Fix duplicate logs in run_simulation

src/py/flwr/common/logger.py

@@ -188,3 +188,29 @@ def warn_deprecated_feature(name: str) -> None:
         """,
         name,
     )
+
+
+def set_logger_propagation(
+    child_logger: logging.Logger, value: bool = True
+) -> logging.Logger:
+    """Set the logger propagation attribute.
+
+    Parameters
+    ----------
+    child_logger : logging.Logger
+        Child logger object
+    value : bool
+        Boolean setting for propagation. If True, both parent and child logger
+        display messages. Otherwise, only the child logger displays a message.
+        This False setting prevents duplicate logs in Colab notebooks.
+        Reference: https://stackoverflow.com/a/19561320
+
+    Returns
+    -------
+    logging.Logger
+        Child logger object with updated propagation setting
+    """
+    child_logger.propagate = value
+    if not child_logger.propagate:
+        child_logger.log(logging.DEBUG, "Logger propagate set to False")
+    return child_logger

src/py/flwr/simulation/app.py

@@ -15,6 +15,8 @@
 """Flower simulation app."""
 
 
+import asyncio
+import logging
 import sys
 import threading
 import traceback
@@ -27,7 +29,7 @@
 
 from flwr.client import ClientFn
 from flwr.common import EventType, event
-from flwr.common.logger import log
+from flwr.common.logger import log, set_logger_propagation
 from flwr.server.client_manager import ClientManager
 from flwr.server.history import History
 from flwr.server.server import Server, init_defaults, run_fl
@@ -156,6 +158,7 @@ def start_simulation(
         is an advanced feature. For all details, please refer to the Ray documentation:
         https://docs.ray.io/en/latest/ray-core/scheduling/index.html
 
+
     Returns
     -------
     hist : flwr.server.history.History
@@ -167,6 +170,18 @@ def start_simulation(
         {"num_clients": len(clients_ids) if clients_ids is not None else num_clients},
     )
 
+    # Set logger propagation
+    loop: Optional[asyncio.AbstractEventLoop] = None
+    try:
+        loop = asyncio.get_running_loop()
+    except RuntimeError:
+        loop = None
+    finally:
+        if loop and loop.is_running():
+            # Set logger propagation to False to prevent duplicated log output in Colab.
+            logger = logging.getLogger("flwr")
+            _ = set_logger_propagation(logger, False)
+
     # Initialize server and server config
     initialized_server, initialized_config = init_defaults(
         server=server,

src/py/flwr/simulation/run_simulation.py

@@ -28,6 +28,7 @@
 
 from flwr.client import ClientApp
 from flwr.common import EventType, event, log
+from flwr.common.logger import set_logger_propagation
 from flwr.common.typing import ConfigsRecordValues
 from flwr.server.driver import Driver, GrpcDriver
 from flwr.server.run_serverapp import run
@@ -364,6 +365,8 @@ def _run_simulation(
 
     finally:
         if run_in_thread:
+            # Set logger propagation to False to prevent duplicated log output in Colab.
+            logger = set_logger_propagation(logger, False)
             log(DEBUG, "Starting Simulation Engine on a new thread.")
             simulation_engine_th = threading.Thread(target=_main_loop, args=args)
             simulation_engine_th.start()