plugin convenience function for getting the entry point object (#9275)

* plugin convenience function for getting the entry point object

* simpler code, thanks to Jake

Co-authored-by: Jake Lishman <[email protected]>

* typehint and docstring

* reno update

* thanks Jake

* Fix docs build

---------

Co-authored-by: Jake Lishman <[email protected]>

qiskit/transpiler/preset_passmanagers/plugin.py

@@ -161,10 +161,11 @@ def pass_manager(self, pass_manager_config, optimization_level):
    PassManagerStagePlugin
    PassManagerStagePluginManager
    list_stage_plugins
+   passmanager_stage_plugins
 """
 
 import abc
-from typing import List, Optional
+from typing import List, Optional, Dict
 
 import stevedore
 
@@ -301,3 +302,47 @@ def list_stage_plugins(stage_name: str) -> List[str]:
         return plugin_mgr.scheduling_plugins.names()
     else:
         raise TranspilerError(f"Invalid stage name: {stage_name}")
+
+
+def passmanager_stage_plugins(stage: str) -> Dict[str, PassManagerStagePlugin]:
+    """Return a dict with, for each stage name, the class type of the plugin.
+
+    This function is useful for getting more information about a plugin:
+
+    .. code-block:: python
+
+        from qiskit.transpiler.preset_passmanagers.plugin import passmanager_stage_plugins
+        routing_plugins = passmanager_stage_plugins('routing')
+        basic_plugin = routing_plugins['basic']
+        help(basic_plugin)
+
+    .. code-block:: text
+
+        Help on BasicSwapPassManager in module ...preset_passmanagers.builtin_plugins object:
+
+        class BasicSwapPassManager(...preset_passmanagers.plugin.PassManagerStagePlugin)
+         |  Plugin class for routing stage with :class:`~.BasicSwap`
+         |
+         |  Method resolution order:
+         |      BasicSwapPassManager
+         |      ...preset_passmanagers.plugin.PassManagerStagePlugin
+         |      abc.ABC
+         |      builtins.object
+         ...
+
+    Args:
+        stage: The stage name to get
+
+    Returns:
+        dict: the key is the name of the plugin and the value is the class type for each.
+
+    Raises:
+       TranspilerError: If an invalid stage name is specified.
+    """
+    plugin_mgr = PassManagerStagePluginManager()
+    try:
+        manager = getattr(plugin_mgr, f"{stage}_plugins")
+    except AttributeError as exc:
+        raise TranspilerError(f"Passmanager stage {stage} not found") from exc
+
+    return {name: manager[name].obj for name in manager.names()}

releasenotes/notes/entry_point_obj-60625d9d797df1d9.yaml

@@ -0,0 +1,18 @@
+---
+features:
+  - |
+    The function ``passmanager_stage_plugins`` in the module
+    ``qiskit.transpiler.preset_passmanagers.plugin`` was added to obtain a map between
+    plugin names and their  class type.
+    This allows to identify and query passmanager plugin documentation. For example::
+
+        >>> from qiskit.transpiler.preset_passmanagers.plugin import passmanager_stage_plugins
+        >>> passmanager_stage_plugins('routing')['lookahead'].__class__
+        qiskit.transpiler.preset_passmanagers.builtin_plugins.LookaheadSwapPassManager
+
+        >>> help(passmanager_stage_plugins('routing')['lookahead'])
+        Help on BasicSwapPassManager in module qiskit.transpiler.preset_passmanagers.builtin_plugins object:
+
+        class BasicSwapPassManager(qiskit.transpiler.preset_passmanagers.plugin.PassManagerStagePlugin)
+        |  Plugin class for routing stage with :class:`~.BasicSwap`
+        ...

test/python/transpiler/test_stage_plugin.py

@@ -22,9 +22,11 @@
 from qiskit.compiler.transpiler import transpile
 from qiskit.test import QiskitTestCase
 from qiskit.transpiler import PassManager, PassManagerConfig, CouplingMap
+from qiskit.transpiler.preset_passmanagers.builtin_plugins import BasicSwapPassManager
 from qiskit.transpiler.preset_passmanagers.plugin import (
     PassManagerStagePluginManager,
     list_stage_plugins,
+    passmanager_stage_plugins,
 )
 from qiskit.transpiler.exceptions import TranspilerError
 from qiskit.providers.basicaer import QasmSimulatorPy
@@ -51,6 +53,16 @@ def test_list_stage_plugins_invalid_stage_name(self):
         with self.assertRaises(TranspilerError):
             list_stage_plugins("not_a_stage")
 
+    def test_passmanager_stage_plugins(self):
+        """Test entry_point_obj function."""
+        basic_obj = passmanager_stage_plugins("routing")
+        self.assertIsInstance(basic_obj["basic"], BasicSwapPassManager)
+
+    def test_passmanager_stage_plugins_not_found(self):
+        """Test entry_point_obj function with nonexistent stage"""
+        with self.assertRaises(TranspilerError):
+            passmanager_stage_plugins("foo_stage")
+
     def test_build_pm_invalid_plugin_name_valid_stage(self):
         """Test get pm from plugin with invalid plugin name and valid stage."""
         plugin_manager = PassManagerStagePluginManager()