fix (solvers): Add compatibility shim for xpress.Namespaces

xpress.Namespaces was added in version 9.6.

Author: coroa

linopy/solvers.py

@@ -108,6 +108,17 @@
 
     available_solvers.append("xpress")
 
+    # xpress.Namespaces was added in xpress 9.6
+    try:
+        from xpress import Namespaces as xpress_Namespaces
+    except ImportError:
+
+        class xpress_Namespaces:
+            ROW = 1
+            COLUMN = 2
+            SET = 3
+
+
 with contextlib.suppress(ModuleNotFoundError):
     import mosek
 
@@ -1605,13 +1616,13 @@ def solve_problem_from_file(
         def get_solver_solution() -> Solution:
             objective = m.getObjVal()
 
-            var = m.getnamelist(xpress.Namespaces.COLUMN, 0, m.attributes.cols - 1)
+            var = m.getnamelist(xpress_Namespaces.COLUMN, 0, m.attributes.cols - 1)
             sol = pd.Series(m.getSolution(), index=var, dtype=float)
 
             try:
                 _dual = m.getDual()
                 constraints = m.getnamelist(
-                    xpress.Namespaces.ROW, 0, m.attributes.rows - 1
+                    xpress_Namespaces.ROW, 0, m.attributes.rows - 1
                 )
                 dual = pd.Series(_dual, index=constraints, dtype=float)
             except (xpress.SolverError, xpress.ModelError, SystemError):