Fix detecting signature of built-in functions in event handler

src/apify/event_manager.py

@@ -100,9 +100,24 @@ def on(self, event_name: ActorEventTypes, listener: ListenerType) -> Callable:
         if not self._initialized:
             raise RuntimeError('EventManager was not initialized!')
 
-        listener_argument_count = len(inspect.signature(listener).parameters)
-        if listener_argument_count > 1:
-            raise ValueError('The "listener" argument must be a callable which accepts 0 or 1 arguments!')
+        # Detect whether the listener will accept the event_data argument
+        try:
+            signature = inspect.signature(listener)
+        except (ValueError, TypeError):
+            # If we can't determine the listener argument count (e.g. for the built-in `print` function),
+            # let's assume the listener will accept the argument
+            listener_argument_count = 1
+        else:
+            try:
+                dummy_event_data: Dict = {}
+                signature.bind(dummy_event_data)
+                listener_argument_count = 1
+            except TypeError:
+                try:
+                    signature.bind()
+                    listener_argument_count = 0
+                except TypeError:
+                    raise ValueError('The "listener" argument must be a callable which accepts 0 or 1 arguments!')
 
         event_name = _maybe_extract_enum_member_value(event_name)
 

tests/unit/test_event_manager.py

@@ -3,6 +3,7 @@
 import logging
 import time
 from collections import defaultdict
+from pprint import pprint
 from typing import Any, Callable, Dict, Optional, Set
 
 import pytest
@@ -135,10 +136,26 @@ def sync_two_arguments(_arg1: Any, _arg2: Any) -> None:
         async def async_two_arguments(_arg1: Any, _arg2: Any) -> None:
             pass
 
+        def sync_two_arguments_one_default(event_data: Any, _arg2: Any = 'default_value') -> None:
+            nonlocal event_calls
+            event_calls.append(('sync_two_arguments_one_default', event_data))
+
+        async def async_two_arguments_one_default(event_data: Any, _arg2: Any = 'default_value') -> None:
+            nonlocal event_calls
+            event_calls.append(('async_two_arguments_one_default', event_data))
+
         event_manager.on(ActorEventTypes.SYSTEM_INFO, sync_no_arguments)
         event_manager.on(ActorEventTypes.SYSTEM_INFO, async_no_arguments)
         event_manager.on(ActorEventTypes.SYSTEM_INFO, sync_one_argument)
         event_manager.on(ActorEventTypes.SYSTEM_INFO, async_one_argument)
+        event_manager.on(ActorEventTypes.SYSTEM_INFO, sync_two_arguments_one_default)
+        event_manager.on(ActorEventTypes.SYSTEM_INFO, async_two_arguments_one_default)
+
+        # built-in functions should work too
+        event_manager.on(ActorEventTypes.SYSTEM_INFO, print)
+
+        # functions from the standard library should work too
+        event_manager.on(ActorEventTypes.SYSTEM_INFO, pprint)
 
         with pytest.raises(ValueError, match='The "listener" argument must be a callable which accepts 0 or 1 arguments!'):
             event_manager.on(ActorEventTypes.SYSTEM_INFO, sync_two_arguments)  # type: ignore[arg-type]
@@ -148,11 +165,13 @@ async def async_two_arguments(_arg1: Any, _arg2: Any) -> None:
         event_manager.emit(ActorEventTypes.SYSTEM_INFO, 'DUMMY_SYSTEM_INFO')
         await asyncio.sleep(0.1)
 
-        assert len(event_calls) == 4
+        assert len(event_calls) == 6
         assert ('sync_no_arguments', None) in event_calls
         assert ('async_no_arguments', None) in event_calls
         assert ('sync_one_argument', 'DUMMY_SYSTEM_INFO') in event_calls
         assert ('async_one_argument', 'DUMMY_SYSTEM_INFO') in event_calls
+        assert ('sync_two_arguments_one_default', 'DUMMY_SYSTEM_INFO') in event_calls
+        assert ('async_two_arguments_one_default', 'DUMMY_SYSTEM_INFO') in event_calls
 
     async def test_event_async_handling_local(self) -> None:
         config = Configuration()