Python Observer Pattern: Examples, Tips?


Question

Are there any exemplary examples of the GoF Observer implemented in Python? I have a bit code which currently has bits of debugging code laced through the key class (currently generating messages to stderr if a magic env is set). Additionally, the class has an interface for incrementally return results as well as storing them (in memory) for post processing. (The class itself is a job manager for concurrently executing commands on remote machines over ssh).

Currently the usage of the class looks something like:

job = SSHJobMan(hostlist, cmd)
job.start()
while not job.done():
    for each in job.poll():
        incrementally_process(job.results[each])
        time.sleep(0.2) # or other more useful work
post_process(job.results)

An alernative usage model is:

job = SSHJobMan(hostlist, cmd)
job.wait()  # implicitly performs a start()
process(job.results)

This all works fine for the current utility. However it does lack flexibility. For example I currently support a brief output format or a progress bar as incremental results, I also support brief, complete and "merged message" outputs for the post_process() function.

However, I'd like to support multiple results/output streams (progress bar to the terminal, debugging and warnings to a log file, outputs from successful jobs to one file/directory, error messages and other results from non-successful jobs to another, etc).

This sounds like a situation that calls for Observer ... have instances of my class accept registration from other objects and call them back with specific types of events as they occur.

I'm looking at PyPubSub since I saw several references to that in SO related questions. I'm not sure I'm ready to add the external dependency to my utility but I could see value in using their interface as a model for mine if that's going to make it easier for others to use. (The project is intended as both a standalone command line utility and a class for writing other scripts/utilities).

In short I know how to do what I want ... but there are numerous ways to accomplish it. I want suggestions on what's most likely to work for other users of the code in the long run.

The code itself is at: classh.

1
42
1/17/2012 2:52:28 AM

Accepted Answer

However it does lack flexibility.

Well... actually, this looks like a good design to me if an asynchronous API is what you want. It usually is. Maybe all you need is to switch from stderr to Python's logging module, which has a sort of publish/subscribe model of its own, what with Logger.addHandler() and so on.

If you do want to support observers, my advice is to keep it simple. You really only need a few lines of code.

class Event(object):
    pass

class Observable(object):
    def __init__(self):
        self.callbacks = []
    def subscribe(self, callback):
        self.callbacks.append(callback)
    def fire(self, **attrs):
        e = Event()
        e.source = self
        for k, v in attrs.iteritems():
            setattr(e, k, v)
        for fn in self.callbacks:
            fn(e)

Your Job class can subclass Observable. When something of interest happens, call self.fire(type="progress", percent=50) or the like.

48
12/18/2009 3:01:00 AM

I think people in the other answers overdo it. You can easily achieve events in Python with less than 15 lines of code.

You simple have two classes: Event and Observer. Any class that wants to listen for an event, needs to inherit Observer and set to listen (observe) for a specific event. When an Event is instantiated and fired, all observers listening to that event will run the specified callback functions.

class Observer():
    _observers = []
    def __init__(self):
        self._observers.append(self)
        self._observables = {}
    def observe(self, event_name, callback):
        self._observables[event_name] = callback


class Event():
    def __init__(self, name, data, autofire = True):
        self.name = name
        self.data = data
        if autofire:
            self.fire()
    def fire(self):
        for observer in Observer._observers:
            if self.name in observer._observables:
                observer._observables[self.name](self.data)

Example:

class Room(Observer):

    def __init__(self):
        print("Room is ready.")
        Observer.__init__(self) # Observer's init needs to be called
    def someone_arrived(self, who):
        print(who + " has arrived!")

room = Room()
room.observe('someone arrived',  room.someone_arrived)

Event('someone arrived', 'Lenard')

Output:

Room is ready.
Lenard has arrived!

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