Add types to remaining InternalSolver methods

miplearn/solvers/gurobi.py

@@ -17,6 +17,7 @@ from miplearn.solvers.internal import (
     LazyCallback,
     MIPSolveStats,
 )
+from miplearn.types import VarIndex
 
 logger = logging.getLogger(__name__)
 
@@ -230,7 +231,7 @@ class GurobiSolver(InternalSolver):
         else:
             return "max"
 
-    def get_value(self, var_name, index):
+    def get_value(self, var_name: str, index: VarIndex) -> Optional[float]:
         var = self._all_vars[var_name][index]
         return self._get_value(var)
 
@@ -244,26 +245,29 @@ class GurobiSolver(InternalSolver):
         else:
             return c.pi
 
-    def _get_value(self, var):
+    def _get_value(self, var: Any) -> Optional[float]:
         if self.cb_where == self.GRB.Callback.MIPSOL:
             return self.model.cbGetSolution(var)
         elif self.cb_where == self.GRB.Callback.MIPNODE:
             return self.model.cbGetNodeRel(var)
         elif self.cb_where is None:
+            if self.is_infeasible():
+                return None
+            else:
                 return var.x
         else:
             raise Exception(
                 "get_value cannot be called from cb_where=%s" % self.cb_where
             )
 
-    def get_variables(self):
+    def get_empty_solution(self) -> Dict:
         self._raise_if_callback()
-        variables = {}
+        solution: Dict = {}
         for (varname, vardict) in self._all_vars.items():
-            variables[varname] = []
+            solution[varname] = {}
             for (idx, var) in vardict.items():
-                variables[varname] += [idx]
-        return variables
+                solution[varname][idx] = None
+        return solution
 
     def add_constraint(self, constraint, name=""):
         if type(constraint) is tuple:
@@ -325,7 +329,7 @@ class GurobiSolver(InternalSolver):
         else:
             raise Exception("Unknown sense: %s" % sense)
 
-    def get_inequality_slacks(self):
+    def get_inequality_slacks(self) -> Dict[str, float]:
         ineqs = [c for c in self.model.getConstrs() if c.sense != "="]
         return {c.ConstrName: c.Slack for c in ineqs}
 
@@ -341,7 +345,7 @@ class GurobiSolver(InternalSolver):
         c = self.model.getConstrByName(cid)
         c.RHS = rhs
 
-    def relax(self):
+    def relax(self) -> None:
         self.model = self.model.relax()
         self._update_vars()
 

miplearn/solvers/internal.py

@@ -12,6 +12,7 @@ from miplearn.types import (
     IterationCallback,
     LazyCallback,
     MIPSolveStats,
+    VarIndex,
 )
 
 logger = logging.getLogger(__name__)
@@ -196,21 +197,22 @@ class InternalSolver(ABC):
         pass
 
     @abstractmethod
-    def get_value(self, var_name, index):
+    def get_value(self, var_name: str, index: VarIndex) -> Optional[float]:
         """
-        Returns the current value of a decision variable.
+        Returns the value of a given variable in the current solution. If no
+        solution is available, returns None.
         """
         pass
 
     @abstractmethod
-    def relax(self):
+    def relax(self) -> None:
         """
         Drops all integrality constraints from the model.
         """
         pass
 
     @abstractmethod
-    def get_inequality_slacks(self):
+    def get_inequality_slacks(self) -> Dict[str, float]:
         """
         Returns a dictionary mapping constraint name to the constraint slack
         in the current solution.
@@ -218,7 +220,7 @@ class InternalSolver(ABC):
         pass
 
     @abstractmethod
-    def is_infeasible(self):
+    def is_infeasible(self) -> bool:
         """
         Returns True if the model has been proved to be infeasible.
         Must be called after solve.
@@ -226,31 +228,30 @@ class InternalSolver(ABC):
         pass
 
     @abstractmethod
-    def get_dual(self, cid):
+    def get_dual(self, cid: str) -> float:
         """
-        If the model is feasible and has been solved to optimality, returns the optimal
-        value of the dual variable associated with this constraint. If the model is infeasible,
-        returns a portion of the infeasibility certificate corresponding to the given constraint.
+        If the model is feasible and has been solved to optimality, returns the
+        optimal value of the dual variable associated with this constraint. If the
+        model is infeasible, returns a portion of the infeasibility certificate
+        corresponding to the given constraint.
 
-        Must be called after solve.
+        Only available for relaxed problems. Must be called after solve.
         """
         pass
 
     @abstractmethod
-    def get_sense(self):
+    def get_sense(self) -> str:
         """
         Returns the sense of the problem (either "min" or "max").
         """
         pass
 
     @abstractmethod
-    def get_variables(self):
+    def get_empty_solution(self) -> Dict:
+        """
+        Returns a dictionary with the same shape as the one produced by
+        `get_solution`, but with all values set to None. This method is
+        used by the ML components to query what variables are there in
+        the model before a solution is available.
+        """
         pass
-
-    def get_empty_solution(self):
-        solution = {}
-        for (var, indices) in self.get_variables().items():
-            solution[var] = {}
-            for idx in indices:
-                solution[var][idx] = 0.0
-        return solution

miplearn/solvers/pyomo/base.py

@@ -22,6 +22,7 @@ from miplearn.solvers.internal import (
     LazyCallback,
     MIPSolveStats,
 )
+from miplearn.types import VarIndex
 
 logger = logging.getLogger(__name__)
 
@@ -53,20 +54,13 @@ class BasePyomoSolver(InternalSolver):
         self,
         tee: bool = False,
     ) -> LPSolveStats:
-        for var in self._bin_vars:
-            lb, ub = var.bounds
-            var.setlb(lb)
-            var.setub(ub)
-            var.domain = pyomo.core.base.set_types.Reals
-            self._pyomo_solver.update_var(var)
+        self.relax()
         streams: List[Any] = [StringIO()]
         if tee:
             streams += [sys.stdout]
         with RedirectOutput(streams):
             results = self._pyomo_solver.solve(tee=True)
-        for var in self._bin_vars:
-            var.domain = pyomo.core.base.set_types.Binary
-            self._pyomo_solver.update_var(var)
+        self._restore_integrality()
         opt_value = None
         if not self.is_infeasible():
             opt_value = results["Problem"][0]["Lower bound"]
@@ -75,6 +69,11 @@ class BasePyomoSolver(InternalSolver):
             "Log": streams[0].getvalue(),
         }
 
+    def _restore_integrality(self) -> None:
+        for var in self._bin_vars:
+            var.domain = pyomo.core.base.set_types.Binary
+            self._pyomo_solver.update_var(var)
+
     def solve(
         self,
         tee: bool = False,
@@ -166,19 +165,23 @@ class BasePyomoSolver(InternalSolver):
         self._update_vars()
         self._update_constrs()
 
-    def get_value(self, var_name, index):
+    def get_value(self, var_name: str, index: VarIndex) -> Optional[float]:
+        if self.is_infeasible():
+            return None
+        else:
             var = self._varname_to_var[var_name]
             return var[index].value
 
-    def get_variables(self):
-        variables = {}
+    def get_empty_solution(self) -> Dict:
+        solution: Dict = {}
         for var in self.model.component_objects(Var):
-            variables[str(var)] = []
+            svar = str(var)
+            solution[svar] = {}
             for index in var:
                 if var[index].fixed:
                     continue
-                variables[str(var)] += [index]
-        return variables
+                solution[svar][index] = None
+        return solution
 
     def _clear_warm_start(self) -> None:
         for var in self._all_vars:
@@ -273,10 +276,15 @@ class BasePyomoSolver(InternalSolver):
     def is_constraint_satisfied(self, cobj):
         raise Exception("Not implemented")
 
-    def relax(self):
-        raise Exception("not implemented")
+    def relax(self) -> None:
+        for var in self._bin_vars:
+            lb, ub = var.bounds
+            var.setlb(lb)
+            var.setub(ub)
+            var.domain = pyomo.core.base.set_types.Reals
+            self._pyomo_solver.update_var(var)
 
-    def get_inequality_slacks(self):
+    def get_inequality_slacks(self) -> Dict[str, float]:
         raise Exception("not implemented")
 
     def set_constraint_sense(self, cid, sense):

miplearn/solvers/tests/__init__.py

@@ -19,9 +19,9 @@ from miplearn.solvers.pyomo.xpress import XpressPyomoSolver
 class InfeasiblePyomoInstance(PyomoInstance):
     def to_model(self) -> pe.ConcreteModel:
         model = pe.ConcreteModel()
-        model.x = pe.Var(domain=pe.Binary)
-        model.OBJ = pe.Objective(expr=model.x, sense=pe.maximize)
-        model.eq = pe.Constraint(expr=model.x >= 2)
+        model.x = pe.Var([0], domain=pe.Binary)
+        model.OBJ = pe.Objective(expr=model.x[0], sense=pe.maximize)
+        model.eq = pe.Constraint(expr=model.x[0] >= 2)
         return model
 
 

miplearn/solvers/tests/test_internal_solver.py

@@ -7,6 +7,7 @@ from io import StringIO
 from warnings import warn
 
 import pyomo.environ as pe
+from pytest import raises
 
 from miplearn.solvers import RedirectOutput
 from miplearn.solvers.gurobi import GurobiSolver
@@ -92,6 +93,7 @@ def test_internal_solver():
         solver.set_instance(instance, model)
 
         stats = solver.solve_lp()
+        assert not solver.is_infeasible()
         assert round(stats["Optimal value"], 3) == 1287.923
         assert len(stats["Log"]) > 100
 
@@ -102,6 +104,7 @@ def test_internal_solver():
         assert round(solution["x"][3], 3) == 0.000
 
         stats = solver.solve(tee=True)
+        assert not solver.is_infeasible()
         assert len(stats["Log"]) > 100
         assert stats["Lower bound"] == 1183.0
         assert stats["Upper bound"] == 1183.0
@@ -131,6 +134,17 @@ def test_internal_solver():
         assert stats["Lower bound"] == 1030.0
 
         if isinstance(solver, GurobiSolver):
+
+            assert solver.get_sense() == "max"
+            assert solver.get_constraint_sense("cut") == "<"
+            assert solver.get_constraint_sense("eq_capacity") == "<"
+
+            # Verify slacks
+            assert solver.get_inequality_slacks() == {
+                "cut": 0.0,
+                "eq_capacity": 3.0,
+            }
+
             # Extract the new constraint
             cobj = solver.extract_constraint("cut")
 
@@ -160,6 +174,17 @@ def test_internal_solver():
             solver.set_constraint_sense("cut", "=")
             stats = solver.solve()
             assert round(stats["Lower bound"]) == 1179.0
+            assert round(solver.get_dual("eq_capacity")) == 12.0
+
+
+def test_relax():
+    for solver_class in _get_internal_solvers():
+        instance = _get_knapsack_instance(solver_class)
+        solver = solver_class()
+        solver.set_instance(instance)
+        solver.relax()
+        stats = solver.solve()
+        assert round(stats["Lower bound"]) == 1288.0
 
 
 def test_infeasible_instance():
@@ -169,6 +194,7 @@ def test_infeasible_instance():
         solver.set_instance(instance)
         stats = solver.solve()
 
+        assert solver.is_infeasible()
         assert solver.get_solution() is None
         assert stats["Upper bound"] is None
         assert stats["Lower bound"] is None
@@ -176,6 +202,7 @@ def test_infeasible_instance():
         stats = solver.solve_lp()
         assert solver.get_solution() is None
         assert stats["Optimal value"] is None
+        assert solver.get_value("x", 0) is None
 
 
 def test_iteration_cb():

miplearn/types.py

@@ -2,7 +2,7 @@
 #  Copyright (C) 2020, UChicago Argonne, LLC. All rights reserved.
 #  Released under the modified BSD license. See COPYING.md for more details.
 
-from typing import Optional, Dict, Callable, Any
+from typing import Optional, Dict, Callable, Any, Union, List
 
 from mypy_extensions import TypedDict
 
@@ -46,3 +46,5 @@ MIPSolveStats = TypedDict(
 IterationCallback = Callable[[], bool]
 
 LazyCallback = Callable[[Any, Any], None]
+
+VarIndex = Union[str, int, List[Union[str, int]]]