Raise error eagerly on bad unitary synthesis method (Qiskit#11367)

* fix issue Qiskit#11355

* update docstring

* added test

* rename non-method

* Update releasenotes/notes/move-synthesis-plugin-error-61e3683bf5a0c225.yaml

* Reword messages

---------

Co-authored-by: Kevin J. Sung <[email protected]>
Co-authored-by: Jake Lishman <[email protected]>

qiskit/transpiler/passes/synthesis/unitary_synthesis.py

@@ -331,6 +331,11 @@ def __init__(
             target: The optional :class:`~.Target` for the target device the pass
                 is compiling for. If specified this will supersede the values
                 set for ``basis_gates``, ``coupling_map``, and ``backend_props``.
+
+        Raises:
+            TranspilerError: if ``method`` was specified but is not found in the
+                installed plugins list. The list of installed plugins can be queried with
+                :func:`~qiskit.transpiler.passes.synthesis.plugin.unitary_synthesis_plugin_names`
         """
         super().__init__()
         self._basis_gates = set(basis_gates or ())
@@ -358,6 +363,9 @@ def __init__(
 
         self._synth_gates = set(self._synth_gates) - self._basis_gates
 
+        if self.method != "default" and self.method not in self.plugins.ext_plugins:
+            raise TranspilerError(f"Specified method '{self.method}' not found in plugin list")
+
     def run(self, dag: DAGCircuit) -> DAGCircuit:
         """Run the UnitarySynthesis pass on ``dag``.
 
@@ -366,15 +374,7 @@ def run(self, dag: DAGCircuit) -> DAGCircuit:
 
         Returns:
             Output dag with UnitaryGates synthesized to target basis.
-
-        Raises:
-            TranspilerError: if ``method`` was specified for the class and is not
-                found in the installed plugins list. The list of installed
-                plugins can be queried with
-                :func:`~qiskit.transpiler.passes.synthesis.plugin.unitary_synthesis_plugin_names`
         """
-        if self.method != "default" and self.method not in self.plugins.ext_plugins:
-            raise TranspilerError("Specified method: %s not found in plugin list" % self.method)
 
         # If there aren't any gates to synthesize in the circuit we can skip all the iteration
         # and just return.

releasenotes/notes/move-synthesis-plugin-error-61e3683bf5a0c225.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    The :class:`.UnitarySynthesis` transpiler pass will now generate an error on initialization when
+    a nonexistent sythesis plugin is specified, rather than waiting until runtime to raise.  Fixed
+    `#11355 <https://github.com/Qiskit/qiskit/issues/11355>`__.

test/python/transpiler/test_stage_plugin.py

@@ -22,6 +22,7 @@
 from qiskit.compiler.transpiler import transpile
 from qiskit.test import QiskitTestCase
 from qiskit.transpiler import PassManager, PassManagerConfig, CouplingMap
+from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
 from qiskit.transpiler.preset_passmanagers.builtin_plugins import BasicSwapPassManager
 from qiskit.transpiler.preset_passmanagers.plugin import (
     PassManagerStagePluginManager,
@@ -113,3 +114,16 @@ def test_routing_plugins(self, optimization_level, routing_method):
         backend = QasmSimulatorPy()
         counts = backend.run(tqc, shots=1000).result().get_counts()
         self.assertDictAlmostEqual(counts, {"0000": 500, "1111": 500}, delta=100)
+
+    @combine(
+        optimization_level=list(range(4)),
+    )
+    def test_unitary_synthesis_plugins(self, optimization_level):
+        """Test unitary synthesis plugins"""
+        backend = QasmSimulatorPy()
+        with self.assertRaises(TranspilerError):
+            _ = generate_preset_pass_manager(
+                optimization_level=optimization_level,
+                backend=backend,
+                unitary_synthesis_method="not a method",
+            )