fix: guard __del__ against partially-constructed objects (PyPy safe)

On PyPy, the GC may call __del__ on an object whose __init__ raised
before all attributes were set (e.g. SolverGurobi.__init__ raises early
when Gurobi is not installed, before _ownsModel/_venv_loaded are set).
CPython avoids this because reference-counting drops the object
immediately; PyPy's tracing GC processes it later.

Replace bare attribute access in __del__ with getattr(..., default) in:
- SolverGurobi.__del__: guard _ownsModel and _venv_loaded
- SolverHighs.__del__:  guard _model (avoid calling Highs_destroy(NULL))
- SolverCbc.__del__:    guard _model (avoid calling Cbc_deleteModel(NULL))
- SolverOsi.__del__:    guard owns_solver

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: h-g-s

mip/cbc.py

@@ -1645,7 +1645,8 @@ def remove_vars(self, varsList: List[int]):
         cbclib.Cbc_deleteCols(self._model, len(varsList), idx)
 
     def __del__(self):
-        cbclib.Cbc_deleteModel(self._model)
+        if getattr(self, "_model", None) is not None:
+            cbclib.Cbc_deleteModel(self._model)
 
     def get_problem_name(self) -> str:
         namep = self.__name_space
@@ -1776,7 +1777,7 @@ def __clear_sol(self: "SolverOsi"):
         self.__obj_val = None
 
     def __del__(self):
-        if self.owns_solver:
+        if getattr(self, "owns_solver", False):
             cbclib.Osi_deleteSolver(self.osi)
 
     def add_var(

mip/gurobi.py

@@ -428,11 +428,12 @@ def __clear_sol(self):
         self.__obj_val = None
 
     def __del__(self):
-        # freeing Gurobi model and environment
-        if self._ownsModel:
+        # Guard against partially-constructed objects (e.g. PyPy GC may call
+        # __del__ even if __init__ raised before setting these attributes).
+        if getattr(self, "_ownsModel", False):
             if self._model:
                 GRBfreemodel(self._model)
-            if self._env and self._venv_loaded:
+            if self._env and getattr(self, "_venv_loaded", False):
                 GRBfreeenv(self._env)
 
     def add_var(

mip/highs.py

@@ -800,7 +800,8 @@ def __init__(self, model: mip.Model, name: str, sense: str):
 
     def __del__(self):
         self._name_buffer = None
-        self._lib.Highs_destroy(self._model)
+        if getattr(self, "_model", None) is not None:
+            self._lib.Highs_destroy(self._model)
 
     # ── Cache grow / flush helpers ─────────────────────────────────────────