BaseServer.on() returning None instead of original handler

[dokterbob]

While working on types for BaseServer's on(), I noticed that the original implementation returns None when used as a decorator.

AFAIK, this is not the most common pattern: people might want to use the already-decorated value for decorating something else, only to find they can't. It also makes the types unnecessarily complex, over simply always returning the event handler callable.

Just to understand, I guess the set_handler() pattern allows for the following AFAIK undocumented behaviour:

message_handler = sio.on('message')

@message_handler
def my_handler(): ...

After this, my_handler() will have it's original value, just the hook was added to sio's handler.

In any other case, e.g. when handler is specified, it will call set_handler() but not return it's return value. So:

sio.on('message')
def my_handler(): ...
my_handler == None

Why has this decision been made, should the last line read return set_handler(handler)?

python-socketio/src/socketio/base_server.py

Line 127 in 8f0e66c

set_handler(handler)

Related: python/typeshed#13095

[miguelgrinberg]

I think you are misunderstanding how the on() method works.

When used as a decorator, the handler argument is None. In that case set_handler is returned as the decorator's inner function. This function will then be invoked with the user's function to decorate as an argument.

When used as a regular method, you include the handler function in the call, so handler is not going to be None. In that case there is no need to return anything, since you are passing all the information directly. Example:

sio.on('message', my_message_handler, namespace='/chat')

I reviewed the code again and it seems correct to me. Am I missing anything?