Handle sliced assignments

qiskit/circuit/library/standard_gates/u.py

@@ -146,15 +146,23 @@ def __init__(self, gate):
         self._gate = gate
 
     def __setitem__(self, key, value):
-        if isinstance(key, slice):
-            raise TypeError("'CUGate' only supports setting parameters by single index")
         super().__setitem__(key, value)
         self._gate._params[key] = value
-        # Magic numbers: CUGate has 4 parameters, UGate has 3.
-        if key < 0:
-            key = 4 + key
-        if key < 3:
-            self._gate.base_gate.params[key] = value
+        # Magic numbers: CUGate has 4 parameters, UGate has 3, with the last of CUGate's missing.
+        if isinstance(key, slice):
+            # We don't need to worry about the case of the slice being used to insert extra / remove
+            # elements because that would be "undefined behaviour" in a gate already, so we're
+            # within our rights to do anything at all.
+            for i, base_key in enumerate(range(*key.indices(4))):
+                if base_key < 0:
+                    base_key = 4 + base_key
+                if base_key < 3:
+                    self._gate.base_gate.params[base_key] = value[i]
+        else:
+            if key < 0:
+                key = 4 + key
+            if key < 3:
+                self._gate.base_gate.params[key] = value
 
 
 class CUGate(ControlledGate):

test/python/circuit/test_controlled_gate.py

@@ -1193,6 +1193,31 @@ def test_assign_cugate(self):
         self.assertEqual(assigned.data[0].operation.base_gate, expected.data[0].operation.base_gate)
         self.assertEqual(assigned, expected)
 
+    def test_modify_cugate_params_slice(self):
+        """Test that CUGate.params can be modified by a standard slice (without changing the number
+        of elements) and changes propagate to the base gate.  This is only needed for as long as
+        CUGate's `base_gate` is `UGate`, which has the "wrong" number of parameters."""
+        cu = CUGate(0.1, 0.2, 0.3, 0.4)
+        self.assertEqual(cu.params, [0.1, 0.2, 0.3, 0.4])
+        self.assertEqual(cu.base_gate.params, [0.1, 0.2, 0.3])
+
+        cu.params[0:4] = [0.5, 0.4, 0.3, 0.2]
+        self.assertEqual(cu.params, [0.5, 0.4, 0.3, 0.2])
+        self.assertEqual(cu.base_gate.params, [0.5, 0.4, 0.3])
+
+        cu.params[:] = [0.1, 0.2, 0.3, 0.4]
+        self.assertEqual(cu.params, [0.1, 0.2, 0.3, 0.4])
+        self.assertEqual(cu.base_gate.params, [0.1, 0.2, 0.3])
+
+        cu.params[:3] = [0.5, 0.4, 0.3]
+        self.assertEqual(cu.params, [0.5, 0.4, 0.3, 0.4])
+        self.assertEqual(cu.base_gate.params, [0.5, 0.4, 0.3])
+
+        # indices (3, 2, 1, 0), note that the assignment is in reverse.
+        cu.params[-1::-1] = [0.1, 0.2, 0.3, 0.4]
+        self.assertEqual(cu.params, [0.4, 0.3, 0.2, 0.1])
+        self.assertEqual(cu.base_gate.params, [0.4, 0.3, 0.2])
+
     def test_assign_nested_controlled_cu(self):
         """Test assignment of an arbitrary controlled parametrised gate that appears through the
         `Gate.control()` method on an already-controlled gate."""