Fixes potentially outdated value of getVal (#993)

* Fixes potentially outdated value of getVal

* Fix funny error

* Check if stored solution remains best

* typo

* Update changelog

---------

Co-authored-by: Joao-Dionisio <up201606210@up.pt>
Co-authored-by: João Dionísio <57299939+Joao-Dionisio@users.noreply.github.com>
Co-authored-by: Joao-Dionisio <joao.goncalves.dionisio@gmail.com>

Author: mmghannam

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Fixed the type of MatrixExpr.sum(axis=...) result from MatrixVariable to MatrixExpr.
 - Updated IIS result in PyiisfinderExec()
 - Fixed lotsizing_lazy example
+- Fixed incorrect getVal() result when _bestSol.sol was outdated
 ### Changed
 - changed default value of enablepricing flag to True
 ### Removed

src/pyscipopt/scip.pxi

@@ -1068,8 +1068,8 @@ cdef class Solution:
     """Base class holding a pointer to corresponding SCIP_SOL."""
 
     # We are raising an error here to avoid creating a solution without an associated model. See Issue #625
-    def __init__(self, raise_error = False):
-        if not raise_error:
+    def __init__(self, raise_error = True):
+        if raise_error:
             raise ValueError("To create a solution you should use the createSol method of the Model class.")
 
     @staticmethod
@@ -1093,7 +1093,7 @@ cdef class Solution:
         """
         if scip == NULL:
             raise Warning("cannot create Solution with SCIP* == NULL")
-        sol = Solution(True)
+        sol = Solution(raise_error=False)
         sol.sol = scip_sol
         sol.scip = scip
         return sol
@@ -10795,11 +10795,20 @@ cdef class Model:
         A variable is also an expression.
 
         """
-        stage_check = SCIPgetStage(self._scip) not in [SCIP_STAGE_INIT, SCIP_STAGE_FREE]
+        cdef SCIP_SOL* current_best_sol
 
-        if not stage_check or self._bestSol.sol == NULL and SCIPgetStage(self._scip) != SCIP_STAGE_SOLVING:
+        stage_check = SCIPgetStage(self._scip) not in [SCIP_STAGE_INIT, SCIP_STAGE_FREE]
+        if not stage_check:
             raise Warning("Method cannot be called in stage ", self.getStage())
 
+        # Ensure _bestSol is up-to-date (cheap pointer comparison)
+        current_best_sol = SCIPgetBestSol(self._scip)
+        if self._bestSol is None or self._bestSol.sol != current_best_sol:
+            self._bestSol = Solution.create(self._scip, current_best_sol)
+
+        if self._bestSol.sol == NULL and SCIPgetStage(self._scip) != SCIP_STAGE_SOLVING:
+            raise Warning("No solution available")
+
         if isinstance(expr, MatrixExpr):
             result = np.empty(expr.shape, dtype=float)
             for idx in np.ndindex(result.shape):