diff --git a/Include/cpython/coreconfig.h b/Include/cpython/coreconfig.h
index 621a09f..827a19a 100644
--- a/Include/cpython/coreconfig.h
+++ b/Include/cpython/coreconfig.h
@@ -123,7 +123,9 @@
 /* --- _PyCoreConfig ---------------------------------------------- */
 
 typedef struct {
-    _PyPreConfig preconfig;
+    int isolated;
+    int use_environment;
+    int dev_mode;
 
     /* Install signal handlers? Yes by default. */
     int install_signal_handlers;
@@ -375,7 +377,9 @@
 #define _PyCoreConfig_INIT \
     (_PyCoreConfig){ \
         _PyCoreConfig_WINDOWS_INIT \
-        .preconfig = _PyPreConfig_INIT, \
+        .isolated = -1, \
+        .use_environment = -1, \
+        .dev_mode = -1, \
         .install_signal_handlers = 1, \
         .use_hash_seed = -1, \
         .faulthandler = -1, \
diff --git a/Include/cpython/pylifecycle.h b/Include/cpython/pylifecycle.h
index 5f3a522..e32e54c 100644
--- a/Include/cpython/pylifecycle.h
+++ b/Include/cpython/pylifecycle.h
@@ -16,7 +16,7 @@
 
 PyAPI_FUNC(_PyInitError) _Py_PreInitialize(void);
 PyAPI_FUNC(_PyInitError) _Py_PreInitializeFromPreConfig(
-    _PyPreConfig *preconfig);
+    const _PyPreConfig *preconfig);
 PyAPI_FUNC(_PyInitError) _Py_PreInitializeFromConfig(
     const _PyCoreConfig *coreconfig);
 
diff --git a/Include/internal/pycore_coreconfig.h b/Include/internal/pycore_coreconfig.h
index 829ff31..d79f590 100644
--- a/Include/internal/pycore_coreconfig.h
+++ b/Include/internal/pycore_coreconfig.h
@@ -16,12 +16,14 @@
     _PyWstrList xoptions;     /* "-X value" option */
     int use_environment;      /* -E option */
     int isolated;             /* -I option */
+    int dev_mode;             /* -X dev and PYTHONDEVMODE */
 } _PyPreCmdline;
 
 #define _PyPreCmdline_INIT \
     (_PyPreCmdline){ \
         .use_environment = -1, \
-        .isolated = -1}
+        .isolated = -1, \
+        .dev_mode = -1}
 /* Note: _PyPreCmdline_INIT sets other fields to 0/NULL */
 
 PyAPI_FUNC(void) _PyPreCmdline_Clear(_PyPreCmdline *cmdline);
@@ -112,7 +114,7 @@
 PyAPI_FUNC(_PyInitError) _PyCoreConfig_Read(_PyCoreConfig *config);
 PyAPI_FUNC(_PyInitError) _PyCoreConfig_ReadFromArgv(_PyCoreConfig *config,
     const _PyArgv *args);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_Write(const _PyCoreConfig *config);
+PyAPI_FUNC(void) _PyCoreConfig_Write(const _PyCoreConfig *config);
 
 /* --- _PyMainInterpreterConfig ----------------------------------- */
 
diff --git a/Include/internal/pycore_pylifecycle.h b/Include/internal/pycore_pylifecycle.h
index 9514b1c..3214d6b 100644
--- a/Include/internal/pycore_pylifecycle.h
+++ b/Include/internal/pycore_pylifecycle.h
@@ -77,6 +77,9 @@
 
 PyAPI_FUNC(void) _PyGC_DumpShutdownStats(void);
 
+PyAPI_FUNC(_PyInitError) _Py_PreInitializeInPlace(
+    _PyPreConfig *config);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 6e145a5..ff3cfb1 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -272,12 +272,19 @@
         'allocator': None,
         'coerce_c_locale': 0,
         'coerce_c_locale_warn': 0,
-        'dev_mode': 0,
-        'isolated': 0,
-        'use_environment': 1,
         'utf8_mode': 0,
     }
+    COPY_PRE_CONFIG = [
+        'dev_mode',
+        'isolated',
+        'use_environment',
+    ]
+
     DEFAULT_CORE_CONFIG = {
+        'isolated': 0,
+        'use_environment': 1,
+        'dev_mode': 0,
+
         'install_signal_handlers': 1,
         'use_hash_seed': 0,
         'hash_seed': 0,
@@ -363,8 +370,6 @@
         '_Py_HasFileSystemDefaultEncodeErrors': 0,
     }
     COPY_GLOBAL_PRE_CONFIG = [
-        ('Py_IgnoreEnvironmentFlag', 'use_environment', True),
-        ('Py_IsolatedFlag', 'isolated'),
         ('Py_UTF8Mode', 'utf8_mode'),
     ]
     COPY_GLOBAL_CONFIG = [
@@ -376,8 +381,10 @@
         ('Py_FileSystemDefaultEncodeErrors', 'filesystem_errors'),
         ('Py_FileSystemDefaultEncoding', 'filesystem_encoding'),
         ('Py_FrozenFlag', '_frozen'),
+        ('Py_IgnoreEnvironmentFlag', 'use_environment', True),
         ('Py_InspectFlag', 'inspect'),
         ('Py_InteractiveFlag', 'interactive'),
+        ('Py_IsolatedFlag', 'isolated'),
         ('Py_NoSiteFlag', 'site_import', True),
         ('Py_NoUserSiteDirectory', 'user_site_directory', True),
         ('Py_OptimizeFlag', 'optimization_level'),
@@ -415,7 +422,7 @@
         expected['xoptions'] = self.main_xoptions(core_config['xoptions'])
         self.assertEqual(main_config, expected)
 
-    def get_expected_config(self, expected, expected_preconfig, env):
+    def get_expected_config(self, expected, env):
         expected = dict(self.DEFAULT_CORE_CONFIG, **expected)
 
         code = textwrap.dedent('''
@@ -443,7 +450,7 @@
         # when test_embed is run from a venv (bpo-35313)
         args = (sys.executable, '-S', '-c', code)
         env = dict(env)
-        if not expected_preconfig['isolated']:
+        if not expected['isolated']:
             env['PYTHONCOERCECLOCALE'] = '0'
             env['PYTHONUTF8'] = '0'
         proc = subprocess.run(args, env=env,
@@ -509,7 +516,10 @@
         config = json.loads(out)
 
         expected_preconfig = dict(self.DEFAULT_PRE_CONFIG, **expected_preconfig)
-        expected_config = self.get_expected_config(expected_config, expected_preconfig, env)
+        expected_config = self.get_expected_config(expected_config, env)
+        for key in self.COPY_PRE_CONFIG:
+            if key not in expected_preconfig:
+                expected_preconfig[key] = expected_config[key]
 
         self.check_core_config(config, expected_config)
         self.check_pre_config(config, expected_preconfig)
@@ -617,35 +627,36 @@
 
     def test_init_env_dev_mode(self):
         preconfig = dict(self.INIT_ENV_PRECONFIG,
-                      allocator='debug',
-                      dev_mode=1)
+                      allocator='debug')
         config = dict(self.INIT_ENV_CONFIG,
                       dev_mode=1)
         self.check_config("init_env_dev_mode", config, preconfig)
 
-    def test_init_env_dev_mode(self):
+    def test_init_env_dev_mode_alloc(self):
         preconfig = dict(self.INIT_ENV_PRECONFIG,
-                         allocator='malloc',
-                         dev_mode=1)
-        config = dict(self.INIT_ENV_CONFIG)
+                         allocator='malloc')
+        config = dict(self.INIT_ENV_CONFIG,
+                      dev_mode=1)
         self.check_config("init_env_dev_mode_alloc", config, preconfig)
 
     def test_init_dev_mode(self):
         preconfig = {
             'allocator': 'debug',
-            'dev_mode': 1,
         }
         config = {
             'faulthandler': 1,
+            'dev_mode': 1,
         }
         self.check_config("init_dev_mode", config, preconfig)
 
     def test_init_isolated(self):
         preconfig = {
-            'isolated': 1,
-            'use_environment': 0,
+            'isolated': 0,
+            'use_environment': 1,
         }
         config = {
+            'isolated': 1,
+            'use_environment': 0,
             'user_site_directory': 0,
         }
         self.check_config("init_isolated", config, preconfig)
diff --git a/Modules/main.c b/Modules/main.c
index 46bc72a..9fcc76e 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -294,7 +294,7 @@
         goto done;
     }
 
-    err = _Py_PreInitializeFromPreConfig(&config);
+    err = _Py_PreInitializeInPlace(&config);
 
 done:
     _PyPreConfig_Clear(&config);
@@ -311,11 +311,6 @@
         return err;
     }
 
-    err = _PyCoreConfig_Write(config);
-    if (_Py_INIT_FAILED(err)) {
-        return err;
-    }
-
     return _Py_InitializeCore(interp_p, config);
 }
 
@@ -483,7 +478,7 @@
 static void
 pymain_import_readline(const _PyCoreConfig *config)
 {
-    if (config->preconfig.isolated) {
+    if (config->isolated) {
         return;
     }
     if (!config->inspect && RUN_CODE(config)) {
@@ -655,7 +650,7 @@
 static void
 pymain_run_startup(_PyCoreConfig *config, PyCompilerFlags *cf)
 {
-    const char *startup = _Py_GetEnv(config->preconfig.use_environment, "PYTHONSTARTUP");
+    const char *startup = _Py_GetEnv(config->use_environment, "PYTHONSTARTUP");
     if (startup == NULL) {
         return;
     }
@@ -735,7 +730,7 @@
 {
     /* Check this environment variable at the end, to give programs the
        opportunity to set it from Python. */
-    if (!Py_InspectFlag && _Py_GetEnv(config->preconfig.use_environment, "PYTHONINSPECT")) {
+    if (!Py_InspectFlag && _Py_GetEnv(config->use_environment, "PYTHONINSPECT")) {
         Py_InspectFlag = 1;
         config->inspect = 1;
     }
@@ -775,7 +770,7 @@
             goto done;
         }
     }
-    else if (!config->preconfig.isolated) {
+    else if (!config->isolated) {
         PyObject *path0 = NULL;
         if (_PyPathConfig_ComputeSysPath0(&config->argv, &path0)) {
             if (path0 == NULL) {
diff --git a/Programs/_freeze_importlib.c b/Programs/_freeze_importlib.c
index e6c51a4..0818012 100644
--- a/Programs/_freeze_importlib.c
+++ b/Programs/_freeze_importlib.c
@@ -77,7 +77,7 @@
     text[text_size] = '\0';
 
     _PyCoreConfig config = _PyCoreConfig_INIT;
-    config.preconfig.use_environment = 0;
+    config.use_environment = 0;
     config.user_site_directory = 0;
     config.site_import = 0;
     config.program_name = L"./_freeze_importlib";
diff --git a/Programs/_testembed.c b/Programs/_testembed.c
index ab5802d..70ef45f 100644
--- a/Programs/_testembed.c
+++ b/Programs/_testembed.c
@@ -397,6 +397,22 @@
 
 static int test_init_from_config(void)
 {
+    _PyInitError err;
+
+    _PyPreConfig preconfig = _PyPreConfig_INIT;
+
+    putenv("PYTHONMALLOC=malloc_debug");
+    preconfig.allocator = "malloc";
+
+    putenv("PYTHONUTF8=0");
+    Py_UTF8Mode = 0;
+    preconfig.utf8_mode = 1;
+
+    err = _Py_PreInitializeFromPreConfig(&preconfig);
+    if (_Py_INIT_FAILED(err)) {
+        _Py_ExitInitError(err);
+    }
+
     /* Test _Py_InitializeFromConfig() */
     _PyCoreConfig config = _PyCoreConfig_INIT;
     config.install_signal_handlers = 0;
@@ -407,9 +423,6 @@
     config.use_hash_seed = 1;
     config.hash_seed = 123;
 
-    putenv("PYTHONMALLOC=malloc_debug");
-    config.preconfig.allocator = "malloc";
-
     /* dev_mode=1 is tested in test_init_dev_mode() */
 
     putenv("PYTHONFAULTHANDLER=");
@@ -430,10 +443,6 @@
 
     /* FIXME: test coerce_c_locale and coerce_c_locale_warn */
 
-    putenv("PYTHONUTF8=0");
-    Py_UTF8Mode = 0;
-    config.preconfig.utf8_mode = 1;
-
     putenv("PYTHONPYCACHEPREFIX=env_pycache_prefix");
     config.pycache_prefix = L"conf_pycache_prefix";
 
@@ -521,7 +530,7 @@
     Py_FrozenFlag = 0;
     config._frozen = 1;
 
-    _PyInitError err = _Py_InitializeFromConfig(&config);
+    err = _Py_InitializeFromConfig(&config);
     /* Don't call _PyCoreConfig_Clear() since all strings are static */
     if (_Py_INIT_FAILED(err)) {
         _Py_ExitInitError(err);
@@ -607,20 +616,30 @@
 
 static int test_init_isolated(void)
 {
+    _PyInitError err;
+
+    _PyPreConfig preconfig = _PyPreConfig_INIT;
+
+    /* Set coerce_c_locale and utf8_mode to not depend on the locale */
+    preconfig.coerce_c_locale = 0;
+    preconfig.utf8_mode = 0;
+
+    err = _Py_PreInitializeFromPreConfig(&preconfig);
+    if (_Py_INIT_FAILED(err)) {
+        _Py_ExitInitError(err);
+    }
+
     /* Test _PyCoreConfig.isolated=1 */
     _PyCoreConfig config = _PyCoreConfig_INIT;
 
     Py_IsolatedFlag = 0;
-    config.preconfig.isolated = 1;
+    config.isolated = 1;
 
-    /* Set coerce_c_locale and utf8_mode to not depend on the locale */
-    config.preconfig.coerce_c_locale = 0;
-    config.preconfig.utf8_mode = 0;
     /* Use path starting with "./" avoids a search along the PATH */
     config.program_name = L"./_testembed";
 
     test_init_env_dev_mode_putenvs();
-    _PyInitError err = _Py_InitializeFromConfig(&config);
+    err = _Py_InitializeFromConfig(&config);
     if (_Py_INIT_FAILED(err)) {
         _Py_ExitInitError(err);
     }
@@ -635,7 +654,7 @@
     _PyCoreConfig config = _PyCoreConfig_INIT;
     putenv("PYTHONFAULTHANDLER=");
     putenv("PYTHONMALLOC=");
-    config.preconfig.dev_mode = 1;
+    config.dev_mode = 1;
     config.program_name = L"./_testembed";
     _PyInitError err = _Py_InitializeFromConfig(&config);
     if (_Py_INIT_FAILED(err)) {
diff --git a/Python/coreconfig.c b/Python/coreconfig.c
index 1245aef..2e6eb40 100644
--- a/Python/coreconfig.c
+++ b/Python/coreconfig.c
@@ -469,8 +469,6 @@
 void
 _PyCoreConfig_Clear(_PyCoreConfig *config)
 {
-    _PyPreConfig_Clear(&config->preconfig);
-
 #define CLEAR(ATTR) \
     do { \
         PyMem_RawFree(ATTR); \
@@ -514,10 +512,6 @@
 {
     _PyCoreConfig_Clear(config);
 
-    if (_PyPreConfig_Copy(&config->preconfig, &config2->preconfig) < 0) {
-        return -1;
-    }
-
 #define COPY_ATTR(ATTR) config->ATTR = config2->ATTR
 #define COPY_STR_ATTR(ATTR) \
     do { \
@@ -544,6 +538,9 @@
         } \
     } while (0)
 
+    COPY_ATTR(isolated);
+    COPY_ATTR(use_environment);
+    COPY_ATTR(dev_mode);
     COPY_ATTR(install_signal_handlers);
     COPY_ATTR(use_hash_seed);
     COPY_ATTR(hash_seed);
@@ -613,7 +610,7 @@
 static const char*
 _PyCoreConfig_GetEnv(const _PyCoreConfig *config, const char *name)
 {
-    return _Py_GetEnv(config->preconfig.use_environment, name);
+    return _Py_GetEnv(config->use_environment, name);
 }
 
 
@@ -622,9 +619,9 @@
                         wchar_t **dest,
                         wchar_t *wname, char *name)
 {
-    assert(config->preconfig.use_environment >= 0);
+    assert(config->use_environment >= 0);
 
-    if (!config->preconfig.use_environment) {
+    if (!config->use_environment) {
         *dest = NULL;
         return 0;
     }
@@ -668,8 +665,6 @@
 void
 _PyCoreConfig_GetGlobalConfig(_PyCoreConfig *config)
 {
-    _PyPreConfig_GetGlobalConfig(&config->preconfig);
-
 #define COPY_FLAG(ATTR, VALUE) \
         if (config->ATTR == -1) { \
             config->ATTR = VALUE; \
@@ -679,6 +674,8 @@
             config->ATTR = !(VALUE); \
         }
 
+    COPY_FLAG(isolated, Py_IsolatedFlag);
+    COPY_NOT_FLAG(use_environment, Py_IgnoreEnvironmentFlag);
     COPY_FLAG(bytes_warning, Py_BytesWarningFlag);
     COPY_FLAG(inspect, Py_InspectFlag);
     COPY_FLAG(interactive, Py_InteractiveFlag);
@@ -714,6 +711,8 @@
             VAR = !config->ATTR; \
         }
 
+    COPY_FLAG(isolated, Py_IsolatedFlag);
+    COPY_NOT_FLAG(use_environment, Py_IgnoreEnvironmentFlag);
     COPY_FLAG(bytes_warning, Py_BytesWarningFlag);
     COPY_FLAG(inspect, Py_InspectFlag);
     COPY_FLAG(interactive, Py_InteractiveFlag);
@@ -924,7 +923,7 @@
 static _PyInitError
 config_read_env_vars(_PyCoreConfig *config)
 {
-    int use_env = config->preconfig.use_environment;
+    int use_env = config->use_environment;
 
     /* Get environment variables */
     _Py_get_env_flag(use_env, &config->parser_debug, "PYTHONDEBUG");
@@ -1149,7 +1148,8 @@
 
 
 static _PyInitError
-config_init_stdio_encoding(_PyCoreConfig *config)
+config_init_stdio_encoding(_PyCoreConfig *config,
+                           const _PyPreConfig *preconfig)
 {
     /* If Py_SetStandardStreamEncoding() have been called, use these
         parameters. */
@@ -1219,7 +1219,7 @@
     }
 
     /* UTF-8 Mode uses UTF-8/surrogateescape */
-    if (config->preconfig.utf8_mode) {
+    if (preconfig->utf8_mode) {
         if (config->stdio_encoding == NULL) {
             config->stdio_encoding = _PyMem_RawStrdup("utf-8");
             if (config->stdio_encoding == NULL) {
@@ -1254,10 +1254,10 @@
 
 
 static _PyInitError
-config_init_fs_encoding(_PyCoreConfig *config)
+config_init_fs_encoding(_PyCoreConfig *config, const _PyPreConfig *preconfig)
 {
 #ifdef MS_WINDOWS
-    if (config->preconfig.legacy_windows_fs_encoding) {
+    if (preconfig->legacy_windows_fs_encoding) {
         /* Legacy Windows filesystem encoding: mbcs/replace */
         if (config->filesystem_encoding == NULL) {
             config->filesystem_encoding = _PyMem_RawStrdup("mbcs");
@@ -1292,7 +1292,7 @@
     }
 #else
     if (config->filesystem_encoding == NULL) {
-        if (config->preconfig.utf8_mode) {
+        if (preconfig->utf8_mode) {
             /* UTF-8 Mode use: utf-8/surrogateescape */
             config->filesystem_encoding = _PyMem_RawStrdup("utf-8");
             /* errors defaults to surrogateescape above */
@@ -1341,12 +1341,8 @@
 {
     _PyInitError err;
 
-    err = _Py_PreInitializeFromConfig(config);
-    if (_Py_INIT_FAILED(err)) {
-        return err;
-    }
-
-    _PyPreCmdline_GetPreConfig(cmdline, &_PyRuntime.preconfig);
+    const _PyPreConfig *preconfig = &_PyRuntime.preconfig;
+    _PyPreCmdline_GetPreConfig(cmdline, preconfig);
     _PyPreCmdline_GetCoreConfig(cmdline, config);
 
     err = _PyPreCmdline_Read(cmdline);
@@ -1360,19 +1356,16 @@
         return _Py_INIT_NO_MEMORY();
     }
 
-    if (_PyPreConfig_Copy(&config->preconfig, &_PyRuntime.preconfig) < 0) {
-        return _Py_INIT_NO_MEMORY();
-    }
-
     _PyCoreConfig_GetGlobalConfig(config);
 
-    assert(config->preconfig.use_environment >= 0);
+    assert(config->use_environment >= 0);
 
-    if (config->preconfig.isolated > 0) {
+    if (config->isolated > 0) {
+        config->use_environment = 0;
         config->user_site_directory = 0;
     }
 
-    if (config->preconfig.use_environment) {
+    if (config->use_environment) {
         err = config_read_env_vars(config);
         if (_Py_INIT_FAILED(err)) {
             return err;
@@ -1421,7 +1414,7 @@
     }
 
     /* default values */
-    if (config->preconfig.dev_mode) {
+    if (config->dev_mode) {
         if (config->faulthandler < 0) {
             config->faulthandler = 1;
         }
@@ -1438,13 +1431,13 @@
     }
 
     if (config->filesystem_encoding == NULL || config->filesystem_errors == NULL) {
-        err = config_init_fs_encoding(config);
+        err = config_init_fs_encoding(config, preconfig);
         if (_Py_INIT_FAILED(err)) {
             return err;
         }
     }
 
-    err = config_init_stdio_encoding(config);
+    err = config_init_stdio_encoding(config, preconfig);
     if (_Py_INIT_FAILED(err)) {
         return err;
     }
@@ -1456,7 +1449,7 @@
         }
     }
 
-    assert(config->preconfig.use_environment >= 0);
+    assert(config->use_environment >= 0);
     assert(config->filesystem_encoding != NULL);
     assert(config->filesystem_errors != NULL);
     assert(config->stdio_encoding != NULL);
@@ -1544,22 +1537,11 @@
 
    - set Py_xxx global configuration variables
    - initialize C standard streams (stdin, stdout, stderr) */
-_PyInitError
+void
 _PyCoreConfig_Write(const _PyCoreConfig *config)
 {
     _PyCoreConfig_SetGlobalConfig(config);
     config_init_stdio(config);
-
-    /* Write the new pre-configuration into _PyRuntime */
-    PyMemAllocatorEx old_alloc;
-    _PyMem_SetDefaultAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
-    int res = _PyPreConfig_Copy(&_PyRuntime.preconfig, &config->preconfig);
-    PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
-    if (res < 0) {
-        return _Py_INIT_NO_MEMORY();
-    }
-
-    return _Py_INIT_OK();
 }
 
 
@@ -1604,6 +1586,9 @@
 #define SET_ITEM_WSTRLIST(LIST) \
     SET_ITEM(#LIST, _PyWstrList_AsList(&config->LIST))
 
+    SET_ITEM_INT(isolated);
+    SET_ITEM_INT(use_environment);
+    SET_ITEM_INT(dev_mode);
     SET_ITEM_INT(install_signal_handlers);
     SET_ITEM_INT(use_hash_seed);
     SET_ITEM_UINT(hash_seed);
@@ -1945,7 +1930,7 @@
      * the lowest precedence entries first so that later entries override them.
      */
 
-    if (config->preconfig.dev_mode) {
+    if (config->dev_mode) {
         if (_PyWstrList_Append(&config->warnoptions, L"default")) {
             return _Py_INIT_NO_MEMORY();
         }
@@ -2101,7 +2086,7 @@
         return err;
     }
 
-    if (config->preconfig.use_environment) {
+    if (config->use_environment) {
         err = cmdline_init_env_warnoptions(cmdline, config);
         if (_Py_INIT_FAILED(err)) {
             return err;
@@ -2178,8 +2163,7 @@
 
     /* pre config */
     PyInterpreterState *interp = _PyInterpreterState_Get();
-    const _PyCoreConfig *core_config = _PyInterpreterState_GetCoreConfig(interp);
-    const _PyPreConfig *pre_config = &core_config->preconfig;
+    const _PyPreConfig *pre_config = &_PyRuntime.preconfig;
     dict = _PyPreConfig_AsDict(pre_config);
     if (dict == NULL) {
         goto error;
@@ -2190,6 +2174,7 @@
     Py_CLEAR(dict);
 
     /* core config */
+    const _PyCoreConfig *core_config = _PyInterpreterState_GetCoreConfig(interp);
     dict = _PyCoreConfig_AsDict(core_config);
     if (dict == NULL) {
         goto error;
diff --git a/Python/pathconfig.c b/Python/pathconfig.c
index f0b13fd..7fea7c3 100644
--- a/Python/pathconfig.c
+++ b/Python/pathconfig.c
@@ -331,7 +331,7 @@
 #endif
 
     if (path_config.isolated != -1) {
-        config->preconfig.isolated = path_config.isolated;
+        config->isolated = path_config.isolated;
     }
     if (path_config.site_import != -1) {
         config->site_import = path_config.site_import;
diff --git a/Python/preconfig.c b/Python/preconfig.c
index 8b685ce..d336352 100644
--- a/Python/preconfig.c
+++ b/Python/preconfig.c
@@ -143,6 +143,23 @@
 
     COPY_ATTR(use_environment);
     COPY_ATTR(isolated);
+    COPY_ATTR(dev_mode);
+
+#undef COPY_ATTR
+}
+
+
+void
+_PyPreCmdline_SetPreConfig(const _PyPreCmdline *cmdline, _PyPreConfig *config)
+{
+#define COPY_ATTR(ATTR) \
+    if (cmdline->ATTR != -1) { \
+        config->ATTR = cmdline->ATTR; \
+    }
+
+    COPY_ATTR(use_environment);
+    COPY_ATTR(isolated);
+    COPY_ATTR(dev_mode);
 
 #undef COPY_ATTR
 }
@@ -152,12 +169,13 @@
 _PyPreCmdline_GetCoreConfig(_PyPreCmdline *cmdline, const _PyCoreConfig *config)
 {
 #define COPY_ATTR(ATTR) \
-    if (config->preconfig.ATTR != -1) { \
-        cmdline->ATTR = config->preconfig.ATTR; \
+    if (config->ATTR != -1) { \
+        cmdline->ATTR = config->ATTR; \
     }
 
     COPY_ATTR(use_environment);
     COPY_ATTR(isolated);
+    COPY_ATTR(dev_mode);
 
 #undef COPY_ATTR
 }
@@ -167,12 +185,13 @@
 _PyPreCmdline_SetCoreConfig(const _PyPreCmdline *cmdline, _PyCoreConfig *config)
 {
 #define COPY_ATTR(ATTR) \
-    if (config->preconfig.ATTR == -1 && cmdline->ATTR != -1) { \
-        config->preconfig.ATTR = cmdline->ATTR; \
+    if (config->ATTR == -1 && cmdline->ATTR != -1) { \
+        config->ATTR = cmdline->ATTR; \
     }
 
     COPY_ATTR(use_environment);
     COPY_ATTR(isolated);
+    COPY_ATTR(dev_mode);
 
 #undef COPY_ATTR
 }
@@ -206,13 +225,13 @@
 
     COPY_ATTR(isolated);
     COPY_ATTR(use_environment);
+    COPY_ATTR(dev_mode);
     COPY_ATTR(coerce_c_locale);
     COPY_ATTR(coerce_c_locale_warn);
 #ifdef MS_WINDOWS
     COPY_ATTR(legacy_windows_fs_encoding);
 #endif
     COPY_ATTR(utf8_mode);
-    COPY_ATTR(dev_mode);
     COPY_STR_ATTR(allocator);
 
 #undef COPY_ATTR
@@ -567,21 +586,6 @@
 }
 
 
-void
-_PyPreCmdline_SetPreConfig(const _PyPreCmdline *cmdline, _PyPreConfig *config)
-{
-#define COPY_ATTR(ATTR) \
-    if (cmdline->ATTR != -1) { \
-        config->ATTR = cmdline->ATTR; \
-    }
-
-    COPY_ATTR(use_environment);
-    COPY_ATTR(isolated);
-
-#undef COPY_ATTR
-}
-
-
 PyObject*
 _PyPreConfig_AsDict(const _PyPreConfig *config)
 {
@@ -712,7 +716,7 @@
     if (coreconfig) {
         _PyPreCmdline_GetCoreConfig(&cmdline, coreconfig);
         if (config->dev_mode == -1) {
-            config->dev_mode = coreconfig->preconfig.dev_mode;
+            config->dev_mode = coreconfig->dev_mode;
         }
     }
 
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index e08f290..b12fa82 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -286,9 +286,10 @@
     "locales is recommended.\n";
 
 static void
-_emit_stderr_warning_for_legacy_locale(const _PyCoreConfig *core_config)
+_emit_stderr_warning_for_legacy_locale(void)
 {
-    if (core_config->preconfig.coerce_c_locale_warn && _Py_LegacyLocaleDetected()) {
+    const _PyPreConfig *preconfig = &_PyRuntime.preconfig;
+    if (preconfig->coerce_c_locale_warn && _Py_LegacyLocaleDetected()) {
         PySys_FormatStderr("%s", _C_LOCALE_WARNING);
     }
 }
@@ -675,6 +676,8 @@
 {
     PyInterpreterState *interp;
 
+    _PyCoreConfig_Write(core_config);
+
     _PyInitError err = pycore_init_runtime(core_config);
     if (_Py_INIT_FAILED(err)) {
         return err;
@@ -720,54 +723,64 @@
                const _PyCoreConfig *coreconfig)
 {
     _PyInitError err;
+    _PyPreConfig local_config = _PyPreConfig_INIT;
+    if (!config) {
+        config = &local_config;
+    }
 
     err = _PyRuntime_Initialize();
     if (_Py_INIT_FAILED(err)) {
-        return err;
+        goto done;
     }
 
     if (_PyRuntime.pre_initialized) {
         /* If it's already configured: ignored the new configuration */
-        return _Py_INIT_OK();
-    }
-
-    if (!src_config && coreconfig) {
-        src_config = &coreconfig->preconfig;
+        err = _Py_INIT_OK();
+        goto done;
     }
 
     if (src_config) {
         if (_PyPreConfig_Copy(config, src_config) < 0) {
-            return _Py_INIT_ERR("failed to copy pre config");
+            err = _Py_INIT_ERR("failed to copy pre config");
+            goto done;
         }
     }
 
     err = _PyPreConfig_Read(config, NULL, coreconfig);
     if (_Py_INIT_FAILED(err)) {
-        return err;
+        goto done;
     }
 
     err = _PyPreConfig_Write(config);
     if (_Py_INIT_FAILED(err)) {
-        return err;
+        goto done;
     }
 
     _PyRuntime.pre_initialized = 1;
-    return _Py_INIT_OK();
+    err = _Py_INIT_OK();
+
+done:
+    _PyPreConfig_Clear(&local_config);
+    return err;
 }
 
 
 _PyInitError
 _Py_PreInitialize(void)
 {
-    _PyPreConfig config = _PyPreConfig_INIT;
-    _PyInitError err = pyinit_preinit(&config, NULL, NULL);
-    _PyPreConfig_Clear(&config);
-    return err;
+    return pyinit_preinit(NULL, NULL, NULL);
 }
 
 
 _PyInitError
-_Py_PreInitializeFromPreConfig(_PyPreConfig *config)
+_Py_PreInitializeFromPreConfig(const _PyPreConfig *src_config)
+{
+    return pyinit_preinit(NULL, src_config, NULL);
+}
+
+
+_PyInitError
+_Py_PreInitializeInPlace(_PyPreConfig *config)
 {
     return pyinit_preinit(config, NULL, NULL);
 }
@@ -776,10 +789,7 @@
 _PyInitError
 _Py_PreInitializeFromConfig(const _PyCoreConfig *coreconfig)
 {
-    _PyPreConfig config = _PyPreConfig_INIT;
-    _PyInitError err = pyinit_preinit(&config, NULL, coreconfig);
-    _PyPreConfig_Clear(&config);
-    return err;
+    return pyinit_preinit(NULL, NULL, coreconfig);
 }
 
 
@@ -964,7 +974,7 @@
     }
 
 #ifndef MS_WINDOWS
-    _emit_stderr_warning_for_legacy_locale(core_config);
+    _emit_stderr_warning_for_legacy_locale();
 #endif
 
     return _Py_INIT_OK();
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 3df4d44..12ec7d5 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -2158,6 +2158,7 @@
 {
     int pos = 0;
     PyObject *seq;
+    const _PyPreConfig *preconfig = &_PyRuntime.preconfig;
     const _PyCoreConfig *config = &_PyInterpreterState_GET_UNSAFE()->core_config;
 
     seq = PyStructSequence_New(&FlagsType);
@@ -2174,16 +2175,16 @@
     SetFlag(!config->write_bytecode);
     SetFlag(!config->user_site_directory);
     SetFlag(!config->site_import);
-    SetFlag(!config->preconfig.use_environment);
+    SetFlag(!config->use_environment);
     SetFlag(config->verbose);
     /* SetFlag(saw_unbuffered_flag); */
     /* SetFlag(skipfirstline); */
     SetFlag(config->bytes_warning);
     SetFlag(config->quiet);
     SetFlag(config->use_hash_seed == 0 || config->hash_seed != 0);
-    SetFlag(config->preconfig.isolated);
-    PyStructSequence_SET_ITEM(seq, pos++, PyBool_FromLong(config->preconfig.dev_mode));
-    SetFlag(config->preconfig.utf8_mode);
+    SetFlag(config->isolated);
+    PyStructSequence_SET_ITEM(seq, pos++, PyBool_FromLong(config->dev_mode));
+    SetFlag(preconfig->utf8_mode);
 #undef SetFlag
 
     if (PyErr_Occurred()) {