fix: error handlers that take args: Args crashed instead of running

slack_bolt/kwargs_injection/args.py

@@ -111,6 +111,8 @@ def handle_buttons(args):
     """`next()` utility function, which tells the middleware chain that it can continue with the next one"""
     next_: Callable[[], None]
     """An alias of `next()` for avoiding the Python built-in method overrides in middleware functions"""
+    error: Optional[Exception]
+    """The exception raised by a listener or middleware; only set in error handlers"""
 
     def __init__(
         self,
@@ -144,6 +146,7 @@ def __init__(
         # the naming conflict with the built-in one affects
         # only the internals of this method
         next: Callable[[], None],
+        error: Optional[Exception] = None,
         **kwargs,  # noqa
     ):
         self.logger: logging.Logger = logger
@@ -177,3 +180,4 @@ def __init__(
 
         self.next: Callable[[], None] = next
         self.next_: Callable[[], None] = next
+        self.error: Optional[Exception] = error

slack_bolt/kwargs_injection/async_args.py

@@ -110,6 +110,8 @@ async def handle_buttons(args):
     """`next()` utility function, which tells the middleware chain that it can continue with the next one"""
     next_: Callable[[], Awaitable[None]]
     """An alias of `next()` for avoiding the Python built-in method overrides in middleware functions"""
+    error: Optional[Exception]
+    """The exception raised by a listener or middleware; only set in error handlers"""
 
     def __init__(
         self,
@@ -140,6 +142,7 @@ def __init__(
         save_thread_context: Optional[AsyncSaveThreadContext] = None,
         say_stream: Optional[AsyncSayStream] = None,
         next: Callable[[], Awaitable[None]],
+        error: Optional[Exception] = None,
         **kwargs,  # noqa
     ):
         self.logger: Logger = logger
@@ -173,3 +176,4 @@ def __init__(
 
         self.next: Callable[[], Awaitable[None]] = next
         self.next_: Callable[[], Awaitable[None]] = next
+        self.error: Optional[Exception] = error

slack_bolt/kwargs_injection/async_utils.py

@@ -65,10 +65,6 @@ def build_async_required_kwargs(
         # error handler
         "error": error,  # Exception
     }
-    if not next_keys_required:
-        all_available_args.pop("next")
-        all_available_args.pop("next_")
-
     all_available_args["payload"] = (
         all_available_args["options"]
         or all_available_args["shortcut"]
@@ -98,7 +94,12 @@ def build_async_required_kwargs(
                 # We are sure that we should skip manipulating this arg
                 required_arg_names.pop(0)
 
-    kwargs: Dict[str, Any] = {k: v for k, v in all_available_args.items() if k in required_arg_names}
+    # next/next_ are only injectable where a middleware chain exists.
+    # They stay in all_available_args so an `args` object can still be built.
+    injectable_names = set(required_arg_names)
+    if not next_keys_required:
+        injectable_names -= {"next", "next_"}
+    kwargs: Dict[str, Any] = {k: v for k, v in all_available_args.items() if k in injectable_names}
     found_arg_names = kwargs.keys()
     for name in required_arg_names:
         if name == "args":

slack_bolt/kwargs_injection/utils.py

@@ -64,10 +64,6 @@ def build_required_kwargs(
         # error handler
         "error": error,  # Exception
     }
-    if not next_keys_required:
-        all_available_args.pop("next")
-        all_available_args.pop("next_")
-
     all_available_args["payload"] = (
         all_available_args["options"]
         or all_available_args["shortcut"]
@@ -97,7 +93,12 @@ def build_required_kwargs(
                 # We are sure that we should skip manipulating this arg
                 required_arg_names.pop(0)
 
-    kwargs: Dict[str, Any] = {k: v for k, v in all_available_args.items() if k in required_arg_names}
+    # next/next_ are only injectable where a middleware chain exists.
+    # They stay in all_available_args so an `args` object can still be built.
+    injectable_names = set(required_arg_names)
+    if not next_keys_required:
+        injectable_names -= {"next", "next_"}
+    kwargs: Dict[str, Any] = {k: v for k, v in all_available_args.items() if k in injectable_names}
     found_arg_names = kwargs.keys()
     for name in required_arg_names:
         if name == "args":

tests/scenario_tests/test_error_handler.py

@@ -5,7 +5,7 @@
 from slack_sdk import WebClient
 from slack_sdk.signature import SignatureVerifier
 
-from slack_bolt import BoltRequest, BoltResponse
+from slack_bolt import Args, BoltRequest, BoltResponse
 from slack_bolt.app import App
 from slack_bolt.error import BoltUnhandledRequestError
 from tests.mock_web_api_server import (
@@ -153,6 +153,26 @@ def failing_listener():
         assert response.status == 500
         assert response.headers["x-test-result"] == ["1"]
 
+    def test_custom_with_args(self):
+        called = []
+
+        def error_handler(args: Args):
+            assert isinstance(args.error, Exception)
+            args.response.headers["x-test-result"] = ["1"]
+            called.append(True)
+
+        def failing_listener():
+            raise Exception("Something wrong!")
+
+        app = App(client=self.web_client, signing_secret=self.signing_secret)
+        app.error(error_handler)
+        app.action("a")(failing_listener)
+
+        response = app.dispatch(self.build_valid_request())
+        assert response.status == 500
+        assert response.headers["x-test-result"] == ["1"]
+        assert called == [True]
+
     def test_unhandled_errors(self):
         app = App(
             client=self.web_client,

tests/scenario_tests_async/test_error_handler.py

@@ -7,6 +7,7 @@
 from slack_sdk.web.async_client import AsyncWebClient
 
 from slack_bolt import BoltResponse
+from slack_bolt.kwargs_injection.async_args import AsyncArgs
 from slack_bolt.async_app import AsyncApp
 from slack_bolt.error import BoltUnhandledRequestError
 from slack_bolt.request.async_request import AsyncBoltRequest
@@ -163,6 +164,27 @@ async def failing_listener():
         assert response.status == 500
         assert response.headers["x-test-result"] == ["1"]
 
+    @pytest.mark.asyncio
+    async def test_custom_with_args(self):
+        called = []
+
+        async def error_handler(args: AsyncArgs):
+            assert isinstance(args.error, Exception)
+            args.response.headers["x-test-result"] = ["1"]
+            called.append(True)
+
+        async def failing_listener():
+            raise Exception("Something wrong!")
+
+        app = AsyncApp(client=self.web_client, signing_secret=self.signing_secret)
+        app.error(error_handler)
+        app.action("a")(failing_listener)
+
+        response = await app.async_dispatch(self.build_valid_request())
+        assert response.status == 500
+        assert response.headers["x-test-result"] == ["1"]
+        assert called == [True]
+
     @pytest.mark.asyncio
     async def test_unhandled_errors(self):
         app = AsyncApp(