Add AMPL.get_iis to return variables and constraints in IIS

Author: fdabrandao

CHANGELOG.md

@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## 0.14.0 - 2024-05-##
 - Allow assigning values to indexed sets from a dictionary with the lists of members
   for every index.
+- Add AMPL.get_iis() to return dictionaries with variables and constraints in IIS.
 
 ## 0.13.3 - 2024-02-20
 - Fix issues with AMPL.solve(verbose=False) when the solver argument was not set.

amplpy/ampl.py

@@ -891,6 +891,45 @@ def solve_result_num(self):
         """
         return self.get_value("solve_result_num")
 
+    def get_iis(self):
+        """
+        Get IIS attributes for all variables and constraints.
+
+        Returns:
+            Tuple with a dictionary for variables in the IIS and another for the constraints.
+
+        Usage example:
+
+        .. code-block:: python
+
+            from amplpy import AMPL
+            ampl = AMPL()
+            ampl.eval(
+                r\"\"\"
+            var x >= 0;
+            var y >= 0;
+            maximize obj: x+y;
+            s.t. s: x+y <= -5;
+            \"\"\"
+            )
+            ampl.option["presolve"] = 0  # disable AMPL presolve
+            ampl.solve(solver="gurobi", gurobi_options="outlev=1 iis=1")
+            if ampl.solve_result == "infeasible":
+                var_iis, con_iis = ampl.get_iis()
+                print(var_iis, con_iis)
+        """
+        df_var = dict(
+            self.get_data(
+                "{i in 1.._nvars: _var[i].iis != 'non'} (_varname[i], _var[i].iis)"
+            ).to_list(skip_index=True)
+        )
+        df_con = dict(
+            self.get_data(
+                "{i in 1.._ncons: _con[i].iis != 'non'} (_conname[i], _con[i].iis)"
+            ).to_list(skip_index=True)
+        )
+        return df_var, df_con
+
     def _start_recording(self, filename):
         """
         Start recording the session to a file for debug purposes.

amplpy/tests/test_ampl.py

@@ -418,6 +418,23 @@ def test_issue56(self):
             ampl.get_output("for{i in 0..2} { display i;}"),
         )
 
+    def test_iis(self):
+        ampl = self.ampl
+        ampl.eval(
+            r"""
+            var x >= 0;
+            var y >= 0;
+            maximize obj: x+y;
+            s.t. s: x+y <= -5;
+            """
+        )
+        ampl.option["presolve"] = 0
+        ampl.solve(solver="gurobi", gurobi_options="outlev=0 iis=1")
+        if ampl.solve_result == "infeasible":
+            var_iis, con_iis = ampl.get_iis()
+            self.assertEqual(var_iis, {"x": "low", "y": "low"})
+            self.assertEqual(con_iis, {"s": "mem"})
+
 
 if __name__ == "__main__":
     unittest.main()