Merge branch 'master' of github.com:cvxgrp/cvxpy

* 'master' of github.com:cvxgrp/cvxpy:
  ENH: Clearer error when rejecting a solver. (cvxpy#329)
  examples: ADMM Lasso: fix missing variables (cvxpy#325)

cvxpy/problems/solvers/solver.py

@@ -126,16 +126,36 @@ def validate_solver(self, constraints):
             raise SolverError("The solver %s is not installed." % self.name())
         # Check the solver can solve the problem.
         constr_map = SymData.filter_constraints(constraints)
-        if ((constr_map[s.BOOL] or constr_map[s.INT]) and
-            not self.MIP_CAPABLE) or \
-           (constr_map[s.SDP] and not self.SDP_CAPABLE) or \
-           (constr_map[s.EXP] and not self.EXP_CAPABLE) or \
-           (constr_map[s.SOC] and not self.SOCP_CAPABLE) or \
-           (len(constraints) == 0 and self.name() in [s.SCS,
-                                                      s.GLPK]):
-            raise SolverError(
-                "The solver %s cannot solve the problem." % self.name()
-            )
+
+        if (constr_map[s.BOOL] or constr_map[s.INT]) and not self.MIP_CAPABLE:
+            self._reject_problem("it cannot solve mixed-integer problems")
+        elif constr_map[s.SDP] and not self.SDP_CAPABLE:
+            self._reject_problem("it cannot solve semidefinite problems")
+        elif constr_map[s.EXP] and not self.EXP_CAPABLE:
+            self._reject_problem("it cannot solve exponential cone problems")
+        elif constr_map[s.SOC] and not self.SOCP_CAPABLE:
+            self._reject_problem("it cannot solve second-order cone problems")
+        elif len(constraints) == 0 and self.name() in (s.SCS, s.GLPK):
+            self._reject_problem("it cannot solve unconstrained problems")
+
+    def _reject_problem(self, reason):
+        """Raise an error indicating that the solver cannot solve a problem.
+
+        Parameters
+        ----------
+        reason : str
+            A short description of the reason the problem cannot be solved by
+            this solver.
+
+        Raises
+        ------
+        cvxpy.SolverError
+            An error explaining why the problem could not be solved.
+        """
+        message = "The solver {} cannot solve the problem because {}.".format(
+            self.name(), reason
+        )
+        raise SolverError(message)
 
     def validate_cache(self, objective, constraints, cached_data):
         """Clears the cache if the objective or constraints changed.

cvxpy/tests/test_problem.py

@@ -176,13 +176,17 @@ def test_solver_stats(self):
         self.assertGreater(stats.setup_time, 0)
         self.assertGreater(stats.num_iters, 0)
 
-
     def test_get_problem_data(self):
         """Test get_problem_data method.
         """
         with self.assertRaises(Exception) as cm:
             Problem(Maximize(Bool())).get_problem_data(s.ECOS)
-        self.assertEqual(str(cm.exception), "The solver ECOS cannot solve the problem.")
+
+        expected = (
+            "The solver ECOS cannot solve the problem because"
+            " it cannot solve mixed-integer problems."
+        )
+        self.assertEqual(str(cm.exception), expected)
 
         data = Problem(Maximize(exp(self.a) + 2)).get_problem_data(s.SCS)
         dims = data["dims"]
@@ -1270,18 +1274,30 @@ def test_invalid_solvers(self):
         """
         with self.assertRaises(Exception) as cm:
             Problem(Minimize(Bool())).solve(solver=s.ECOS)
-        self.assertEqual(str(cm.exception),
-                         "The solver ECOS cannot solve the problem.")
+
+        expected = (
+            "The solver ECOS cannot solve the problem "
+            "because it cannot solve mixed-integer problems."
+        )
+        self.assertEqual(str(cm.exception), expected)
 
         with self.assertRaises(Exception) as cm:
             Problem(Minimize(lambda_max(self.a))).solve(solver=s.ECOS)
-        self.assertEqual(str(cm.exception),
-                         "The solver ECOS cannot solve the problem.")
+
+        expected = (
+            "The solver ECOS cannot solve the problem "
+            "because it cannot solve semidefinite problems."
+        )
+        self.assertEqual(str(cm.exception), expected)
 
         with self.assertRaises(Exception) as cm:
             Problem(Minimize(self.a)).solve(solver=s.SCS)
-        self.assertEqual(str(cm.exception),
-                         "The solver SCS cannot solve the problem.")
+
+        expected = (
+            "The solver SCS cannot solve the problem "
+            "because it cannot solve unconstrained problems."
+        )
+        self.assertEqual(str(cm.exception), expected)
 
     def test_reshape(self):
         """Tests problems with reshape.

examples/admm_lasso.py

@@ -5,6 +5,8 @@
 # Problem data.
 m = 100
 n = 75
+gamma = 1.0
+NUM_PROCS = 2
 np.random.seed(1)
 A = np.random.randn(m, n)
 b = np.random.randn(m, 1)