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Happy Software Engineering

Track down `flask-socketio` logger

Flask supports the SocketIO connection via flask-socketio library. It’s written by Miguel Grinberg (@miguelgrinberg) as well as its underlining python-engineio and python-socketio.

I tracked down the path to set the custom engineio logger.

engineio_logger argument of flask-socketio

As written in the flask-socketio documentation (Getting Started — Flask-SocketIO documentation), when SocketIO is initialized, we can customize the logging via engineio_logger parameter.

It looks like it takes one boolean value, True | False, but actually can take the third type logging.Logger.

These arguments can be set to True to output logs to stderr, or to an object compatible with Python’s logging package where the logs should be emitted to. A value of False disables logging.

In flask-socketio

The parameter engineio_logger will be taken as **kwargs and set to server_options, and then passed to socketio.Server() here: Flask-SocketIO/src/flask_socketio/init.py at main · miguelgrinberg/Flask-SocketIO.

The next place is python-socketio library code.

In python-socketio

In the python-socketio library, the server takes the above options as **kwargs. And it just passes through to engineio.Server() just renaming from engineio_logger to logger (returned from _engineio_server_class()): python-socketio/src/socketio/server.py at main · miguelgrinberg/python-socketio.

This is the almost same way as flask-socketio!

In python-engineio

Finally, we entered the final library python-engineio. In fact, how it is passed is the same way again.

After checking the given type, it set the given logger to engineio options: python-engineio/src/engineio/server.py at main · miguelgrinberg/python-engineio


  • Flask SocketIO server can take just a standard Python logging.Logger as engineio_logger argument.
  • Going through flask-socketio -> python-socketio -> python-engineio.