Enable adding listeners to event before the event is registered

jupyter_events/logger.py

@@ -142,12 +142,16 @@ def register_event_schema(self, schema: SchemaType) -> None:
 
         Get this registered schema using the EventLogger.schema.get() method.
         """
-
         event_schema = self.schemas.register(schema)  # type:ignore[arg-type]
         key = event_schema.id
-        self._modifiers[key] = set()
-        self._modified_listeners[key] = set()
-        self._unmodified_listeners[key] = set()
+        # It's possible that listeners and modifiers have been added for this
+        # schema before the schema is registered.
+        if key not in self._modifiers:
+            self._modifiers[key] = set()
+        if key not in self._modified_listeners:
+            self._modified_listeners[key] = set()
+        if key not in self._unmodified_listeners:
+            self._unmodified_listeners[key] = set()
 
     def register_handler(self, handler: logging.Handler) -> None:
         """Register a new logging handler to the Event Logger.
@@ -205,7 +209,11 @@ def add_modifier(
         # If the schema ID and version is given, only add
         # this modifier to that schema
         if schema_id:
-            self._modifiers[schema_id].add(modifier)
+            # If the schema hasn't been added yet,
+            # start a placeholder set.
+            modifiers = self._modifiers.get(schema_id, set())
+            modifiers.add(modifier)
+            self._modifiers[schema_id] = modifiers
             return
         for id_ in self._modifiers:
             if schema_id is None or id_ == schema_id:
@@ -264,9 +272,16 @@ def add_listener(
         # this modifier to that schema
         if schema_id:
             if modified:
-                self._modified_listeners[schema_id].add(listener)
+                # If the schema hasn't been added yet,
+                # start a placeholder set.
+                listeners = self._modified_listeners.get(schema_id, set())
+                listeners.add(listener)
+                self._modified_listeners[schema_id] = listeners
                 return
-            self._unmodified_listeners[schema_id].add(listener)
+            listeners = self._unmodified_listeners.get(schema_id, set())
+            listeners.add(listener)
+            self._unmodified_listeners[schema_id] = listeners
+            return
         for id_ in self.schemas.schema_ids:
             if schema_id is None or id_ == schema_id:
                 if modified:

tests/test_listeners.py

@@ -5,7 +5,7 @@
 
 import pytest
 
-from jupyter_events.logger import EventLogger
+from jupyter_events.logger import EventLogger, SchemaNotRegistered
 from jupyter_events.schema import EventSchema
 
 from .utils import SCHEMA_PATH
@@ -138,3 +138,41 @@ async def my_listener(logger: EventLogger, schema_id: str, data: dict) -> None:
     assert listener_was_called
     # Check that the active listeners are cleaned up.
     assert len(event_logger._active_listeners) == 0
+
+
+@pytest.mark.parametrize(
+    # Make sure no schemas are added at the start of this test.
+    "jp_event_schemas",
+    [
+        # Empty events list.
+        []
+    ],
+)
+async def test_listener_added_before_schemas_passes(jp_event_logger, schema):
+    # Ensure there are no schemas listed.
+    assert len(jp_event_logger.schemas.schema_ids) == 0
+
+    listener_was_called = False
+
+    async def my_listener(logger: EventLogger, schema_id: str, data: dict) -> None:
+        nonlocal listener_was_called
+        listener_was_called = True
+
+    # Add the listener without any schemas
+    jp_event_logger.add_listener(schema_id=schema.id, listener=my_listener)
+
+    # Proof that emitting the event won't success
+    with pytest.warns(SchemaNotRegistered):
+        jp_event_logger.emit(schema_id=schema.id, data={"prop": "hello, world"})
+
+    assert not listener_was_called
+
+    # Now register the event and emit.
+    jp_event_logger.register_event_schema(schema)
+
+    # Try emitting the event again and ensure the listener saw it.
+    jp_event_logger.emit(schema_id=schema.id, data={"prop": "hello, world"})
+    await jp_event_logger.gather_listeners()
+    assert listener_was_called
+    # Check that the active listeners are cleaned up.
+    assert len(jp_event_logger._active_listeners) == 0