python: fix dangling pointer and CSX state after Reset/ReadFromXML; add tests

- Reset() now nulls __CSX.thisptr and clears __CSX before calling C++
  Reset, preventing the old Python wrapper from holding a freed pointer
- ReadFromXML() clears the Python-side CSX reference inline before
  delegating to C++, avoiding an extra C++ Reset() call
- GetCSX() nulls the old wrapper's thisptr when the C++ pointer has
  changed unexpectedly, as a safety net against unforeseen state changes
- Add test_getcsx_not_none_after_read and test_getcsx_replaced_after_read
  to cover the fix from PR #218 and guard against regressions

Signed-off-by: Thorsten Liebig <thorsten.liebig@gmx.de>
Assisted-by: Claude Sonnet 4.6

Author: thliebig

python/Tests/test_XML_RoundTrip.py

@@ -161,6 +161,39 @@ def test_read_resets_state(self):
         fdtd = openEMS(NrTS=9999)
         self.assertTrue(fdtd.ReadFromXML(self.fn))
 
+    def test_getcsx_not_none_after_read(self):
+        fdtd = openEMS()
+        fdtd.ReadFromXML(self.fn)
+        self.assertIsNotNone(fdtd.GetCSX())
+
+    def test_getcsx_replaced_after_read(self):
+        csx_before = _make_csx()
+        csx_before.AddMetal('pre_existing_metal')
+        mat = csx_before.AddMaterial('pre_existing_dielectric')
+        mat.SetMaterialProperty(epsilon=99)
+        csx_before.GetGrid().SetDeltaUnit(1.0)  # intentionally wrong unit
+        fdtd = openEMS()
+        fdtd.SetCSX(csx_before)
+
+        fdtd.ReadFromXML(self.fn)  # self.fn has no metals/dielectrics, DeltaUnit=1e-3
+
+        csx_after = fdtd.GetCSX()
+        self.assertIsNotNone(csx_after)
+
+        # Grid delta unit must reflect the XML, not csx_before
+        self.assertAlmostEqual(csx_after.GetGrid().GetDeltaUnit(), 1e-3)
+
+        fn_out = os.path.join(tempfile.gettempdir(), 'test_getcsx_replaced.xml')
+        try:
+            fdtd.Write2XML(fn_out)
+            with open(fn_out) as f:
+                xml_str = f.read()
+            self.assertNotIn('pre_existing_metal', xml_str)
+            self.assertNotIn('pre_existing_dielectric', xml_str)
+        finally:
+            if os.path.exists(fn_out):
+                os.remove(fn_out)
+
 
 class Test_XML_RoundTrip(unittest.TestCase):
     """Write → Read → Write cycle: second file should be identical to first."""

python/openEMS/openEMS.pyx

@@ -115,6 +115,9 @@ cdef class openEMS:
             self.__CSX.thisptr = NULL
 
     def Reset(self):
+        if self.__CSX is not None:
+            self.__CSX.thisptr = NULL
+            self.__CSX = None
         self.thisptr.Reset()
 
     def SetNumberOfTimeSteps(self, val):
@@ -436,6 +439,8 @@ cdef class openEMS:
             return None
 
         if self.__CSX is None or self.__CSX.thisptr != ptr:
+            if self.__CSX is not None:
+                self.__CSX.thisptr = NULL  # prevent dangling pointer on the old wrapper
             csx = ContinuousStructure.__new__(ContinuousStructure)
             csx.thisptr = ptr
             self.__CSX = csx
@@ -579,4 +584,7 @@ cdef class openEMS:
 
         :param file: xml file name
         """
+        if self.__CSX is not None:
+            self.__CSX.thisptr = NULL
+            self.__CSX = None
         return self.thisptr.ReadFromXML(file.encode('UTF-8'))