diff --git a/.clang-format b/.clang-format
index 11d0dcd..e0958e5 100644
--- a/.clang-format
+++ b/.clang-format
@@ -1 +1,12 @@
 BasedOnStyle: WebKit
+QualifierAlignment: 'Custom'
+QualifierOrder: [
+  'friend',
+  'static',
+  'constexpr',
+  'inline',
+  'volatile',
+  'restrict',
+  'const',
+  'type',
+]
diff --git a/CMakeLists.txt b/CMakeLists.txt
index fa5b942..c9e8d48 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -25,7 +25,7 @@ option(PPPLUGIN_ENABLE_UNREACHABLE_SANITIZE
        "Enable compilation with unreachable sanitize flags" OFF)
 
 find_package(Boost 1.61.0 REQUIRED COMPONENTS headers filesystem)
-find_package(Python 3.0 REQUIRED COMPONENTS Development)
+find_package(Python 3.9...<3.12 REQUIRED COMPONENTS Development)
 find_package(Lua 5.2 REQUIRED)
 
 set(THREADS_PREFER_PTHREAD_FLAG ON)
@@ -60,7 +60,6 @@ set(DEBUG_FLAGS "-g3")
 if(${PPPLUGIN_ENABLE_CPP17_COMPATIBILITY})
   find_package(fmt 8.1.0 REQUIRED)
   set(CMAKE_CXX_STANDARD 17)
-  # TODO: generate C++17 compatible header files instead
   add_definitions("-DPPPLUGIN_CPP17_COMPATIBILITY")
 else()
   set(CMAKE_CXX_STANDARD 20)
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index 890a974..f18ec2e 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -4,3 +4,5 @@ add_subdirectory(lua_plugin)
 add_subdirectory(multi_language_plugin)
 add_subdirectory(multi_return_lua_plugin)
 add_subdirectory(python_plugin)
+
+add_subdirectory(testing) # TODO: to be removed
diff --git a/examples/lua_plugin/CMakeLists.txt b/examples/lua_plugin/CMakeLists.txt
index 4d6708b..09c45f4 100644
--- a/examples/lua_plugin/CMakeLists.txt
+++ b/examples/lua_plugin/CMakeLists.txt
@@ -5,4 +5,6 @@ add_custom_command(
   TARGET lua_plugin_manager
   POST_BUILD
   COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/plugin.lua
-          $<TARGET_FILE_DIR:lua_plugin_manager>)
+          $<TARGET_FILE_DIR:lua_plugin_manager>/plugin_1.lua
+  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/plugin.lua
+          $<TARGET_FILE_DIR:lua_plugin_manager>/plugin_2.lua)
diff --git a/examples/lua_plugin/lua_plugin_manager.cpp b/examples/lua_plugin/lua_plugin_manager.cpp
index 8cc8f9d..88a95b0 100644
--- a/examples/lua_plugin/lua_plugin_manager.cpp
+++ b/examples/lua_plugin/lua_plugin_manager.cpp
@@ -36,9 +36,11 @@ int main(int argc, char* argv[])
             // only load files ending with ".lua" and execute in separate thread
             if (path.extension() == ".lua") {
                 if (auto plugin = manager.loadLuaPlugin(path)) {
-                    threads.emplace_back([plugin = std::move(*plugin)]() mutable {
+                    threads.emplace_back([plugin = std::move(*plugin), plugin_number = threads.size()]() mutable {
+                        // data race due to shared resource "stdout",
+                        // but Lua plugins are otherwise thread-safe
                         std::ignore = plugin.call<void>("initialize");
-                        std::ignore = plugin.call<void>("loop", "2");
+                        std::ignore = plugin.call<void>("loop", plugin_number);
                     });
                 }
             }
diff --git a/examples/python_plugin/python_plugin_manager.cpp b/examples/python_plugin/python_plugin_manager.cpp
index 6d79f72..f195cc6 100644
--- a/examples/python_plugin/python_plugin_manager.cpp
+++ b/examples/python_plugin/python_plugin_manager.cpp
@@ -41,7 +41,7 @@ int main(int argc, char* argv[])
                 if (plugin) {
                     threads.emplace_back([plugin = std::move(*plugin), plugin_number = plugin_number++]() mutable {
                         // ignore calling errors
-                        std::ignore = plugin.call<void>("initialize", plugin_number);
+                        plugin.call<void>("initialize", plugin_number).valueOrElse([](ppplugin::CallError error) { std::cerr << error.what() << std::endl; });
                         std::ignore = plugin.call<void>("loop", std::to_string(plugin_number));
                     });
                 } else {
diff --git a/examples/testing/CMakeLists.txt b/examples/testing/CMakeLists.txt
new file mode 100644
index 0000000..b5b7e2e
--- /dev/null
+++ b/examples/testing/CMakeLists.txt
@@ -0,0 +1,10 @@
+add_executable(testing testing.cpp)
+target_link_libraries(testing ${LIBRARY_TARGET})
+
+add_custom_command(
+  TARGET testing
+  POST_BUILD
+  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/plugin.py
+          $<TARGET_FILE_DIR:testing>/plugin_1.py
+  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/plugin.py
+          $<TARGET_FILE_DIR:testing>/plugin_2.py)
diff --git a/examples/testing/plugin.py b/examples/testing/plugin.py
new file mode 100644
index 0000000..27aa021
--- /dev/null
+++ b/examples/testing/plugin.py
@@ -0,0 +1,14 @@
+import time
+
+
+def initialize(something):
+    print("initialize: ", something)
+
+
+def loop(something):
+    while True:
+        print("loop: ", something)
+        time.sleep(1)
+
+
+print("loading")
diff --git a/examples/testing/testing.cpp b/examples/testing/testing.cpp
new file mode 100644
index 0000000..7f86a3c
--- /dev/null
+++ b/examples/testing/testing.cpp
@@ -0,0 +1,64 @@
+#include "plugin_manager.h"
+#include "python/plugin.h"
+#include "python/python_interpreter.h"
+
+#include <exception>
+#include <filesystem>
+#include <functional>
+#include <iostream>
+#include <thread>
+#include <utility>
+#include <vector>
+
+int main(int argc, char* argv[])
+{
+    std::vector<std::thread> threads;
+    try {
+        if (argc < 1) {
+            return -1;
+        }
+        // path to plugins can be passed via command line;
+        // if no path was specified, the location of the executable is used instead
+        std::filesystem::path plugin_dir;
+        if (argc < 2) {
+            plugin_dir = std::filesystem::path { argv[0] }.parent_path();
+        } else {
+            plugin_dir = std::filesystem::path { argv[1] };
+        }
+        ppplugin::GenericPluginManager<ppplugin::PythonPlugin> const manager;
+        int plugin_number {};
+        std::vector<ppplugin::PythonPlugin> const plugins;
+
+        // checkout: https://stackoverflow.com/questions/26061298/python-multi-thread-multi-interpreter-c-api#26570708
+        // recursively traverse filesystem to find scripts
+        const std::filesystem::recursive_directory_iterator dir_iterator { plugin_dir };
+        for (const auto& entry : dir_iterator) {
+            if (!entry.is_regular_file()) {
+                continue;
+            }
+            const auto& path = entry.path();
+            // only load files ending with ".lua" and execute in separate thread
+            if (path.extension() == ".py") {
+                ppplugin::PythonInterpreter interpreter;
+                if (auto error = interpreter.load(path.string())) {
+                    std::cerr << "failed to load " << path << '\n';
+                    // return -1;
+                }
+                threads.emplace_back([manager, path, plugin_number = plugin_number++, interpreter = std::move(interpreter)]() mutable {
+                    interpreter.call<void>("initialize", std::hash<std::thread::id> {}(std::this_thread::get_id()));
+                    interpreter.call<void>("loop", std::hash<std::thread::id> {}(std::this_thread::get_id()));
+                });
+            }
+        }
+        for (auto& thread : threads) {
+            thread.join();
+        }
+    } catch (const std::exception& exception) {
+        std::cerr << "A fatal error occurred: '" << exception.what() << "'\n";
+        return 1;
+    } catch (...) {
+        std::cerr << "An unknown fatal error occurred!";
+        return 1;
+    }
+    return 0;
+}
diff --git a/include/errors.h b/include/errors.h
index 01d69da..635984e 100644
--- a/include/errors.h
+++ b/include/errors.h
@@ -47,7 +47,7 @@ class CallError {
     {
     }
 
-    [[nodiscard]] CallError error() const { return error_; }
+    [[nodiscard]] CallError::Code error() const { return error_; }
     [[nodiscard]] const std::string& what() const { return what_; }
 #ifndef PPPLUGIN_CPP17_COMPATIBILITY
     [[nodiscard]] const std::source_location& location() const
diff --git a/include/lua/lua_state.h b/include/lua/lua_state.h
index 3bc80f1..a908639 100644
--- a/include/lua/lua_state.h
+++ b/include/lua/lua_state.h
@@ -134,7 +134,12 @@ class LuaState {
     std::optional<std::function<Func>> popFunction(bool always_pop = false);
 
     void pushOne(double value);
-    void pushOne(int value);
+    void pushOne(unsigned int value) { pushOne(static_cast<long long>(value)); }
+    void pushOne(int value) { pushOne(static_cast<long long>(value)); }
+    void pushOne(unsigned long value) { pushOne(static_cast<long long>(value)); }
+    void pushOne(long value) { pushOne(static_cast<long long>(value)); }
+    void pushOne(unsigned long long value) { pushOne(static_cast<long long>(value)); }
+    void pushOne(long long value);
     void pushOne(const char* value);
     void pushOne(std::string_view value);
     void pushOne(bool value);
@@ -259,7 +264,6 @@ auto LuaState::topFunction()
     }
     return std::optional<decltype(top_function)> { std::nullopt };
 }
-
 } // namespace ppplugin
 
 #endif // PPPLUGIN_LUA_STATE_H
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 9a02424..872b264 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -19,6 +19,9 @@ target_link_libraries(
   PUBLIC Python::Python
   PRIVATE ${LUA_LIBRARIES})
 if(${PPPLUGIN_ENABLE_CPP17_COMPATIBILITY})
+  # TODO: generate C++17 compatible header files instead
+  # target_compile_options(${LIBRARY_TARGET} PUBLIC
+  # "-DPPPLUGIN_CPP17_COMPATIBILITY") # TODO: fix
   target_link_libraries(${LIBRARY_TARGET} PUBLIC fmt::fmt)
 endif()
 target_include_directories(
diff --git a/src/boost_dll_loader.cpp b/src/boost_dll_loader.cpp
index 62fc600..859e1c7 100644
--- a/src/boost_dll_loader.cpp
+++ b/src/boost_dll_loader.cpp
@@ -10,6 +10,11 @@
 #include <boost/filesystem/path.hpp>
 
 namespace {
+/**
+ * Helper function to determine if given function can be called in given library.
+ *
+ * @return std::nullopt if function is callable, otherwise an error
+ */
 std::optional<ppplugin::CallError> isCallable(const boost::dll::shared_library& library, const std::string& function_name)
 {
     if (!library.is_loaded()) {
diff --git a/src/lua_script.cpp b/src/lua_script.cpp
index 0ad710e..3169aeb 100644
--- a/src/lua_script.cpp
+++ b/src/lua_script.cpp
@@ -30,7 +30,8 @@ LuaScript::LuaScript()
     luaL_openlibs(state_.state());
     state_.registerPanicHandler([](lua_State* state) -> int {
         auto error = LuaState::wrap(state).top<std::string>();
-        // TODO: don't throw because will cross library boundaries
+        // TODO: don't throw because will cross library boundaries;
+        //       use global error state (thread-local or lock-protected)
         throw std::runtime_error(format("Lua PANIC: '{}'!", error.value_or("?")));
     });
     // TODO: setup lua_setwarnf
diff --git a/src/lua_state.cpp b/src/lua_state.cpp
index 6c75962..424a86c 100644
--- a/src/lua_state.cpp
+++ b/src/lua_state.cpp
@@ -77,7 +77,7 @@ void LuaState::pushOne(double value)
     lua_pushnumber(state(), value);
 }
 
-void LuaState::pushOne(int value)
+void LuaState::pushOne(long long value)
 {
     lua_pushinteger(state(), value);
 }