Merge pull request #1152 from arcondello/fix/overlapping-discrete-fro…

…m-QM

Fix exceptions in `CQM.add_discrete_from_model()`

dimod/constrained.py

@@ -652,7 +652,13 @@ def add_discrete_from_model(self,
             raise ValueError("discrete constraints must have at least two variables")
 
         for v, bias in qm.iter_linear():
-            if not vartype(v) is Vartype.BINARY:
+            if v in self.variables:
+                # it already exists, let's make sure it's not already used
+                if any(v in self.constraints[label].lhs.variables for label in self.discrete):
+                    raise ValueError(f"variable {v!r} is already used in a discrete variable")
+                if self.vartype(v) is not Vartype.BINARY:
+                    raise ValueError(f"variable {v!r} has already been added but is not BINARY")
+            elif not vartype(v) is Vartype.BINARY:
                 raise ValueError("all variables in a discrete constraint must be binary, "
                                  f"{v!r} is {vartype(v).name!r}")
             # we could maybe do a scaling, but let's just keep it simple for now

releasenotes/notes/fix-overlapping-discrete-from-QM-2c015d6300eb5e29.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    ``ConstrainedQuadraticModel.add_discrete()``,
+    ``ConstrainedQuadraticModel.add_constraint_from_iterable()``, and
+    ``ConstrainedQuadraticModel.add_discrete_from_model()``
+    now correctly raise an error when given a binary variable that overlaps with an existing
+    discrete constraint.
+    See `#1149 <https://github.com/dwavesystems/dimod/issues/1149>`_.

tests/test_constrained.py

@@ -156,6 +156,29 @@ def test_qm(self):
         self.assertIn(c, cqm.discrete)
         self.assertTrue(cqm.constraints[c].lhs.is_equal(qm))
 
+    def test_qm_overlap(self):
+        b1 = dimod.Binary('b1')
+        b2 = dimod.Binary('b2')
+        b3 = dimod.Binary('b3')
+
+        cqm = dimod.CQM()
+        cqm.add_discrete(dimod.quicksum([b1, b2]))
+        with self.assertRaises(ValueError):
+            cqm.add_discrete(dimod.quicksum([b1, b3]))
+
+    def test_qm_not_binary(self):
+        x, y = dimod.Binaries('xy')
+        i, j = dimod.Integers('ij')
+        ii = dimod.Binary('i')
+
+        cqm = dimod.CQM()
+        cqm.add_constraint(i <= 5)
+        with self.assertRaises(ValueError):
+            cqm.add_discrete(x + y + ii)
+
+        with self.assertRaises(ValueError):
+            cqm.add_discrete(x + j)
+
     def test_simple(self):
         cqm = CQM()
         cqm.add_discrete('abc')