Use PyConfig_InitPythonConfig instead of PyConfig_InitIsolatedConfig (#4473)

* Use PyConfig_InitPythonConfig instead of PyConfig_InitIsolatedConfig

* add unit test for default python configuration

---------
Co-authored-by: Daniel Jacobs <daniel.jacobs@is4s.com>

include/pybind11/embed.h

@@ -198,9 +198,10 @@ inline void initialize_interpreter(bool init_signal_handlers = true,
         init_signal_handlers, argc, argv, add_program_dir_to_path);
 #else
     PyConfig config;
-    PyConfig_InitIsolatedConfig(&config);
-    config.isolated = 0;
-    config.use_environment = 1;
+    PyConfig_InitPythonConfig(&config);
+    // See PR #4473 for background
+    config.parse_argv = 0;
+
     config.install_signal_handlers = init_signal_handlers ? 1 : 0;
     initialize_interpreter(&config, argc, argv, add_program_dir_to_path);
 #endif

tests/test_embed/test_interpreter.cpp

@@ -184,7 +184,7 @@ TEST_CASE("Custom PyConfig") {
     py::initialize_interpreter();
 }
 
-TEST_CASE("Custom PyConfig with argv") {
+TEST_CASE("scoped_interpreter with PyConfig_InitIsolatedConfig and argv") {
     py::finalize_interpreter();
     {
         PyConfig config;
@@ -199,6 +199,26 @@ TEST_CASE("Custom PyConfig with argv") {
     }
     py::initialize_interpreter();
 }
+
+TEST_CASE("scoped_interpreter with PyConfig_InitPythonConfig and argv") {
+    py::finalize_interpreter();
+    {
+        PyConfig config;
+        PyConfig_InitPythonConfig(&config);
+
+        // `initialize_interpreter() overrides the default value for config.parse_argv (`1`) by
+        // changing it to `0`. This test exercises `scoped_interpreter` with the default config.
+        char *argv[] = {strdup("a.out"), strdup("arg1")};
+        py::scoped_interpreter argv_scope(&config, 2, argv);
+        std::free(argv[0]);
+        std::free(argv[1]);
+        auto module = py::module::import("test_interpreter");
+        auto py_widget = module.attr("DerivedWidget")("The question");
+        const auto &cpp_widget = py_widget.cast<const Widget &>();
+        REQUIRE(cpp_widget.argv0() == "arg1");
+    }
+    py::initialize_interpreter();
+}
 #endif
 
 TEST_CASE("Add program dir to path pre-PyConfig") {