Remove incorrect Py_INCREF/DECREF on Model in catchEvent/dropEvent (#1190)

* Remove spurious Py_INCREF/DECREF on Model in catchEvent/dropEvent

* Apply suggestions from code review

Co-authored-by: DominikKamp <130753997+DominikKamp@users.noreply.github.com>

* Update tests/test_event.py

Co-authored-by: DominikKamp <130753997+DominikKamp@users.noreply.github.com>

* Update tests/test_event.py

Co-authored-by: DominikKamp <130753997+DominikKamp@users.noreply.github.com>

* Add changelog entries for unreleased changes

* Fix incomplete docstring in test_catchEvent_does_not_leak_model

* Update CHANGELOG.md

Co-authored-by: DominikKamp <130753997+DominikKamp@users.noreply.github.com>

---------

Co-authored-by: DominikKamp <130753997+DominikKamp@users.noreply.github.com>

Author: Joao-Dionisio

CHANGELOG.md

@@ -2,7 +2,9 @@
 
 ## Unreleased
 ### Added
+- Added `getMemUsed()`, `getMemTotal()`, and `getMemExternEstim()` methods
 ### Fixed
+- Removed `Py_INCREF`/`Py_DECREF` on `Model` in `catchEvent`/`dropEvent` that caused memory leak for imbalanced usage
 ### Changed
 ### Removed
 

src/pyscipopt/scip.pxi

@@ -11268,7 +11268,6 @@ cdef class Model:
         else:
             raise Warning("event handler not found")
 
-        Py_INCREF(self)
         PY_SCIP_CALL(SCIPcatchEvent(self._scip, eventtype, _eventhdlr, NULL, NULL))
 
     def dropEvent(self, eventtype, Eventhdlr eventhdlr):
@@ -11288,7 +11287,6 @@ cdef class Model:
         else:
             raise Warning("event handler not found")
 
-        Py_DECREF(self)
         PY_SCIP_CALL(SCIPdropEvent(self._scip, eventtype, _eventhdlr, NULL, -1))
 
     def catchVarEvent(self, Variable var, eventtype, Eventhdlr eventhdlr):

tests/test_event.py

@@ -1,4 +1,4 @@
-import pytest, random
+import pytest, weakref, gc, random
 
 from pyscipopt import Model, Eventhdlr, SCIP_RESULT, SCIP_EVENTTYPE, SCIP_PARAMSETTING, quicksum
 
@@ -189,4 +189,42 @@ def eventexec(self, event):
     m.includeEventhdlr(ev, "var_event", "event handler for var events")
 
     with pytest.raises(Exception):
-        m.optimize()
+        m.optimize()
+
+def test_catchEvent_does_not_leak_model():
+    """catchEvent should not artificially increment the Model's reference count.
+
+    Previously, catchEvent called Py_INCREF(self) on the Model, and dropEvent
+    called Py_DECREF(self). In practice these calls are often unbalanced — event
+    handlers commonly catch events without a matching drop (e.g. calling
+    catchEvent in eventinit but omitting dropEvent in eventexit). Each unmatched
+    catchEvent permanently inflated the Model's refcount, preventing garbage
+    collection.
+    """
+
+    class SimpleEvent(Eventhdlr):
+        def eventinit(self):
+            self.model.catchEvent(SCIP_EVENTTYPE.NODEFOCUSED, self)
+
+        def eventexit(self):
+            pass  # intentionally no dropEvent, which is bad practice
+
+        def eventexec(self, event):
+            pass
+
+    m = Model()
+    m.hideOutput()
+    ev = SimpleEvent()
+    m.includeEventhdlr(ev, "simple", "test event handler")
+    m.addVar("x", obj=1, vtype="I")
+    m.optimize()
+
+    ref = weakref.ref(m)
+
+    del ev
+    gc.collect()
+    assert ref() is not None, "Model was garbage collected — event handler absorbed a reference"
+
+    del m
+    gc.collect()
+    assert ref() is None, "Model was not garbage collected — catchEvent likely leaked a reference"