diff --git a/tests/slic3rutils/python_test_support.hpp b/tests/slic3rutils/python_test_support.hpp
index ab66f05887..29747a88c4 100644
--- a/tests/slic3rutils/python_test_support.hpp
+++ b/tests/slic3rutils/python_test_support.hpp
@@ -6,6 +6,7 @@
 #include <boost/filesystem.hpp>
 #include <memory.h>
 #include <stdexcept>
+#include <string>
 #include <pybind11/embed.h>
 #include <pybind11/pybind11.h>
 
@@ -25,8 +26,15 @@ void ensure_python_initialized()
     config.parse_argv = 0;
 
     const auto python_home = boost::dll::program_location().parent_path() / "python";
+#ifdef _WIN32
+    const auto stdlib = python_home / "Lib";
+#else
+    const auto stdlib = python_home / "lib" /
+                        ("python" + std::to_string(PY_MAJOR_VERSION) + "." + std::to_string(PY_MINOR_VERSION));
+#endif
 
-    if (boost::filesystem::exists(python_home)) {
+    // Only a real runtime: a stray python/ folder (packages a test left behind) is not a home.
+    if (boost::filesystem::exists(stdlib / "encodings")) {
         const std::string home = python_home.string();
         const PyStatus status  = PyConfig_SetBytesString(&config, &config.home, home.c_str());
 
diff --git a/tests/slic3rutils/test_plugin_capability_config.cpp b/tests/slic3rutils/test_plugin_capability_config.cpp
index f224f92308..39ecd1002d 100644
--- a/tests/slic3rutils/test_plugin_capability_config.cpp
+++ b/tests/slic3rutils/test_plugin_capability_config.cpp
@@ -38,6 +38,9 @@ namespace {
 // before this destructor's shutdown() runs.
 struct ScopedPluginManager
 {
+    // Before initialize(): the interpreter creates {data_dir}/python/packages and {data_dir}/log,
+    // which would otherwise land in the working directory.
+    ScopedDataDir python_data_dir{"plugin-python"};
     bool initialized = PluginManager::instance().initialize();
 
     ~ScopedPluginManager()
diff --git a/tests/slic3rutils/test_plugin_cloud_metadata.cpp b/tests/slic3rutils/test_plugin_cloud_metadata.cpp
index c34b596ee0..241ef3ed90 100644
--- a/tests/slic3rutils/test_plugin_cloud_metadata.cpp
+++ b/tests/slic3rutils/test_plugin_cloud_metadata.cpp
@@ -31,6 +31,9 @@ namespace {
 // same as any other plugin.
 struct ScopedManagerShutdown
 {
+    // Before initialize(): the interpreter creates {data_dir}/python/packages and {data_dir}/log,
+    // which would otherwise land in the working directory.
+    ScopedDataDir python_data_dir{"plugin-python"};
     bool initialized = PluginManager::instance().initialize();
 
     ~ScopedManagerShutdown()
diff --git a/tests/slic3rutils/test_plugin_lifecycle.cpp b/tests/slic3rutils/test_plugin_lifecycle.cpp
index bac3a19ea9..c57e12fb43 100644
--- a/tests/slic3rutils/test_plugin_lifecycle.cpp
+++ b/tests/slic3rutils/test_plugin_lifecycle.cpp
@@ -41,6 +41,9 @@ namespace {
 // Declare this FIRST in a test so it is destroyed last.
 struct ScopedPluginManager
 {
+    // Before initialize(): the interpreter creates {data_dir}/python/packages and {data_dir}/log,
+    // which would otherwise land in the working directory.
+    ScopedDataDir python_data_dir{"plugin-python"};
     bool initialized = false;
 
     ScopedPluginManager() { initialized = PluginManager::instance().initialize(); }
diff --git a/tests/slic3rutils/test_plugin_printer_agent.cpp b/tests/slic3rutils/test_plugin_printer_agent.cpp
index 2065360080..29c11c7fec 100644
--- a/tests/slic3rutils/test_plugin_printer_agent.cpp
+++ b/tests/slic3rutils/test_plugin_printer_agent.cpp
@@ -12,6 +12,8 @@
 #include <memory>
 #include <string>
 
+#include "plugin_test_utils.hpp"
+
 namespace py = pybind11;
 using namespace Slic3r;
 
@@ -21,6 +23,9 @@ namespace {
 // into Python unless PythonInterpreter::instance() reports initialized.
 struct ScopedPluginManager
 {
+    // Before initialize(): the interpreter creates {data_dir}/python/packages and {data_dir}/log,
+    // which would otherwise land in the working directory.
+    ScopedDataDir python_data_dir{"plugin-python"};
     bool initialized = PluginManager::instance().initialize();
 
     ~ScopedPluginManager()
diff --git a/tests/slic3rutils/test_slicing_pipeline_config.cpp b/tests/slic3rutils/test_slicing_pipeline_config.cpp
index ff8cddcba6..37984cff34 100644
--- a/tests/slic3rutils/test_slicing_pipeline_config.cpp
+++ b/tests/slic3rutils/test_slicing_pipeline_config.cpp
@@ -35,6 +35,9 @@ namespace {
 
 struct ScopedPluginManager
 {
+    // Before initialize(): the interpreter creates {data_dir}/python/packages and {data_dir}/log,
+    // which would otherwise land in the working directory.
+    ScopedDataDir python_data_dir{"plugin-python"};
     bool initialized = false;
 
     ScopedPluginManager() { initialized = PluginManager::instance().initialize(); }