Lazy: Minor fixes; make it compatible with Pyomo

tests/test_lazy_pyomo.py

@@ -0,0 +1,44 @@
+#  MIPLearn: Extensible Framework for Learning-Enhanced Mixed-Integer Optimization
+#  Copyright (C) 2020-2023, UChicago Argonne, LLC. All rights reserved.
+#  Released under the modified BSD license. See COPYING.md for more details.
+import logging
+from typing import Any, Hashable, List
+
+import pyomo.environ as pe
+
+from miplearn.solvers.pyomo import PyomoModel
+
+logger = logging.getLogger(__name__)
+
+
+def _build_model() -> PyomoModel:
+    m = pe.ConcreteModel()
+    m.x = pe.Var(bounds=(0, 5), domain=pe.Integers)
+    m.obj = pe.Objective(expr=-m.x)
+    m.cons = pe.ConstraintList()
+
+    def lazy_separate(model: PyomoModel) -> List[Hashable]:
+        model.solver.cbGetSolution(vars=[m.x])
+        if m.x.value > 0.5:
+            return [m.x.value]
+        else:
+            return []
+
+    def lazy_enforce(model: PyomoModel, violations: List[Any]) -> None:
+        for v in violations:
+            model.add_constr(m.cons.add(m.x <= round(v - 1)))
+
+    return PyomoModel(
+        m,
+        "gurobi_persistent",
+        lazy_separate=lazy_separate,
+        lazy_enforce=lazy_enforce,
+    )
+
+
+def test_pyomo_callback() -> None:
+    model = _build_model()
+    model.optimize()
+    assert model.lazy_constrs_ is not None
+    assert len(model.lazy_constrs_) > 0
+    assert model.inner.x.value == 0.0