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I am creating a project in Python and I would like to add a monitoring system that makes use of events and event handlers. I would like this system to be available throughout the project. I have the following operations in mind:

  • Defining an event. The event can take some data as parameters.
  • Defining a monitor. A monitor registers for a certain event. Multiple monitors can register for the same event. I want to create different kinds of monitors, e.g. one to print data, one to create plots with the data, etc. Thus a monitor should be a class, capable of holding all the data it collects until some method (e.g. print, create-log, ...) is called.
  • Defining an event-handler for a monitor-event pair. This defines how the given monitor will respond to the given event. This action will mostly be: add this data to the data-list of the instance of some monitor class.
  • A notify function that can notify when an event has occurred. This will trigger the event-handler for all monitors that are registered for that event. Ideally, the notify function should be callable from anywhere in the project.

How can I create such a system? Are there any libraries that can help me with this? I am especially wondering how I can make this system in such a way that it is transparently available throughout the project.

JNevens
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    Is this link something corresponding to your need ? https://stackoverflow.com/questions/1092531/event-system-in-python – Ludovic Guerra Jun 21 '17 at 12:46
  • @LudovicGuerra This is certainly helpful, e.g. Blinker looks nice. But I am looking for the handler to be a class and not a simple function. In this class, I would collect all the data and then call a function, e.g. to export to CSV or to create a plot of the logged data. – JNevens Jun 22 '17 at 18:25

4 Answers4

2

Check out Reactive Python (RxPy)

ReactiveX

GitHub/python

Maxim Krabov
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2

You can do most of what you want with forty lines of Python code. This is my own design that I use all the time. The function names are chosen to make it a drop-in replacement for Qt's "signals" and "slots".

It's simple to use. You create a PSignal. You register handlers by calling the connect method. A handler can be any callable. When an event occurs you emit a signal (i.e., notify an event) by calling the emit function. Every registered callable runs at that point. The object calling emit doesn't know, or care, whether anyone is listening or what happens if they are.

You can also disconnect a handler.

There is a lot of debugging code because I discovered that otherwise certain errors can be difficult to track down.

In your question you wanted each handler to be a monitor, and in my design handlers are just functions. But it seems to me that your "monitor" concept is independent of the event/handler mechanism. You're going to have to write functions to make your application go, and it should be pretty easy to make those functions call your monitors.

The code is extensively tested with Python 3.3.

#! python3
import traceback

class PSignal:
    def __init__(self, debug=False):
        self.debug = debug
        self.__handlers = []

    def clear(self):
        """Deletes all the handlers."""
        self.__handlers.clear()

    def connect(self, f):
        """f is a python function."""
        if not callable(f):
            raise ValueError("Object {!r} is not callable".format(f))
        self.__handlers.append(f)
        if self.debug:
            print("PSIGNAL: Connecting", f, self.__handlers)

    def disconnect(self, f):
        for f1 in self.__handlers:
            if f == f1:
                self.__handlers.remove(f)
                return

    def emit(self, *x, **y):
        self._emit(*x, **y)

    def check_debug(self):
        if self.debug and self.__handlers:
            print("PSIGNAL: Signal emitted")
            traceback.print_stack()

    def _emit(self, *x, **y):
        self.check_debug()
        for f in self.__handlers:
            try:
                if self.debug:
                    print("PSIGNAL: emit", f, len(x), x, y)
                f(*x, **y)
            except Exception:
                print("PSIGNAL: Error in signal", f)
                traceback.print_exc()
Paul Cornelius
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0

You can create your own system using a distribuited messaging system like zmq and the "publisher subscriber" pattern.

I've made something like that building a customizable workflow engine (Flows, https://github.com/mastro35/flows )

Bye D.

mastro35
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0

I use this one for health monitoring, which allows the user to specify callbacks, and allows for both threaded, active monitors, and passive monitors:

https://gist.github.com/earonesty/4ccf8fc9bde6feac30e5c155e54dfa5f

I pasted the code below, without the tests (more than the code):

class MonitorInstance:
    def __init__(self, parent, label, func, threshold, active, metric):
        self.parent = parent
        self.label = label
        self.func = func
        self.threshold = threshold
        self.active = active
        self.metric = metric
        self.__errors = None

    def ok(self):
        if self.__errors is None or self.__errors:
            self.parent._ok(self)
        self.__errors = 0
        if self.metric:
            self.metric.set(0)

    def error(self):
        if not self.__errors:
            self.parent._error(self)

        if self.__errors is None:
            self.__errors = 0

        self.__errors += 1

        if self.metric:
            self.metric.inc()

    def check(self):
        try:
            self.func()
            self.ok()
        except Exception as e:
            log.error("%s error: %s", self.label, e)
            self.error()

    @property
    def healthy(self):
        return self.__errors < self.threshold

DEFAULT_THRESHOLD = 1           # errors to cause fault
DEFAULT_CHECKSECS = 5           # time in secs between checks

class Monitor:
    def __init__(self, health_callback=None, check_secs=DEFAULT_CHECKSECS, use_thread=False):
        self.active = []        # active moniors
        self.alerts = set()     # thresholds currently triggered (not healthy)
        self.health_callback = health_callback
        self.healthy = False    # default: not healthy unless a monitor is added!
        self.check_secs = check_secs
        self.last_check = 0

        if use_thread:
            assert self.check_secs > 0, "threads need to sleep"
            threading.Thread(target=self._thread_loop, daemon=True).start()

    def add(self, label, check, threshold=DEFAULT_THRESHOLD, active=False, metric=None):
        inst = MonitorInstance(self, label, check, threshold, active, metric)
        if active:
            self.active.append(inst)
        inst.check()
        return inst

    def _error(self, inst):
        self.alerts.add(inst)
        if self.healthy:
            self._callback(False)
        self.healthy = False

    def _thread_loop(self):
        while True:
            self.check()
            time.sleep(self.check_secs)

    def _callback(self, value):
        if not self.health_callback is None:
            try:
                self.health_callback(value)
            except:
                # health callback should always succeed!
                log.exception("deadlyexes: error calling %s", self.health_callback)

    def _ok(self, inst):
        self.alerts.discard(inst)
        if not self.healthy and not self.alerts:
            self._callback(True)
            self.healthy = True

    def check(self, force=False):
        if not force and (time.time() < (self.last_check + self.check_secs)):
            return False

        # returns true if check was done
        checked=False
        # convert to list prevents modifying iterators
        for inst in list(self.alerts) + self.active:
            try:
                checked=True
                inst.check()
            except:
                pass
        return checked
Erik Aronesty
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