Fix detecting signature of built-in functions in event handler (#86)

I found out that `inspect.signature()` cannot read the signature of
built-in methods, so e.g. `Actor.on(ActorEventTypes.SYSTEM_INFO, print)`
would not work.

This fixes it by rewriting the signature-processing to ignore errors
when the signature can't be read, and also to better check methods which
have default arguments, by just trying to bind the arguments to the
method signature.

Author: fnesveda

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()