Fix the Tcl/Tk setup of the example's plot tests

examples/embedded_python/README.md

@@ -0,0 +1,17 @@
+# Embedded Python example
+
+`tb_example.vhd` calls Python from a VHDL testbench: `exec` and `eval`, function calls, NumPy
+arrays, Python models and verification components, error reporting and plots. See the
+[user guide](../../docs/user_guide.rst) for the API.
+
+## Running
+
+```bash
+python run.py --without-attributes .expected_failure --without-attributes .optional_deps
+```
+
+runs every test that needs no extra Python packages and no user input. Without the options,
+`run.py` also runs the tests that fail by design and says which packages the others need.
+
+The three PySimpleGUI tests are interactive: they open dialogs and wait for an answer, so they
+need a display and someone to answer them.

examples/embedded_python/tb_example.vhd

@@ -71,31 +71,6 @@ begin
     variable coefficients : real_vector(0 to 2);
     variable table : integer_vector_ptr_t;
 
-    procedure set_tcl_installation is
-    begin
-      exec("from os import environ");
-      exec("from sys import prefix");
-      exec("from pathlib import Path");
-      exec("old_environ = environ");
-      exec(
-        "if (Path(prefix) / 'lib' / 'tcl8.6').exists():" +
-        "    environ['TCL_LIBRARY'] = str(Path(prefix) / 'lib' / 'tcl8.6')" +
-        "else:" +
-        "    environ['TCL_LIBRARY'] = str(Path(prefix) / 'tcl' / 'tcl8.6')"
-      );
-      exec(
-        "if (Path(prefix) / 'lib' / 'tk8.6').exists():" +
-        "    environ['TK_LIBRARY'] = str(Path(prefix) / 'lib' / 'tk8.6')" +
-        "else:" +
-        "    environ['TK_LIBRARY'] = str(Path(prefix) / 'tcl' / 'tk8.6')"
-      );
-    end;
-
-    procedure unset_tcl_installation is
-    begin
-      exec("environ = old_environ");
-    end;
-
     procedure query_if(expr : boolean; check_result : check_result_t) is
       variable logger : logger_t;
       variable log_level : log_level_t;
@@ -163,10 +138,8 @@ begin
   begin
     test_runner_setup(runner, runner_cfg);
 
-    -- To avoid mixup with the Riviera-PRO TCL installation I had to
-    -- set the TCL_LIBRARY and TK_LIBRARY environment variables
-    -- to the Python installation. TODO: Find a better way if possible
-    set_tcl_installation;
+    -- For the tests showing a Tk window: the Tcl/Tk of the Python installation
+    import_module_from_file(join(tb_path(runner_cfg), "tcl_library.py"), "tcl_library");
 
     show(display_handler, debug);
 
@@ -712,6 +685,12 @@ begin
       -- Simulators have a limited set of capabilities when it comes to
       -- Visualize simulation output beyond signal waveforms. Python has
       -- almost endless capabilities
+      --
+      -- On Windows, a simulator with a Tcl of its own, like NVC or Questa, can
+      -- crash when matplotlib opens a Tk window and Python has another Tcl
+      -- version: Pillow and matplotlib use the first Tcl they find in the
+      -- process. Use a Python with the Tcl version of the simulator, or another
+      -- backend, for example MPLBACKEND=QtAgg after pip install PySide6.
       ---------------------------------------------------------------------
       elsif run("Test simple plot") then
         exec("from matplotlib import pyplot as plt"); -- Matplotlib is installed with pip install matplotlib
@@ -743,8 +722,6 @@ begin
     end loop;
     -- @formatter:on
 
-    -- Revert to old environment variables
-    unset_tcl_installation;
     test_runner_cleanup(runner);
   end process;
 

examples/embedded_python/tcl_library.py

@@ -0,0 +1,18 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# Copyright (c) 2014-2026, Lars Asplund lars.anders.asplund@gmail.com
+
+"""
+Points Tcl/Tk to the scripts of the Python installation. Tcl looks for them next
+to the executable, which here is the simulator.
+"""
+
+from os import environ
+from pathlib import Path
+from sys import base_prefix
+
+for var, pattern in [("TCL_LIBRARY", "*/tcl[0-9]*/init.tcl"), ("TK_LIBRARY", "*/tk[0-9]*/tk.tcl")]:
+    for script in Path(base_prefix).glob(pattern):
+        environ[var] = str(script.parent)

src/vunit_python_bridge/__init__.py

@@ -58,6 +58,7 @@ def setup(context):
     # pylint: disable=import-outside-toplevel
     from .bridge import setup as setup_bridge
     from .foreign_application import setup_vhpi_application
+    from .native_library import add_python_libraries_to_library_path
     from . import simulator_hooks
 
     simulator_name = context.simulator_name
@@ -73,6 +74,8 @@ def setup(context):
             f"it has no foreign language interface for {simulator_name}"
         )
 
+    add_python_libraries_to_library_path()
+
     if interface == "VHPI":
         # Riviera-PRO/Active-HDL, the simulators served by the VHPI application
         context.add_source_files(context.library.name, [VHDL_PATH / "python_pkg_vhpi.vhd"])

src/vunit_python_bridge/native_library.py

@@ -406,6 +406,25 @@ def add_python_dll_to_path() -> None:
         os.environ["PATH"] = os.pathsep.join([item for item in paths if item] + [directory])
 
 
+def add_python_libraries_to_library_path() -> None:
+    """
+    Standalone builds of Python (python-build-standalone) keep the shared libraries their extension
+    modules load, Tcl/Tk for tkinter, next to libpython. On Linux only the python executable finds
+    them, through its RPATH, not a simulator embedding the interpreter. They have no SONAME either,
+    so loading them from Python first does not help: the simulator processes VUnit starts must find
+    them through LD_LIBRARY_PATH.
+    """
+    # ponytail: changes LD_LIBRARY_PATH of the whole VUnit process, only for its children to find Tcl/Tk
+    if not sys.platform.startswith("linux"):
+        return
+    directory = Path(sys.base_prefix) / "lib"
+    if not any(directory.glob("libtcl*.so")):
+        return
+    paths = os.environ.get("LD_LIBRARY_PATH", "").split(os.pathsep)
+    if str(directory) not in paths:
+        os.environ["LD_LIBRARY_PATH"] = os.pathsep.join([item for item in paths if item] + [str(directory)])
+
+
 def _build_windows_library(root: Path, simulator_prefix: Optional[Path] = None) -> Path:
     """
     Build the library on Windows with gcc (see windows_gcc) against the headers and the import

tests/test_python_bridge.py

@@ -880,6 +880,32 @@ def test_add_python_dll_to_path_appends_once(self):
             native_library.add_python_dll_to_path()
             self.assertEqual(os.environ["PATH"], os.pathsep.join(["first", "pydir"]))
 
+    def test_add_python_libraries_to_library_path_only_for_a_python_with_its_own_tcl(self):
+        with create_tempdir() as tempdir:
+            lib = tempdir / "lib"
+            lib.mkdir()
+            with (
+                mock.patch("sys.platform", "linux"),
+                mock.patch("sys.base_prefix", str(tempdir)),
+                mock.patch.dict(os.environ, {"LD_LIBRARY_PATH": "first"}),
+            ):
+                # A distribution Python: Tcl/Tk are in the system library directories
+                native_library.add_python_libraries_to_library_path()
+                self.assertEqual(os.environ["LD_LIBRARY_PATH"], "first")
+                # A standalone build: Tcl/Tk next to libpython, added once
+                (lib / "libtcl9.0.so").touch()
+                native_library.add_python_libraries_to_library_path()
+                native_library.add_python_libraries_to_library_path()
+                self.assertEqual(os.environ["LD_LIBRARY_PATH"], os.pathsep.join(["first", str(lib)]))
+            for platform in ("darwin", "win32"):
+                with (
+                    mock.patch("sys.platform", platform),
+                    mock.patch("sys.base_prefix", str(tempdir)),
+                    mock.patch.dict(os.environ, {"LD_LIBRARY_PATH": "first"}),
+                ):
+                    native_library.add_python_libraries_to_library_path()
+                    self.assertEqual(os.environ["LD_LIBRARY_PATH"], "first", platform)
+
 
 class TestWindows64Bit(unittest.TestCase):
     """