-34170: _PyCoreConfig_Read() defaults to argc=0 (GH-8595)
https://github.com/python/cpython/commit/ea68d83933e6de6cabfb115ec1b8888301947369

New changeset 6c785c0ebdadc84d80a53d896c38fd7ada8ae1f6 by Victor Stinner in branch 'master':
bpo-34170: Add Python/coreconfig.c for _PyCoreConfig (GH-8607)
https://github.com/python/cpython/commit/6c785c0ebdadc84d80a53d896c38fd7ada8ae1f6

New changeset 80b762f010097ab8137782e5fbdc89c5c620ed4e by Victor Stinner in branch 'master':
bpo-31650: Remove _Py_CheckHashBasedPycsMode global config var (GH-8608)
https://github.com/python/cpython/commit/80b762f010097ab8137782e5fbdc89c5c620ed4e

"""
TODO list, open questions:

* Should isolated=1 always imply config->ignore_environment=1 and config->user_site_directory = 0?
* Should we rename debug to parser_debug? "debug" is a very generic term, whereas this flag is really specific to the parser
* Remove _Py_CheckHashBasedPycsMode: replace it with config._check_hash_pycs_mode?
"""

=> DONE!

New changeset 72ec3193b5118a2ccc8be8bf03d7b74691c6a264 by Victor Stinner in branch 'master':
bpo-34170: Cleanup pymain_run_filename() (GH-8631)
https://github.com/python/cpython/commit/72ec3193b5118a2ccc8be8bf03d7b74691c6a264

New changeset 5a953fd0ab4d0f792f4c1537616b2ce8ee40d976 by Victor Stinner in branch 'master':
bpo-34170: _PyCoreConfig_Read() don't replace coerce_c_locale (GH-8658)
https://github.com/python/cpython/commit/5a953fd0ab4d0f792f4c1537616b2ce8ee40d976

New changeset d8078626770a8d358eb83d7928c12d75ff4e821a by Victor Stinner in branch 'master':
bpo-34170: Fix pymain_run_file() (GH-8660)
https://github.com/python/cpython/commit/d8078626770a8d358eb83d7928c12d75ff4e821a

New changeset 89487f51b8d6ba8a55f5de0ed689e46fefe73cc9 by Victor Stinner in branch 'master':
bpo-34207: Fix pymain_read_conf() for UTF-8 Mode (GH-8868)
https://github.com/python/cpython/commit/89487f51b8d6ba8a55f5de0ed689e46fefe73cc9

New changeset 80a0ebaa8385988b1d2dbbbb71ed1f548fc32ad7 by Victor Stinner in branch '3.7':
bpo-34207: Fix pymain_read_conf() for UTF-8 Mode (GH-8868) (GH-8870)
https://github.com/python/cpython/commit/80a0ebaa8385988b1d2dbbbb71ed1f548fc32ad7