signalslot 0.0.2

Tight coupling

Consider such a code in your_client.py:

import your_service
import your_plugin

class YourClient(object):
    def something_happens(self, some_argument):
        your_service.something_happens(some_argument)
        your_plugin.something_happens(some_argument)

The problem with that code is that it ties your_client with your_service and your_plugin, which makes it harder to test and maintain.

An improvement would be to acheive the same while keeping components loosely coupled.

Observer pattern

You could implement an Observer pattern in YourClient by adding boilerplate code:

class YourClient(object):
    def __init__(self):
        self.observers = []

    def register_observer(self, observer):
        self.observers.append(observer)

    def something_happens(self, some_argument):
        for observer in self.observers:
            observer.something_happens(some_argument)

This implementation is a bit dumb, it doesn’t check the compatibility of observers for example, also it’s additionnal code you’d have to test.

And, this would work if you have control on instanciation of YourClient, ie.:

your_client = YourClient()
your_client.register_observer(your_service)
your_client.register_observer(your_plugin)

If YourClient is used by a framework with IoC (Inversion of Control) then it’s a lot harder. Example:

client = some_framework.Service.create(
    client='your_client')

As you can see in the above example, it’s a lot harder to get your hands on the YourClient instance and call register_observer.

Using signalslot

Consider such a signal definition in your_client/signals.py

import signalslot

something_happens = signalslot.Signal(args=['some_argument'])

You can then connect slots to this signal in each loosely coupled module. your_service/slots.py and your_plugin/slots.py would contain something like:

from your_client.signals import something_happens

def something_happens(some_argument):
    # ....

something_happens.connect(something_happens)

Then, your_service.something_happens and your_plugin.something_happens would be called every time the something_happens signal is emited, ie.:

from . import signals

class YourClient(object):
    def something_happens(self, some_argument):
        signals.something_happens.emit(some_argument)

All you have to do is import your_client and your_plugin somewhere before YourClient to ensure that the connect() calls are executed, ie.:

from your_service import slots
from your_plugin import slots

client = some_framework.Service.create(
    client='your_client')