Test usability of HiGHS solver through GAMSModel

.github/workflows/pytest.yaml

@@ -76,7 +76,7 @@ jobs:
 
     - uses: iiasa/actions/setup-gams@main
       with:
-        version: 25.1.1
+        version: 41.5.0
 
     - name: Set RETICULATE_PYTHON
       # Use the environment variable set by the setup-python action, above.

ixmp/model/gams.py

@@ -167,6 +167,7 @@ class GAMSModel(Model):
         "out_file": str(Path("{cwd}", "{model_name}_out.gdx")),
         "solve_args": ['--in="{in_file}"', '--out="{out_file}"'],
         # Not formatted
+        "LP": None,
         "gams_args": [],
         "check_solution": True,
         "comment": None,
@@ -252,6 +253,8 @@ def run(self, scenario):
         # Input and output file names
         self.in_file = Path(self.format_option("in_file"))
         self.out_file = Path(self.format_option("out_file"))
+        # LP solver; only pass if given
+        self.solve_args.extend([f"LP={self.LP}"] if self.LP else [])
 
         # Assemble the full command: executable, model file, model-specific arguments,
         # and general GAMS arguments

ixmp/tests/test_model.py

@@ -122,13 +122,19 @@ def test_filename_invalid_char(self, dantzig, char):
             dict(comment=None),
             dict(equ_list=None, var_list=["x"]),
             dict(equ_list=["demand", "supply"], var_list=[]),
+            dict(LP="HiGHS"),
         ],
-        ids=["null-comment", "null-list", "empty-list"],
+        ids=["null-comment", "null-list", "empty-list", "HiGHS"],
     )
-    def test_GAMSModel_solve(test_data_path, dantzig, kwargs):
+    def test_GAMSModel_solve(self, dantzig, kwargs):
         """Options to GAMSModel are handled without error."""
         dantzig.clone().solve(**kwargs, quiet=True)
 
+    def test_solve_highs(self, capfd, test_data_path, dantzig):
+        dantzig.clone().solve(LP="HiGHS")
+        captured = capfd.readouterr()
+        assert "HiGHS run time" in captured.out
+
     def test_error_message(self, test_data_path, test_mp):
         """GAMSModel.solve() displays a user-friendly message on error."""
         # Empty Scenario