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()) {