diff --git a/Modules/main.c b/Modules/main.c
index 1f1bf53..6a707c4 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -1364,9 +1364,24 @@
 }
 
 
+void
+_PyCoreConfig_GetGlobalConfig(_PyCoreConfig *config)
+{
+#define COPY_FLAG(ATTR, VALUE) \
+        if (config->ATTR == -1) { \
+            config->ATTR = VALUE; \
+        }
+
+    COPY_FLAG(ignore_environment, Py_IgnoreEnvironmentFlag);
+    COPY_FLAG(utf8_mode, Py_UTF8Mode);
+
+#undef COPY_FLAG
+}
+
+
 /* Get Py_xxx global configuration variables */
 static void
-pymain_get_global_config(_PyMain *pymain, _Py_CommandLineDetails *cmdline)
+cmdline_get_global_config(_Py_CommandLineDetails *cmdline)
 {
     cmdline->bytes_warning = Py_BytesWarningFlag;
     cmdline->debug = Py_DebugFlag;
@@ -1385,15 +1400,24 @@
     cmdline->legacy_windows_stdio = Py_LegacyWindowsStdioFlag;
 #endif
     cmdline->check_hash_pycs_mode = _Py_CheckHashBasedPycsMode ;
+}
 
-    pymain->config.ignore_environment = Py_IgnoreEnvironmentFlag;
-    pymain->config.utf8_mode = Py_UTF8Mode;
+
+void
+_PyCoreConfig_SetGlobalConfig(const _PyCoreConfig *config)
+{
+    Py_IgnoreEnvironmentFlag = config->ignore_environment;
+    Py_UTF8Mode = config->utf8_mode;
+
+    /* Random or non-zero hash seed */
+    Py_HashRandomizationFlag = (config->use_hash_seed == 0 ||
+                                config->hash_seed != 0);
 }
 
 
 /* Set Py_xxx global configuration variables */
 static void
-pymain_set_global_config(_PyMain *pymain, _Py_CommandLineDetails *cmdline)
+cmdline_set_global_config(_Py_CommandLineDetails *cmdline)
 {
     Py_BytesWarningFlag = cmdline->bytes_warning;
     Py_DebugFlag = cmdline->debug;
@@ -1412,13 +1436,6 @@
     Py_LegacyWindowsFSEncodingFlag = cmdline->legacy_windows_fs_encoding;
     Py_LegacyWindowsStdioFlag = cmdline->legacy_windows_stdio;
 #endif
-
-    Py_IgnoreEnvironmentFlag = pymain->config.ignore_environment;
-    Py_UTF8Mode = pymain->config.utf8_mode;
-
-    /* Random or non-zero hash seed */
-    Py_HashRandomizationFlag = (pymain->config.use_hash_seed == 0 ||
-                                pymain->config.hash_seed != 0);
 }
 
 
@@ -1632,7 +1649,7 @@
 
 
 static _PyInitError
-pymain_init_tracemalloc(_PyCoreConfig *config)
+config_init_tracemalloc(_PyCoreConfig *config)
 {
     int nframe;
     int valid;
@@ -1714,6 +1731,27 @@
 }
 
 
+/* Set global variable variables from environment variables */
+void
+_Py_Initialize_ReadEnvVarsNoAlloc(void)
+{
+    _Py_CommandLineDetails cmdline;
+    memset(&cmdline, 0, sizeof(cmdline));
+
+    cmdline_get_global_config(&cmdline);
+    if (cmdline.isolated) {
+        Py_IgnoreEnvironmentFlag = 1;
+        cmdline.no_user_site_directory = 1;
+    }
+    if (!Py_IgnoreEnvironmentFlag) {
+        cmdline_get_env_flags(&cmdline);
+    }
+    cmdline_set_global_config(&cmdline);
+
+    /* no need to call pymain_clear_cmdline(), no memory has been allocated */
+}
+
+
 static _PyInitError
 config_init_home(_PyCoreConfig *config)
 {
@@ -1741,17 +1779,15 @@
 static _PyInitError
 config_init_hash_seed(_PyCoreConfig *config)
 {
-    if (config->use_hash_seed < 0) {
-        const char *seed_text = config_get_env_var("PYTHONHASHSEED");
-        int use_hash_seed;
-        unsigned long hash_seed;
-        if (_Py_ReadHashSeed(seed_text, &use_hash_seed, &hash_seed) < 0) {
-            return _Py_INIT_USER_ERR("PYTHONHASHSEED must be \"random\" "
-                                     "or an integer in range [0; 4294967295]");
-        }
-        config->use_hash_seed = use_hash_seed;
-        config->hash_seed = hash_seed;
+    const char *seed_text = config_get_env_var("PYTHONHASHSEED");
+    int use_hash_seed;
+    unsigned long hash_seed;
+    if (_Py_ReadHashSeed(seed_text, &use_hash_seed, &hash_seed) < 0) {
+        return _Py_INIT_USER_ERR("PYTHONHASHSEED must be \"random\" "
+                                 "or an integer in range [0; 4294967295]");
     }
+    config->use_hash_seed = use_hash_seed;
+    config->hash_seed = hash_seed;
     return _Py_INIT_OK();
 }
 
@@ -1759,12 +1795,6 @@
 static _PyInitError
 config_init_utf8_mode(_PyCoreConfig *config)
 {
-    /* The option was already set by Py_UTF8Mode,
-       Py_LegacyWindowsFSEncodingFlag or PYTHONLEGACYWINDOWSFSENCODING. */
-    if (config->utf8_mode >= 0) {
-        return _Py_INIT_OK();
-    }
-
     const wchar_t *xopt = config_get_xoption(config, L"utf8");
     if (xopt) {
         wchar_t *sep = wcschr(xopt, L'=');
@@ -1808,7 +1838,11 @@
 static _PyInitError
 config_read_env_vars(_PyCoreConfig *config)
 {
-    config->allocator = config_get_env_var("PYTHONMALLOC");
+    assert(!config->ignore_environment);
+
+    if (config->allocator == NULL) {
+        config->allocator = config_get_env_var("PYTHONMALLOC");
+    }
 
     if (config_get_env_var("PYTHONDUMPREFS")) {
         config->dump_refs = 1;
@@ -1817,16 +1851,18 @@
         config->malloc_stats = 1;
     }
 
-    const char *env = config_get_env_var("PYTHONCOERCECLOCALE");
-    if (env) {
-        if (strcmp(env, "0") == 0) {
-            config->coerce_c_locale = 0;
-        }
-        else if (strcmp(env, "warn") == 0) {
-            config->coerce_c_locale_warn = 1;
-        }
-        else {
-            config->coerce_c_locale = 1;
+    if (config->coerce_c_locale < 0) {
+        const char *env = config_get_env_var("PYTHONCOERCECLOCALE");
+        if (env) {
+            if (strcmp(env, "0") == 0) {
+                config->coerce_c_locale = 0;
+            }
+            else if (strcmp(env, "warn") == 0) {
+                config->coerce_c_locale_warn = 1;
+            }
+            else {
+                config->coerce_c_locale = 1;
+            }
         }
     }
 
@@ -1837,9 +1873,11 @@
     }
     config->module_search_path_env = path;
 
-    _PyInitError err = config_init_hash_seed(config);
-    if (_Py_INIT_FAILED(err)) {
-        return err;
+    if (config->use_hash_seed < 0) {
+        _PyInitError err = config_init_hash_seed(config);
+        if (_Py_INIT_FAILED(err)) {
+            return err;
+        }
     }
 
     return _Py_INIT_OK();
@@ -1850,9 +1888,11 @@
 config_read_complex_options(_PyCoreConfig *config)
 {
     /* More complex options configured by env var and -X option */
-    if (config_get_env_var("PYTHONFAULTHANDLER")
-       || config_get_xoption(config, L"faulthandler")) {
-        config->faulthandler = 1;
+    if (config->faulthandler < 0) {
+        if (config_get_env_var("PYTHONFAULTHANDLER")
+           || config_get_xoption(config, L"faulthandler")) {
+            config->faulthandler = 1;
+        }
     }
     if (config_get_env_var("PYTHONPROFILEIMPORTTIME")
        || config_get_xoption(config, L"importtime")) {
@@ -1862,13 +1902,13 @@
         config_get_env_var("PYTHONDEVMODE"))
     {
         config->dev_mode = 1;
-        config->faulthandler = 1;
-        config->allocator = "debug";
     }
 
-    _PyInitError err = pymain_init_tracemalloc(config);
-    if (_Py_INIT_FAILED(err)) {
-        return err;
+    if (config->tracemalloc < 0) {
+        _PyInitError err = config_init_tracemalloc(config);
+        if (_Py_INIT_FAILED(err)) {
+            return err;
+        }
     }
     return _Py_INIT_OK();
 }
@@ -1892,10 +1932,12 @@
 
     /* Set Py_IgnoreEnvironmentFlag for Py_GETENV() */
     _PyCoreConfig *config = &pymain->config;
-    Py_IgnoreEnvironmentFlag = config->ignore_environment;
+    Py_IgnoreEnvironmentFlag = config->ignore_environment || cmdline->isolated;
 
     /* Get environment variables */
-    cmdline_get_env_flags(cmdline);
+    if (!Py_IgnoreEnvironmentFlag) {
+        cmdline_get_env_flags(cmdline);
+    }
 
     err = cmdline_init_env_warnoptions(cmdline);
     if (_Py_INIT_FAILED(err)) {
@@ -1940,6 +1982,8 @@
 static int
 pymain_read_conf(_PyMain *pymain, _Py_CommandLineDetails *cmdline)
 {
+    _PyCoreConfig *config = &pymain->config;
+    _PyCoreConfig save_config = _PyCoreConfig_INIT;
     int res = -1;
 
     char *oldloc = _PyMem_RawStrdup(setlocale(LC_ALL, NULL));
@@ -1953,10 +1997,15 @@
 
     int locale_coerced = 0;
     int loops = 0;
-    int init_ignore_env = pymain->config.ignore_environment;
+    int init_ignore_env = config->ignore_environment;
+
+    if (_PyCoreConfig_Copy(&save_config, config) < 0) {
+        pymain->err = _Py_INIT_NO_MEMORY();
+        goto done;
+    }
 
     while (1) {
-        int utf8_mode = pymain->config.utf8_mode;
+        int init_utf8_mode = config->utf8_mode;
         int encoding_changed = 0;
 
         /* Watchdog to prevent an infinite loop */
@@ -1987,20 +2036,20 @@
          * See the documentation of the PYTHONCOERCECLOCALE setting for more
          * details.
          */
-        if (pymain->config.coerce_c_locale == 1 && !locale_coerced) {
+        if (config->coerce_c_locale == 1 && !locale_coerced) {
             locale_coerced = 1;
-            _Py_CoerceLegacyLocale(&pymain->config);
+            _Py_CoerceLegacyLocale(config);
             encoding_changed = 1;
         }
 
-        if (utf8_mode == -1) {
-            if (pymain->config.utf8_mode == 1) {
+        if (init_utf8_mode == -1) {
+            if (config->utf8_mode == 1) {
                 /* UTF-8 Mode enabled */
                 encoding_changed = 1;
             }
         }
         else {
-            if (pymain->config.utf8_mode != utf8_mode) {
+            if (config->utf8_mode != init_utf8_mode) {
                 encoding_changed = 1;
             }
         }
@@ -2013,12 +2062,18 @@
            Py_DecodeLocale(). Reset Py_IgnoreEnvironmentFlag, modified by
            pymain_read_conf_impl(). Reset Py_IsolatedFlag and Py_NoSiteFlag
            modified by _PyCoreConfig_Read(). */
-        Py_UTF8Mode = pymain->config.utf8_mode;
+        int new_utf8_mode = config->utf8_mode;
         Py_IgnoreEnvironmentFlag = init_ignore_env;
-        _PyCoreConfig_Clear(&pymain->config);
+        if (_PyCoreConfig_Copy(config, &save_config) < 0) {
+            pymain->err = _Py_INIT_NO_MEMORY();
+            goto done;
+        }
         pymain_clear_cmdline(pymain, cmdline);
         memset(cmdline, 0, sizeof(*cmdline));
-        pymain_get_global_config(pymain, cmdline);
+
+        cmdline_get_global_config(cmdline);
+        _PyCoreConfig_GetGlobalConfig(config);
+        config->utf8_mode = new_utf8_mode;
 
         /* The encoding changed: read again the configuration
            with the new encoding */
@@ -2026,6 +2081,7 @@
     res = 0;
 
 done:
+    _PyCoreConfig_Clear(&save_config);
     if (oldloc != NULL) {
         setlocale(LC_ALL, oldloc);
         PyMem_RawFree(oldloc);
@@ -2038,10 +2094,6 @@
 static void
 config_init_locale(_PyCoreConfig *config)
 {
-    if (config->utf8_mode >= 0 && config->coerce_c_locale >= 0) {
-        return;
-    }
-
     if (_Py_LegacyLocaleDetected()) {
         /* POSIX locale: enable C locale coercion and UTF-8 Mode */
         if (config->utf8_mode < 0) {
@@ -2050,15 +2102,6 @@
         if (config->coerce_c_locale < 0) {
             config->coerce_c_locale = 1;
         }
-        return;
-    }
-
-    /* By default, C locale coercion and UTF-8 Mode are disabled */
-    if (config->coerce_c_locale < 0) {
-        config->coerce_c_locale = 0;
-    }
-    if (config->utf8_mode < 0) {
-        config->utf8_mode = 0;
     }
 }
 
@@ -2175,9 +2218,14 @@
 {
     _PyInitError err;
 
-    err = config_read_env_vars(config);
-    if (_Py_INIT_FAILED(err)) {
-        return err;
+    _PyCoreConfig_GetGlobalConfig(config);
+
+    assert(config->ignore_environment >= 0);
+    if (!config->ignore_environment) {
+        err = config_read_env_vars(config);
+        if (_Py_INIT_FAILED(err)) {
+            return err;
+        }
     }
 
     /* -X options */
@@ -2193,26 +2241,29 @@
         return err;
     }
 
-    err = config_init_utf8_mode(config);
-    if (_Py_INIT_FAILED(err)) {
-        return err;
+    if (config->utf8_mode < 0) {
+        err = config_init_utf8_mode(config);
+        if (_Py_INIT_FAILED(err)) {
+            return err;
+        }
     }
 
-    err = config_init_home(config);
-    if (_Py_INIT_FAILED(err)) {
-        return err;
+    if (config->home == NULL) {
+        err = config_init_home(config);
+        if (_Py_INIT_FAILED(err)) {
+            return err;
+        }
     }
 
-    err = config_init_program_name(config);
-    if (_Py_INIT_FAILED(err)) {
-        return err;
+    if (config->program_name == NULL) {
+        err = config_init_program_name(config);
+        if (_Py_INIT_FAILED(err)) {
+            return err;
+        }
     }
 
-    config_init_locale(config);
-
-    /* Signal handlers are installed by default */
-    if (config->install_signal_handlers < 0) {
-        config->install_signal_handlers = 1;
+    if (config->utf8_mode < 0 || config->coerce_c_locale < 0) {
+        config_init_locale(config);
     }
 
     if (!config->_disable_importlib) {
@@ -2221,6 +2272,39 @@
             return err;
         }
     }
+
+    /* default values */
+    if (config->dev_mode) {
+        if (config->faulthandler < 0) {
+            config->faulthandler = 1;
+        }
+        if (config->allocator == NULL) {
+            config->allocator = "debug";
+        }
+    }
+    if (config->install_signal_handlers < 0) {
+        config->install_signal_handlers = 1;
+    }
+    if (config->use_hash_seed < 0) {
+        config->use_hash_seed = 0;
+        config->hash_seed = 0;
+    }
+    if (config->faulthandler < 0) {
+        config->faulthandler = 0;
+    }
+    if (config->tracemalloc < 0) {
+        config->tracemalloc = 0;
+    }
+    if (config->coerce_c_locale < 0) {
+        config->coerce_c_locale = 0;
+    }
+    if (config->utf8_mode < 0) {
+        config->utf8_mode = 0;
+    }
+    if (config->argc < 0) {
+        config->argc = 0;
+    }
+
     return _Py_INIT_OK();
 }
 
@@ -2289,6 +2373,7 @@
         config->LEN = config2->LEN; \
     } while (0)
 
+    COPY_ATTR(install_signal_handlers);
     COPY_ATTR(ignore_environment);
     COPY_ATTR(use_hash_seed);
     COPY_ATTR(hash_seed);
@@ -2302,6 +2387,9 @@
     COPY_ATTR(show_alloc_count);
     COPY_ATTR(dump_refs);
     COPY_ATTR(malloc_stats);
+
+    COPY_ATTR(coerce_c_locale);
+    COPY_ATTR(coerce_c_locale_warn);
     COPY_ATTR(utf8_mode);
 
     COPY_STR_ATTR(module_search_path_env);
@@ -2457,14 +2545,14 @@
 
 
 static int
-pymain_init_python_main(_PyMain *pymain)
+pymain_init_python_main(_PyMain *pymain, PyInterpreterState *interp)
 {
     _PyInitError err;
 
     _PyMainInterpreterConfig main_config = _PyMainInterpreterConfig_INIT;
-    err = _PyMainInterpreterConfig_Read(&main_config, &pymain->config);
+    err = _PyMainInterpreterConfig_Read(&main_config, &interp->core_config);
     if (!_Py_INIT_FAILED(err)) {
-        err = _Py_InitializeMainInterpreter(&main_config);
+        err = _Py_InitializeMainInterpreter(interp, &main_config);
     }
     _PyMainInterpreterConfig_Clear(&main_config);
 
@@ -2615,11 +2703,17 @@
     _Py_CommandLineDetails cmdline;
     memset(&cmdline, 0, sizeof(cmdline));
 
-    pymain_get_global_config(pymain, &cmdline);
+    cmdline_get_global_config(&cmdline);
+    _PyCoreConfig_GetGlobalConfig(&pymain->config);
 
     int res = pymain_cmdline_impl(pymain, &cmdline);
 
-    pymain_set_global_config(pymain, &cmdline);
+    cmdline_set_global_config(&cmdline);
+    _PyCoreConfig_SetGlobalConfig(&pymain->config);
+    if (Py_IsolatedFlag) {
+        Py_IgnoreEnvironmentFlag = 1;
+        Py_NoUserSiteDirectory = 1;
+    }
 
     pymain_clear_cmdline(pymain, &cmdline);
 
@@ -2649,16 +2743,13 @@
 
     pymain_init_stdio(pymain);
 
-    /* bpo-34008: For backward compatibility reasons, calling Py_Main() after
-       Py_Initialize() ignores the new configuration. */
-    if (!_PyRuntime.initialized) {
-        pymain->err = _Py_InitializeCore(&pymain->config);
-        if (_Py_INIT_FAILED(pymain->err)) {
-            _Py_FatalInitError(pymain->err);
-        }
+    PyInterpreterState *interp;
+    pymain->err = _Py_InitializeCore(&interp, &pymain->config);
+    if (_Py_INIT_FAILED(pymain->err)) {
+        _Py_FatalInitError(pymain->err);
     }
 
-    if (pymain_init_python_main(pymain) < 0) {
+    if (pymain_init_python_main(pymain, interp) < 0) {
         _Py_FatalInitError(pymain->err);
     }