[Python] Don't rely on possible memory leaks in T(Seq)Collection tests

Some unit tests rely on the behavior that when adding object to `TLists`,
Python ownership is always dropped, resulting in memory leaks if the ownership
is not correctly handled on the C++ side.

To make these unit tests transparently ready for the moment where the
ownership is handled correctly, this commit suggests to use some global
lists that keep around Python references until the end of the test. So
the effect on the lifetime is the same as having the "controlled" memory
leaks.

Author: guitargeek

bindings/pyroot/pythonizations/test/tcollection_listmethods.py

@@ -11,14 +11,16 @@ class TCollectionListMethods(unittest.TestCase):
 
     num_elems = 3
 
+    _global_objects = []
+
     # Helpers
     def create_tcollection(self):
         c = ROOT.TList()
         for _ in range(self.num_elems):
             o = ROOT.TObject()
-            # Prevent immediate deletion of C++ TObjects
-            ROOT.SetOwnership(o, False)
             c.Add(o)
+            # To prevent deletion of the objects (TList is by default non-owning)
+            self._global_objects.append(o)
 
         return c
 

bindings/pyroot/pythonizations/test/tseqcollection_itemaccess.py

@@ -12,14 +12,16 @@ class TSeqCollectionItemAccess(unittest.TestCase):
 
     num_elems = 3
 
+    _global_objects = []
+
     # Helpers
     def create_tseqcollection(self):
         sc = ROOT.TList()
         for _ in range(self.num_elems):
             o = ROOT.TObject()
-            # Prevent immediate deletion of C++ TObjects
-            ROOT.SetOwnership(o, False)
             sc.Add(o)
+            # To prevent deletion of the objects (TList is by default non-owning)
+            self._global_objects.append(o)
 
         return sc
 

bindings/pyroot/pythonizations/test/tseqcollection_listmethods.py

@@ -11,11 +11,16 @@ class TSeqCollectionListMethods(unittest.TestCase):
 
     num_elems = 3
 
+    _global_objects = []
+
     # Helpers
     def create_tseqcollection(self):
         sc = ROOT.TList()
         for i in reversed(range(self.num_elems)):
-            sc.Add(ROOT.TObjString(str(i)))
+            o = ROOT.TObjString(str(i))
+            sc.Add(o)
+            # To prevent deletion of the objects (TList is by default non-owning)
+            self._global_objects.append(o)
 
         return sc
 
@@ -49,6 +54,10 @@ def test_insert(self):
         self.assertEqual(sc.GetEntries(), self.num_elems + 4)
         self.assertEqual(sc.At(self.num_elems + 3), o4)
 
+        # Clear before the added element might be garbage collected,
+        # to avoid dangling pointer access.
+        sc.Clear()
+
     def test_pop(self):
         sc = self.create_tseqcollection()
         l1 = [elem for elem in sc]
@@ -82,8 +91,10 @@ def test_pop(self):
         with self.assertRaises(TypeError):
             sc2.pop(1.0)
 
-        # Pop a repeated element
-        sc2.append(ROOT.TObjString("2"))
+        # Pop a repeated element.
+        # Keep Python reference so the added element lives beyond the sc2.pop() call:
+        new_elem = ROOT.TObjString("2")
+        sc2.append(new_elem)
         elem = sc2.pop()
         self.assertEqual(sc2.At(0), elem)