Fix Optimizer reconstruction from settings (#8381) (#8385)

* fix settings in optimizers

* add reno

* don't duplicate args

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit d9af1d1)

Co-authored-by: Julien Gacon <[email protected]>

qiskit/algorithms/optimizers/scipy_optimizer.py

@@ -86,11 +86,23 @@ def get_support_level(self):
 
     @property
     def settings(self) -> Dict[str, Any]:
-        settings = {
-            "max_evals_grouped": self._max_evals_grouped,
-            "options": self._options,
-            **self._kwargs,
-        }
+        options = self._options.copy()
+        if hasattr(self, "_OPTIONS"):
+            # all _OPTIONS should be keys in self._options, but add a failsafe here
+            attributes = [
+                option
+                for option in self._OPTIONS  # pylint: disable=no-member
+                if option in options.keys()
+            ]
+
+            settings = {attr: options.pop(attr) for attr in attributes}
+        else:
+            settings = {}
+
+        settings["max_evals_grouped"] = self._max_evals_grouped
+        settings["options"] = options
+        settings.update(self._kwargs)
+
         # the subclasses don't need the "method" key as the class type specifies the method
         if self.__class__ == SciPyOptimizer:
             settings["method"] = self._method

releasenotes/notes/fix-optimizer-settings-881585bfa8130cb7.yaml

@@ -0,0 +1,15 @@
+---
+fixes:
+  - |
+    Fix a bug in the :class:`~.Optimizer` classes where re-constructing a new optimizer instance
+    from a previously exisiting :attr:`~.Optimizer.settings` reset both the new and previous 
+    optimizer settings to the defaults. This notably led to a bug if :class:`~.Optimizer` objects
+    were send as input to Qiskit Runtime programs.
+
+    Now optimizer objects are correctly reconstructed::
+
+      >>> from qiskit.algorithms.optimizers import COBYLA
+      >>> original = COBYLA(maxiter=1)
+      >>> reconstructed = COBYLA(**original.settings)
+      >>> reconstructed._options["maxiter"]
+      1  # used to be 1000!

test/python/algorithms/optimizers/test_optimizers.py

@@ -226,6 +226,31 @@ def test_scipy(self, method, options):
         self.assertEqual(from_dict._method, method.lower())
         self.assertEqual(from_dict._options, options)
 
+    def test_independent_reconstruction(self):
+        """Test the SciPyOptimizers don't reset all settings upon creating a new instance.
+
+        COBYLA is used as representative example here."""
+
+        kwargs = {"coffee": "without sugar"}
+        options = {"tea": "with milk"}
+        optimizer = COBYLA(maxiter=1, options=options, **kwargs)
+        serialized = optimizer.settings
+        from_dict = COBYLA(**serialized)
+
+        with self.subTest(msg="test attributes"):
+            self.assertEqual(from_dict.settings["maxiter"], 1)
+
+        with self.subTest(msg="test options"):
+            # options should only contain values that are *not* already in the initializer
+            # (e.g. should not contain maxiter)
+            self.assertEqual(from_dict.settings["options"], {"tea": "with milk"})
+
+        with self.subTest(msg="test kwargs"):
+            self.assertEqual(from_dict.settings["coffee"], "without sugar")
+
+        with self.subTest(msg="option ids differ"):
+            self.assertNotEqual(id(serialized["options"]), id(from_dict.settings["options"]))
+
     def test_adam(self):
         """Test ADAM is serializable."""