Capture solver log

src/python/miplearn/solvers.py

@@ -3,8 +3,10 @@
 #  Released under the modified BSD license. See COPYING.md for more details.
 
 import logging
+import sys
 from abc import ABC
 from copy import deepcopy
+from io import StringIO
 
 import pyomo.core.kernel.objective
 import pyomo.environ as pe
@@ -40,6 +42,33 @@ def _parallel_solve(instance_idx):
     }
 
 
+class RedirectOutput(object):
+    def __init__(self, streams):
+        self.streams = streams
+        self._original_stdout = sys.stdout
+        self._original_stderr = sys.stderr
+        sys.stdout = self
+        sys.stderr = self
+
+    def __del__(self):
+        sys.stdout = self._original_stdout
+        sys.stderr = self._original_stderr
+
+    def write(self, data):
+        for stream in self.streams:
+            stream.write(data)
+
+    def flush(self):
+        for stream in self.streams:
+            stream.flush()
+
+    def __enter__(self):
+        pass
+
+    def __exit__(self, _type, _value, _traceback):
+        pass
+
+
 class InternalSolver(ABC):
     """
     The MIP solver used internaly by LearningSolver.
@@ -217,13 +246,18 @@ class InternalSolver(ABC):
         -------
         dict
             A dictionary of solver statistics containing the following keys:
-            "Lower bound", "Upper bound", "Wallclock time", "Nodes" and "Sense".
+            "Lower bound", "Upper bound", "Wallclock time", "Nodes", "Sense"
+            and "Log".
         """
         total_wallclock_time = 0
+        streams = [StringIO()]
+        if tee:
+            streams += [sys.stdout]
         self.instance.found_violations = []
         while True:
             logger.debug("Solving MIP...")
-            results = self._pyomo_solver.solve(tee=tee)
+            with RedirectOutput(streams):
+                results = self._pyomo_solver.solve(tee=True)
             total_wallclock_time += results["Solver"][0]["Wallclock time"]
             if not hasattr(self.instance, "find_violations"):
                 break
@@ -243,6 +277,7 @@ class InternalSolver(ABC):
             "Wallclock time": total_wallclock_time,
             "Nodes": 1,
             "Sense": self._obj_sense,
+            "Log": streams[0].getvalue()
         }
 
 
@@ -280,8 +315,13 @@ class GurobiSolver(InternalSolver):
             self._pyomo_solver.set_callback(cb)
             self.instance.found_violations = []
         print(self._is_warm_start_available)
-        results = self._pyomo_solver.solve(tee=tee,
+
-                                           warmstart=self._is_warm_start_available)
+        streams = [StringIO()]
+        if tee:
+            streams += [sys.stdout]
+        with RedirectOutput(streams):
+            results = self._pyomo_solver.solve(tee=True,
+                                               warmstart=self._is_warm_start_available)
         self._pyomo_solver.set_callback(None)
         node_count = int(self._pyomo_solver._solver_model.getAttr("NodeCount"))
         return {
@@ -290,6 +330,7 @@ class GurobiSolver(InternalSolver):
             "Wallclock time": results["Solver"][0]["Wallclock time"],
             "Nodes": max(1, node_count),
             "Sense": self._obj_sense,
+            "Log": streams[0].getvalue(),
         }    
             
 

src/python/miplearn/tests/test_solver.py

@@ -34,7 +34,8 @@ def test_internal_solver():
             }
         })
 
-        stats = solver.solve()
+        stats = solver.solve(tee=True)
+        assert len(stats["Log"]) > 100
         assert stats["Lower bound"] == 1183.0
         assert stats["Upper bound"] == 1183.0
         assert stats["Sense"] == "max"