diff --git a/Include/cpython/coreconfig.h b/Include/cpython/coreconfig.h
index f05eddad912..ca71c15b67d 100644
--- a/Include/cpython/coreconfig.h
+++ b/Include/cpython/coreconfig.h
@@ -115,27 +115,8 @@ typedef struct {
     PyMemAllocatorName allocator;
 } _PyPreConfig;
 
-#ifdef MS_WINDOWS
-#  define _PyPreConfig_WINDOWS_INIT \
-        .legacy_windows_fs_encoding = -1,
-#else
-#  define _PyPreConfig_WINDOWS_INIT
-#endif
-
-#define _PyPreConfig_INIT \
-    (_PyPreConfig){ \
-        _PyPreConfig_WINDOWS_INIT \
-        ._config_version = _Py_CONFIG_VERSION, \
-        .isolated = -1, \
-        .use_environment = -1, \
-        .configure_locale = 1, \
-        .utf8_mode = -2, \
-        .dev_mode = -1, \
-        .allocator = PYMEM_ALLOCATOR_NOT_SET}
-
-PyAPI_FUNC(void) _PyPreConfig_Init(_PyPreConfig *config);
 PyAPI_FUNC(void) _PyPreConfig_InitPythonConfig(_PyPreConfig *config);
-PyAPI_FUNC(void) _PyPreConfig_InitIsolateConfig(_PyPreConfig *config);
+PyAPI_FUNC(void) _PyPreConfig_InitIsolatedConfig(_PyPreConfig *config);
 
 
 /* --- _PyCoreConfig ---------------------------------------------- */
@@ -419,47 +400,23 @@ typedef struct {
 
 } _PyCoreConfig;
 
-#ifdef MS_WINDOWS
-#  define _PyCoreConfig_WINDOWS_INIT \
-        .legacy_windows_stdio = -1,
-#else
-#  define _PyCoreConfig_WINDOWS_INIT
-#endif
-
-#define _PyCoreConfig_INIT \
-    (_PyCoreConfig){ \
-        _PyCoreConfig_WINDOWS_INIT \
-        ._config_version = _Py_CONFIG_VERSION, \
-        .isolated = -1, \
-        .use_environment = -1, \
-        .dev_mode = -1, \
-        .install_signal_handlers = 1, \
-        .use_hash_seed = -1, \
-        .faulthandler = -1, \
-        .tracemalloc = -1, \
-        .use_module_search_paths = 0, \
-        .parse_argv = 0, \
-        .site_import = -1, \
-        .bytes_warning = -1, \
-        .inspect = -1, \
-        .interactive = -1, \
-        .optimization_level = -1, \
-        .parser_debug= -1, \
-        .write_bytecode = -1, \
-        .verbose = -1, \
-        .quiet = -1, \
-        .user_site_directory = -1, \
-        .configure_c_stdio = 0, \
-        .buffered_stdio = -1, \
-        ._install_importlib = 1, \
-        .check_hash_pycs_mode = NULL, \
-        .pathconfig_warnings = -1, \
-        ._init_main = 1}
-/* Note: _PyCoreConfig_INIT sets other fields to 0/NULL */
-
-PyAPI_FUNC(void) _PyCoreConfig_Init(_PyCoreConfig *config);
 PyAPI_FUNC(_PyInitError) _PyCoreConfig_InitPythonConfig(_PyCoreConfig *config);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_InitIsolateConfig(_PyCoreConfig *config);
+PyAPI_FUNC(_PyInitError) _PyCoreConfig_InitIsolatedConfig(_PyCoreConfig *config);
+PyAPI_FUNC(void) _PyCoreConfig_Clear(_PyCoreConfig *);
+PyAPI_FUNC(_PyInitError) _PyCoreConfig_SetString(
+    wchar_t **config_str,
+    const wchar_t *str);
+PyAPI_FUNC(_PyInitError) _PyCoreConfig_DecodeLocale(
+    wchar_t **config_str,
+    const char *str);
+PyAPI_FUNC(_PyInitError) _PyCoreConfig_Read(_PyCoreConfig *config);
+PyAPI_FUNC(_PyInitError) _PyCoreConfig_SetArgv(
+    _PyCoreConfig *config,
+    Py_ssize_t argc,
+    char **argv);
+PyAPI_FUNC(_PyInitError) _PyCoreConfig_SetWideArgv(_PyCoreConfig *config,
+    Py_ssize_t argc,
+    wchar_t **argv);
 
 #ifdef __cplusplus
 }
diff --git a/Include/internal/pycore_coreconfig.h b/Include/internal/pycore_coreconfig.h
index ea4418f5061..edde7b1d8d8 100644
--- a/Include/internal/pycore_coreconfig.h
+++ b/Include/internal/pycore_coreconfig.h
@@ -121,8 +121,6 @@ PyAPI_FUNC(_PyInitError) _PyPreCmdline_Read(_PyPreCmdline *cmdline,
 /* --- _PyPreConfig ----------------------------------------------- */
 
 PyAPI_FUNC(void) _PyPreConfig_Init(_PyPreConfig *config);
-PyAPI_FUNC(void) _PyPreConfig_InitPythonConfig(_PyPreConfig *config);
-PyAPI_FUNC(void) _PyPreConfig_InitIsolatedConfig(_PyPreConfig *config);
 PyAPI_FUNC(void) _PyPreConfig_Copy(_PyPreConfig *config,
     const _PyPreConfig *config2);
 PyAPI_FUNC(PyObject*) _PyPreConfig_AsDict(const _PyPreConfig *config);
@@ -135,34 +133,18 @@ PyAPI_FUNC(_PyInitError) _PyPreConfig_Write(const _PyPreConfig *config);
 
 /* --- _PyCoreConfig ---------------------------------------------- */
 
-PyAPI_FUNC(void) _PyCoreConfig_Clear(_PyCoreConfig *);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_InitPythonConfig(_PyCoreConfig *config);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_InitIsolatedConfig(_PyCoreConfig *config);
+PyAPI_FUNC(void) _PyCoreConfig_Init(_PyCoreConfig *config);
 PyAPI_FUNC(_PyInitError) _PyCoreConfig_Copy(
     _PyCoreConfig *config,
     const _PyCoreConfig *config2);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_SetString(
-    wchar_t **config_str,
-    const wchar_t *str);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_DecodeLocale(
-    wchar_t **config_str,
-    const char *str);
 PyAPI_FUNC(_PyInitError) _PyCoreConfig_InitPathConfig(_PyCoreConfig *config);
 PyAPI_FUNC(_PyInitError) _PyCoreConfig_SetPathConfig(
     const _PyCoreConfig *config);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_Read(_PyCoreConfig *config);
 PyAPI_FUNC(void) _PyCoreConfig_Write(const _PyCoreConfig *config,
     _PyRuntimeState *runtime);
 PyAPI_FUNC(_PyInitError) _PyCoreConfig_SetPyArgv(
     _PyCoreConfig *config,
     const _PyArgv *args);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_SetArgv(
-    _PyCoreConfig *config,
-    Py_ssize_t argc,
-    char **argv);
-PyAPI_FUNC(_PyInitError) _PyCoreConfig_SetWideArgv(_PyCoreConfig *config,
-    Py_ssize_t argc,
-    wchar_t **argv);
 
 
 /* --- Function used for testing ---------------------------------- */
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index c389df85fb6..6b77a2d1fd8 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -285,6 +285,16 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         'coerce_c_locale_warn': 0,
         'utf8_mode': 0,
     }
+    ISOLATED_PRE_CONFIG = dict(DEFAULT_PRE_CONFIG,
+        configure_locale=0,
+        isolated=1,
+        use_environment=0,
+        utf8_mode=0,
+        dev_mode=0,
+    )
+    if MS_WINDOWS:
+        ISOLATED_PRE_CONFIG['legacy_windows_fs_encoding'] = 0
+
     COPY_PRE_CONFIG = [
         'dev_mode',
         'isolated',
@@ -363,6 +373,24 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
             'legacy_windows_stdio': 0,
         })
 
+    PYTHON_CORE_CONFIG = dict(DEFAULT_CORE_CONFIG,
+        configure_c_stdio=1,
+        parse_argv=1,
+    )
+    ISOLATED_CORE_CONFIG = dict(DEFAULT_CORE_CONFIG,
+        isolated=1,
+        use_environment=0,
+        user_site_directory=0,
+        dev_mode=0,
+        install_signal_handlers=0,
+        use_hash_seed=0,
+        faulthandler=0,
+        tracemalloc=0,
+        pathconfig_warnings=0,
+    )
+    if MS_WINDOWS:
+        ISOLATED_CORE_CONFIG['legacy_windows_stdio'] = 0
+
     # global config
     DEFAULT_GLOBAL_CONFIG = {
         'Py_HasFileSystemDefaultEncoding': 0,
@@ -410,8 +438,15 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
                 xoptions[opt] = True
         return xoptions
 
-    def get_expected_config(self, expected_preconfig, expected, env, add_path=None):
-        expected = dict(self.DEFAULT_CORE_CONFIG, **expected)
+    def get_expected_config(self, expected_preconfig, expected, env, api,
+                            add_path=None):
+        if api == "python":
+            default_config = self.PYTHON_CORE_CONFIG
+        elif api == "isolated":
+            default_config = self.ISOLATED_CORE_CONFIG
+        else:
+            default_config = self.DEFAULT_CORE_CONFIG
+        expected = dict(default_config, **expected)
 
         code = textwrap.dedent('''
             import json
@@ -521,8 +556,8 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
 
         self.assertEqual(config['global_config'], expected)
 
-    def check_config(self, testname, expected_config, expected_preconfig,
-                     add_path=None, stderr=None):
+    def check_config(self, testname, expected_config=None, expected_preconfig=None,
+                     add_path=None, stderr=None, api="default"):
         env = dict(os.environ)
         # Remove PYTHON* environment variables to get deterministic environment
         for key in list(env):
@@ -533,8 +568,18 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         env['PYTHONCOERCECLOCALE'] = '0'
         env['PYTHONUTF8'] = '0'
 
-        expected_preconfig = dict(self.DEFAULT_PRE_CONFIG, **expected_preconfig)
-        expected_config = self.get_expected_config(expected_preconfig, expected_config, env, add_path)
+        if api == "isolated":
+            default_preconfig = self.ISOLATED_PRE_CONFIG
+        else:
+            default_preconfig = self.DEFAULT_PRE_CONFIG
+        if expected_preconfig is None:
+            expected_preconfig = {}
+        expected_preconfig = dict(default_preconfig, **expected_preconfig)
+        if expected_config is None:
+            expected_config = {}
+        expected_config = self.get_expected_config(expected_preconfig,
+                                                   expected_config, env,
+                                                   api, add_path)
         for key in self.COPY_PRE_CONFIG:
             if key not in expected_preconfig:
                 expected_preconfig[key] = expected_config[key]
@@ -677,76 +722,56 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
             'dev_mode': 1,
             'warnoptions': ['default'],
         }
-        self.check_config("init_dev_mode", config, preconfig)
+        self.check_config("init_dev_mode", config, preconfig, api="python")
 
     def test_init_isolated_flag(self):
-        preconfig = {}
         config = {
             'isolated': 1,
             'use_environment': 0,
             'user_site_directory': 0,
         }
-        self.check_config("init_isolated_flag", config, preconfig)
+        self.check_config("init_isolated_flag", config, api="python")
 
     def test_preinit_isolated1(self):
         # _PyPreConfig.isolated=1, _PyCoreConfig.isolated not set
-        preconfig = {}
         config = {
             'isolated': 1,
             'use_environment': 0,
             'user_site_directory': 0,
         }
-        self.check_config("preinit_isolated1", config, preconfig)
+        self.check_config("preinit_isolated1", config)
 
     def test_preinit_isolated2(self):
         # _PyPreConfig.isolated=0, _PyCoreConfig.isolated=1
-        preconfig = {}
         config = {
             'isolated': 1,
             'use_environment': 0,
             'user_site_directory': 0,
         }
-        self.check_config("preinit_isolated2", config, preconfig)
+        self.check_config("preinit_isolated2", config)
 
     def test_init_isolated_config(self):
-        preconfig = {
-            'configure_locale': 0,
-        }
-        config = {
-            'isolated': 1,
-            'use_environment': 0,
-            'user_site_directory': 0,
-            'install_signal_handlers': 0,
-            'pathconfig_warnings': 0,
-        }
-        self.check_config("init_isolated_config", config, preconfig)
+        self.check_config("init_isolated_config", api="isolated")
 
     def test_init_python_config(self):
-        preconfig = {}
-        config = {
-            'configure_c_stdio': 1,
-            'parse_argv': 1,
-        }
-        self.check_config("init_python_config", config, preconfig)
+        self.check_config("init_python_config", api="python")
 
     def test_init_dont_configure_locale(self):
         # _PyPreConfig.configure_locale=0
         preconfig = {
             'configure_locale': 0,
         }
-        self.check_config("init_dont_configure_locale", {}, preconfig)
+        self.check_config("init_dont_configure_locale", {}, preconfig, api="python")
 
     def test_init_read_set(self):
-        preconfig = {}
         core_config = {
             'program_name': './init_read_set',
             'executable': 'my_executable',
         }
-        self.check_config("init_read_set", core_config, preconfig,
+        self.check_config("init_read_set", core_config, api="python",
                           add_path="init_read_set_path")
 
     def test_init_run_main(self):
-        preconfig = {}
         code = ('import _testinternalcapi, json; '
                 'print(json.dumps(_testinternalcapi.get_configs()))')
         core_config = {
@@ -755,10 +780,9 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
             'run_command': code + '\n',
             'parse_argv': 1,
         }
-        self.check_config("init_run_main", core_config, preconfig)
+        self.check_config("init_run_main", core_config, api="python")
 
     def test_init_main(self):
-        preconfig = {}
         code = ('import _testinternalcapi, json; '
                 'print(json.dumps(_testinternalcapi.get_configs()))')
         core_config = {
@@ -768,25 +792,26 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
             'parse_argv': 1,
             '_init_main': 0,
         }
-        self.check_config("init_main", core_config, preconfig,
+        self.check_config("init_main", core_config, api="python",
                           stderr="Run Python code before _Py_InitializeMain")
 
     def test_init_parse_argv(self):
         core_config = {
+            'parse_argv': 1,
             'argv': ['-c', 'arg1', '-v', 'arg3'],
             'program_name': './argv0',
-            'parse_argv': 1,
             'run_command': 'pass\n',
             'use_environment': 0,
         }
-        self.check_config("init_parse_argv", core_config, {})
+        self.check_config("init_parse_argv", core_config, api="python")
 
     def test_init_dont_parse_argv(self):
         core_config = {
+            'parse_argv': 0,
             'argv': ['./argv0', '-E', '-c', 'pass', 'arg1', '-v', 'arg3'],
             'program_name': './argv0',
         }
-        self.check_config("init_dont_parse_argv", core_config, {})
+        self.check_config("init_dont_parse_argv", core_config, api="python")
 
 
 if __name__ == "__main__":
diff --git a/Programs/_freeze_importlib.c b/Programs/_freeze_importlib.c
index d89d66abee7..1a719e2f967 100644
--- a/Programs/_freeze_importlib.c
+++ b/Programs/_freeze_importlib.c
@@ -77,14 +77,12 @@ main(int argc, char *argv[])
     text[text_size] = '\0';
 
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
-    config.use_environment = 0;
-    config.user_site_directory = 0;
+    _PyCoreConfig_InitIsolatedConfig(&config);
+
     config.site_import = 0;
     config.program_name = L"./_freeze_importlib";
     /* Don't install importlib, since it could execute outdated bytecode. */
     config._install_importlib = 0;
-    config.pathconfig_warnings = 0;
     config._init_main = 0;
 
     _PyInitError err = _Py_InitializeFromConfig(&config);
diff --git a/Programs/_testembed.c b/Programs/_testembed.c
index 2352179cc6f..f1bb731dcba 100644
--- a/Programs/_testembed.c
+++ b/Programs/_testembed.c
@@ -5,6 +5,7 @@
 
 #include <Python.h>
 #include "pycore_coreconfig.h"   /* FIXME: PEP 587 makes these functions public */
+#include <Python.h>
 #include "pythread.h"
 #include <inttypes.h>
 #include <stdio.h>
@@ -404,7 +405,7 @@ static int test_init_from_config(void)
     config.use_hash_seed = 1;
     config.hash_seed = 123;
 
-    /* dev_mode=1 is tested in init_dev_mode() */
+    /* dev_mode=1 is tested in test_init_dev_mode() */
 
     putenv("PYTHONFAULTHANDLER=");
     config.faulthandler = 1;
@@ -521,12 +522,12 @@ static int test_init_from_config(void)
 }
 
 
-static int test_init_parse_argv(int parse_argv)
+static int check_init_parse_argv(int parse_argv)
 {
     _PyInitError err;
 
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitPythonConfig(&config);
 
     static wchar_t* argv[] = {
         L"./argv0",
@@ -552,15 +553,15 @@ static int test_init_parse_argv(int parse_argv)
 }
 
 
-static int init_parse_argv(void)
+static int test_init_parse_argv(void)
 {
-    return test_init_parse_argv(1);
+    return check_init_parse_argv(1);
 }
 
 
-static int init_dont_parse_argv(void)
+static int test_init_dont_parse_argv(void)
 {
-    return test_init_parse_argv(0);
+    return check_init_parse_argv(0);
 }
 
 
@@ -603,7 +604,7 @@ static int test_init_env(void)
 }
 
 
-static void test_init_env_dev_mode_putenvs(void)
+static void set_all_env_vars(void)
 {
     test_init_env_putenvs();
     putenv("PYTHONMALLOC=");
@@ -616,7 +617,7 @@ static int test_init_env_dev_mode(void)
 {
     /* Test initialization from environment variables */
     Py_IgnoreEnvironmentFlag = 0;
-    test_init_env_dev_mode_putenvs();
+    set_all_env_vars();
     _testembed_Py_Initialize();
     dump_config();
     Py_Finalize();
@@ -628,7 +629,7 @@ static int test_init_env_dev_mode_alloc(void)
 {
     /* Test initialization from environment variables */
     Py_IgnoreEnvironmentFlag = 0;
-    test_init_env_dev_mode_putenvs();
+    set_all_env_vars();
     putenv("PYTHONMALLOC=malloc");
     _testembed_Py_Initialize();
     dump_config();
@@ -637,13 +638,13 @@ static int test_init_env_dev_mode_alloc(void)
 }
 
 
-static int init_isolated_flag(void)
+static int test_init_isolated_flag(void)
 {
     _PyInitError err;
 
     /* Test _PyCoreConfig.isolated=1 */
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitPythonConfig(&config);
 
     Py_IsolatedFlag = 0;
     config.isolated = 1;
@@ -651,7 +652,7 @@ static int init_isolated_flag(void)
     /* Use path starting with "./" avoids a search along the PATH */
     config.program_name = L"./_testembed";
 
-    test_init_env_dev_mode_putenvs();
+    set_all_env_vars();
     err = _Py_InitializeFromConfig(&config);
     if (_PyInitError_Failed(err)) {
         _Py_ExitInitError(err);
@@ -680,7 +681,7 @@ static int test_preinit_isolated1(void)
     _PyCoreConfig_Init(&config);
     config.program_name = L"./_testembed";
 
-    test_init_env_dev_mode_putenvs();
+    set_all_env_vars();
     err = _Py_InitializeFromConfig(&config);
     if (_PyInitError_Failed(err)) {
         _Py_ExitInitError(err);
@@ -715,7 +716,7 @@ static int test_preinit_isolated2(void)
     /* Use path starting with "./" avoids a search along the PATH */
     config.program_name = L"./_testembed";
 
-    test_init_env_dev_mode_putenvs();
+    set_all_env_vars();
     err = _Py_InitializeFromConfig(&config);
     if (_PyInitError_Failed(err)) {
         _Py_ExitInitError(err);
@@ -726,10 +727,38 @@ static int test_preinit_isolated2(void)
 }
 
 
-static int init_isolated_config(void)
+static void set_all_global_config_variables(void)
+{
+    Py_IsolatedFlag = 0;
+    Py_IgnoreEnvironmentFlag = 0;
+    Py_BytesWarningFlag = 2;
+    Py_InspectFlag = 1;
+    Py_InteractiveFlag = 1;
+    Py_OptimizeFlag = 1;
+    Py_DebugFlag = 1;
+    Py_VerboseFlag = 1;
+    Py_QuietFlag = 1;
+    Py_FrozenFlag = 0;
+    Py_UnbufferedStdioFlag = 1;
+    Py_NoSiteFlag = 1;
+    Py_DontWriteBytecodeFlag = 1;
+    Py_NoUserSiteDirectory = 1;
+#ifdef MS_WINDOWS
+    Py_LegacyWindowsStdioFlag = 1;
+#endif
+}
+
+
+static int test_init_isolated_config(void)
 {
     _PyInitError err;
 
+    /* environment variables must be ignored */
+    set_all_env_vars();
+
+    /* global configuration variables must be ignored */
+    set_all_global_config_variables();
+
     _PyPreConfig preconfig;
     _PyPreConfig_InitIsolatedConfig(&preconfig);
 
@@ -759,10 +788,23 @@ static int init_isolated_config(void)
 }
 
 
-static int init_python_config(void)
+static int test_init_python_config(void)
 {
     _PyInitError err;
 
+    /* global configuration variables must be ignored */
+    set_all_global_config_variables();
+    Py_IsolatedFlag = 1;
+    Py_IgnoreEnvironmentFlag = 1;
+    Py_FrozenFlag = 1;
+    Py_UnbufferedStdioFlag = 1;
+    Py_NoSiteFlag = 1;
+    Py_DontWriteBytecodeFlag = 1;
+    Py_NoUserSiteDirectory = 1;
+#ifdef MS_WINDOWS
+    Py_LegacyWindowsStdioFlag = 1;
+#endif
+
     _PyPreConfig preconfig;
     _PyPreConfig_InitPythonConfig(&preconfig);
 
@@ -788,11 +830,12 @@ static int init_python_config(void)
 }
 
 
-static int init_dont_configure_locale(void)
+static int test_init_dont_configure_locale(void)
 {
     _PyInitError err;
 
-    _PyPreConfig preconfig = _PyPreConfig_INIT;
+    _PyPreConfig preconfig;
+    _PyPreConfig_InitPythonConfig(&preconfig);
     preconfig.configure_locale = 0;
     preconfig.coerce_c_locale = 1;
     preconfig.coerce_c_locale_warn = 1;
@@ -802,7 +845,8 @@ static int init_dont_configure_locale(void)
         _Py_ExitInitError(err);
     }
 
-    _PyCoreConfig config = _PyCoreConfig_INIT;
+    _PyCoreConfig config;
+    _PyCoreConfig_InitPythonConfig(&config);
     config.program_name = L"./_testembed";
     err = _Py_InitializeFromConfig(&config);
     if (_PyInitError_Failed(err)) {
@@ -815,10 +859,10 @@ static int init_dont_configure_locale(void)
 }
 
 
-static int init_dev_mode(void)
+static int test_init_dev_mode(void)
 {
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitPythonConfig(&config);
     putenv("PYTHONFAULTHANDLER=");
     putenv("PYTHONMALLOC=");
     config.dev_mode = 1;
@@ -837,7 +881,7 @@ static int test_init_read_set(void)
 {
     _PyInitError err;
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitPythonConfig(&config);
 
     err = _PyCoreConfig_DecodeLocale(&config.program_name, "./init_read_set");
     if (_PyInitError_Failed(err)) {
@@ -894,7 +938,7 @@ static void configure_init_main(_PyCoreConfig *config)
 static int test_init_run_main(void)
 {
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitPythonConfig(&config);
     configure_init_main(&config);
 
     _PyInitError err = _Py_InitializeFromConfig(&config);
@@ -909,7 +953,7 @@ static int test_init_run_main(void)
 static int test_init_main(void)
 {
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitPythonConfig(&config);
     configure_init_main(&config);
     config._init_main = 0;
 
@@ -939,7 +983,7 @@ static int test_init_main(void)
 static int test_run_main(void)
 {
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitPythonConfig(&config);
 
     wchar_t *argv[] = {L"python3", L"-c",
                        (L"import sys; "
@@ -947,7 +991,6 @@ static int test_run_main(void)
                        L"arg2"};
     config.argv.length = Py_ARRAY_LENGTH(argv);
     config.argv.items = argv;
-    config.parse_argv = 1;
     config.program_name = L"./python3";
 
     _PyInitError err = _Py_InitializeFromConfig(&config);
@@ -988,16 +1031,16 @@ static struct TestCase TestCases[] = {
     { "init_default_config", test_init_default_config },
     { "init_global_config", test_init_global_config },
     { "init_from_config", test_init_from_config },
-    { "init_parse_argv", init_parse_argv },
-    { "init_dont_parse_argv", init_dont_parse_argv },
+    { "init_parse_argv", test_init_parse_argv },
+    { "init_dont_parse_argv", test_init_dont_parse_argv },
     { "init_env", test_init_env },
     { "init_env_dev_mode", test_init_env_dev_mode },
     { "init_env_dev_mode_alloc", test_init_env_dev_mode_alloc },
-    { "init_dont_configure_locale", init_dont_configure_locale },
-    { "init_dev_mode", init_dev_mode },
-    { "init_isolated_flag", init_isolated_flag },
-    { "init_isolated_config", init_isolated_config },
-    { "init_python_config", init_python_config },
+    { "init_dont_configure_locale", test_init_dont_configure_locale },
+    { "init_dev_mode", test_init_dev_mode },
+    { "init_isolated_flag", test_init_isolated_flag },
+    { "init_isolated_config", test_init_isolated_config },
+    { "init_python_config", test_init_python_config },
     { "preinit_isolated1", test_preinit_isolated1 },
     { "preinit_isolated2", test_preinit_isolated2 },
     { "init_read_set", test_init_read_set },
diff --git a/Python/coreconfig.c b/Python/coreconfig.c
index fd457262a82..3678d124067 100644
--- a/Python/coreconfig.c
+++ b/Python/coreconfig.c
@@ -551,14 +551,69 @@ _PyCoreConfig_Clear(_PyCoreConfig *config)
 void
 _PyCoreConfig_Init(_PyCoreConfig *config)
 {
-    *config = _PyCoreConfig_INIT;
+    memset(config, 0, sizeof(*config));
+
+    config->_config_version = _Py_CONFIG_VERSION;
+    config->isolated = -1;
+    config->use_environment = -1;
+    config->dev_mode = -1;
+    config->install_signal_handlers = 1;
+    config->use_hash_seed = -1;
+    config->faulthandler = -1;
+    config->tracemalloc = -1;
+    config->use_module_search_paths = 0;
+    config->parse_argv = 0;
+    config->site_import = -1;
+    config->bytes_warning = -1;
+    config->inspect = -1;
+    config->interactive = -1;
+    config->optimization_level = -1;
+    config->parser_debug= -1;
+    config->write_bytecode = -1;
+    config->verbose = -1;
+    config->quiet = -1;
+    config->user_site_directory = -1;
+    config->configure_c_stdio = 0;
+    config->buffered_stdio = -1;
+    config->_install_importlib = 1;
+    config->check_hash_pycs_mode = NULL;
+    config->pathconfig_warnings = -1;
+    config->_init_main = 1;
+#ifdef MS_WINDOWS
+    config->legacy_windows_stdio = -1;
+#endif
+}
+
+
+static void
+_PyCoreConfig_InitDefaults(_PyCoreConfig *config)
+{
+    _PyCoreConfig_Init(config);
+
+    config->isolated = 0;
+    config->use_environment = 1;
+    config->site_import = 1;
+    config->bytes_warning = 0;
+    config->inspect = 0;
+    config->interactive = 0;
+    config->optimization_level = 0;
+    config->parser_debug= 0;
+    config->write_bytecode = 1;
+    config->verbose = 0;
+    config->quiet = 0;
+    config->user_site_directory = 1;
+    config->buffered_stdio = 1;
+    config->pathconfig_warnings = 1;
+#ifdef MS_WINDOWS
+    config->legacy_windows_stdio = 0;
+#endif
 }
 
 
 _PyInitError
 _PyCoreConfig_InitPythonConfig(_PyCoreConfig *config)
 {
-    _PyCoreConfig_Init(config);
+    _PyCoreConfig_InitDefaults(config);
 
     config->configure_c_stdio = 1;
     config->parse_argv = 1;
@@ -570,30 +625,16 @@ _PyCoreConfig_InitPythonConfig(_PyCoreConfig *config)
 _PyInitError
 _PyCoreConfig_InitIsolatedConfig(_PyCoreConfig *config)
 {
-    _PyCoreConfig_Init(config);
+    _PyCoreConfig_InitDefaults(config);
 
-    /* set to 1 */
     config->isolated = 1;
-    config->site_import = 1;
-    config->write_bytecode = 1;
-    config->buffered_stdio = 1;
-
-    /* set to 0 */
     config->use_environment = 0;
+    config->user_site_directory = 0;
     config->dev_mode = 0;
     config->install_signal_handlers = 0;
     config->use_hash_seed = 0;
     config->faulthandler = 0;
     config->tracemalloc = 0;
-    config->bytes_warning = 0;
-    config->inspect = 0;
-    config->interactive = 0;
-    config->optimization_level = 0;
-    config->parser_debug = 0;
-    config->verbose = 0;
-    config->quiet = 0;
-    config->user_site_directory = 0;
-    config->configure_c_stdio = 0;
     config->pathconfig_warnings = 0;
 #ifdef MS_WINDOWS
     config->legacy_windows_stdio = 0;
diff --git a/Python/frozenmain.c b/Python/frozenmain.c
index 7b232bb8023..a51fb580012 100644
--- a/Python/frozenmain.c
+++ b/Python/frozenmain.c
@@ -40,7 +40,7 @@ Py_FrozenMain(int argc, char **argv)
     }
 
     _PyCoreConfig config;
-    _PyCoreConfig_Init(&config);
+    _PyCoreConfig_InitPythonConfig(&config);
     config.pathconfig_warnings = 0;   /* Suppress errors from getpath.c */
 
     if ((p = Py_GETENV("PYTHONINSPECT")) && *p != '\0')
diff --git a/Python/preconfig.c b/Python/preconfig.c
index b7bcfeb9b2f..0f4bd8ece53 100644
--- a/Python/preconfig.c
+++ b/Python/preconfig.c
@@ -241,8 +241,9 @@ _PyPreCmdline_Read(_PyPreCmdline *cmdline,
     }
 
     /* dev_mode */
-    if ((cmdline && _Py_get_xoption(&cmdline->xoptions, L"dev"))
-        || _Py_GetEnv(cmdline->use_environment, "PYTHONDEVMODE"))
+    if ((cmdline->dev_mode < 0)
+        && (_Py_get_xoption(&cmdline->xoptions, L"dev")
+            || _Py_GetEnv(cmdline->use_environment, "PYTHONDEVMODE")))
     {
         cmdline->dev_mode = 1;
     }
@@ -260,10 +261,22 @@ _PyPreCmdline_Read(_PyPreCmdline *cmdline,
 
 /* --- _PyPreConfig ----------------------------------------------- */
 
+
 void
 _PyPreConfig_Init(_PyPreConfig *config)
 {
-    *config = _PyPreConfig_INIT;
+    memset(config, 0, sizeof(*config));
+
+    config->_config_version = _Py_CONFIG_VERSION;
+    config->isolated = -1;
+    config->use_environment = -1;
+    config->configure_locale = 1;
+    config->utf8_mode = -2;
+    config->dev_mode = -1;
+    config->allocator = PYMEM_ALLOCATOR_NOT_SET;
+#ifdef MS_WINDOWS
+    config->legacy_windows_fs_encoding = -1;
+#endif
 }
 
 
@@ -289,11 +302,11 @@ _PyPreConfig_InitIsolatedConfig(_PyPreConfig *config)
     config->configure_locale = 0;
     config->isolated = 1;
     config->use_environment = 0;
+    config->utf8_mode = 0;
+    config->dev_mode = 0;
 #ifdef MS_WINDOWS
     config->legacy_windows_fs_encoding = 0;
 #endif
-    config->utf8_mode = 0;
-    config->dev_mode = 0;
 }