Event system in Python

Question

What event system for Python do you use  I m already aware of pydispatcher  but I was wondering what else can be found  or is commonly used   I m not interested in event managers that are part of large frameworks  I d rather use a small bare-bones solution that I can easily extend

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I created an EventManager class  code at the end   The syntax is the following   Create an event with no listeners assigned to it EventManager addEvent  eventName          Create an event with listeners assigned to it EventManager addEvent  eventName    fun1  fun2          Create any number event with listeners assigned to them EventManager addEvent  eventName1    e1fun1  e1fun2       eventName2    e2fun1  e2fun2               Add or remove listener to an existing event EventManager eventName    extra fun EventManager eventName -  removed fun   Delete an event del EventManager eventName   Fire the event EventManager eventName    Here is an Example  def hello name       print  quot Hello    quot  format name       def greetings name       print  quot Greetings    quot  format name   EventManager addEvent  salute    greetings    EventManager salute    hello  print  quot  nInitial salute quot  EventManager salute  Oscar    print  quot  nNow remove greetings quot  EventManager salute -  greetings EventManager salute  Oscar    Output   Initial salute  Greetings Oscar  Hello Oscar  Now remove greetings  Hello Oscar  EventManger Code  class EventManager           class Event          def   init   self functions               if type functions  is not list                  raise ValueError  quot functions parameter has to be a list quot               self functions   functions                      def   iadd   self func               self functions append func              return self                      def   isub   self func               self functions remove func              return self                      def   call   self  args   kvargs               for func in self functions   func  args   kvargs                    classmethod     def addEvent cls   kvargs            quot  quot  quot          addEvent  event1    f1 f2       event2    g1 g2                     creates events using   kvargs to create any number of events  Each event recieves a list of functions          where every function in the list recieves the same parameters                   Example                   def hello    print  quot Hello  quot           def world    print  quot World quot                   EventManager addEvent  salute    hello            EventManager salute    world                  EventManager salute                    Output          Hello World          quot  quot  quot          for key in kvargs keys                if type kvargs key   is not list                  raise ValueError  quot value has to be a list quot               else                  kvargs key    cls Event kvargs key                    cls   dict   update kvargs

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We use an EventHook as suggested from Michael Foord in his Event Pattern   Just add EventHooks to your classes with   class MyBroadcaster       def   init              self onChange   EventHook    theBroadcaster   MyBroadcaster      add a listener to the event theBroadcaster onChange    myFunction    remove listener from the event theBroadcaster onChange -  myFunction    fire event theBroadcaster onChange fire     We add the functionality to remove all listener from an object to Michaels class and ended up with this   class EventHook object        def   init   self           self   handlers           def   iadd   self  handler           self   handlers append handler          return self      def   isub   self  handler           self   handlers remove handler          return self      def fire self   args    keywargs           for handler in self   handlers              handler  args    keywargs       def clearObjectHandlers self  inObject           for theHandler in self   handlers              if theHandler im self    inObject                  self -  theHandler

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Some time ago I ve wrote library that might be useful for you   It allows you to have local and global listeners  multiple different ways of registering them  execution priority and so on    from pyeventdispatcher import register  register  foo bar   lambda event  print  second    register  foo bar   lambda event  print  first     -100   dispatch Event  foo bar     id   1      first second   Have a look pyeventdispatcher

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I made a variation of Longpoke s minimalistic approach that also ensures the signatures for both callees and callers   class EventHook object               A simple implementation of the Observer-Pattern      The user can specify an event signature upon inizializazion      defined by kwargs in the form of argumentname class  e g  id int       The arguments  types are not checked in this implementation though      Callables with a fitting signature can be added with    or removed with -       All listeners can be notified by calling the EventHook class with fitting     arguments        gt  gt  gt  event   EventHook id int  data dict       gt  gt  gt  event    lambda id  data  print   d  s     id  data        gt  gt  gt  event id 5  data   foo    bar        5   foo    bar         gt  gt  gt  event   EventHook id int       gt  gt  gt  event    lambda wrong name  None     Traceback  most recent call last                   ValueError  Listener must have these arguments   id int        gt  gt  gt  event   EventHook id int       gt  gt  gt  event    lambda id  None      gt  gt  gt  event wrong name 0      Traceback  most recent call last                   ValueError  This EventHook must be called with these arguments   id int              def   init   self    signature           self  signature   signature         self  argnames   set signature keys            self  handlers           def  kwargs str self           return      join k     v   name   for k  v in self  signature items         def   iadd   self  handler           params   inspect signature handler  parameters         valid   True         argnames   set n for n in params keys            if argnames    self  argnames              valid   False         for p in params values                if p kind    p VAR KEYWORD                  valid   True                 break             if p kind not in  p POSITIONAL OR KEYWORD  p KEYWORD ONLY                   valid   False                 break         if not valid              raise ValueError  Listener must have these arguments    s                                  self  kwargs str            self  handlers append handler          return self      def   isub   self  handler           self  handlers remove handler          return self      def   call   self   args    kwargs           if args or set kwargs keys       self  argnames              raise ValueError  This EventHook must be called with these                                   keyword arguments    s     self  kwargs str            for handler in self  handlers                 handler   kwargs       def   repr   self           return  EventHook  s     self  kwargs str

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You may have a look at pymitter  pypi   Its a small single-file   250 loc  approach  providing namespaces  wildcards and TTL    Here s a basic example   from pymitter import EventEmitter  ee   EventEmitter      decorator usage  ee on  myevent   def handler1 arg      print  handler1 called with   arg    callback usage def handler2 arg       print  handler2 called with   arg ee on  myotherevent   handler2     emit ee emit  myevent    foo     - gt   handler1 called with foo   ee emit  myotherevent    bar     - gt   handler2 called with bar

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If I do code in pyQt I use QT sockets signals paradigm  same is for django  If I m doing async I O I use native select module  If I m usign a SAX python parser I m using event API provided by SAX  So it looks like I m victim of underlying API  -   Maybe you should ask yourself what do you expect from event framework module  My personal preference is to use Socket Signal paradigm from QT  more info about that can be found here

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Here s another module for consideration  It seems a viable choice for more demanding applications      Py-notify is a Python package   providing tools for implementing   Observer programming pattern  These   tools include signals  conditions and   variables       Signals are lists of handlers that are   called when signal is emitted    Conditions are basically boolean   variables coupled with a signal that   is emitted when condition state   changes  They can be combined using   standard logical operators  not  and    etc   into compound conditions    Variables  unlike conditions  can hold   any Python object  not just booleans    but they cannot be combined

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I found this small script on Valued Lessons  It seems to have just the right simplicity power ratio I m after  Peter Thatcher is the author of following code  no licensing is mentioned    class Event      def   init   self           self handlers   set        def handle self  handler           self handlers add handler          return self      def unhandle self  handler           try              self handlers remove handler          except              raise ValueError  Handler is not handling this event  so cannot unhandle it            return self      def fire self   args    kargs           for handler in self handlers              handler  args    kargs       def getHandlerCount self           return len self handlers         iadd     handle       isub     unhandle       call     fire       len      getHandlerCount  class MockFileWatcher      def   init   self           self fileChanged   Event        def watchFiles self           source path    foo          self fileChanged source path   def log file change source path       print   r changed      source path    def log file change2 source path       print   r changed      source path    watcher                MockFileWatcher   watcher fileChanged    log file change2 watcher fileChanged    log file change watcher fileChanged -  log file change2 watcher watchFiles

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PyPI packages As of January 2021  these are the event-related packages available on PyPI  ordered by most recent release date   pymitter 0 3 0  Nov 2020 zope event 4 5 0  Sept 2020 python-dispatch 0 1 31  Aug 2020 RxPy3 1 0 1  June 2020 pluggy 0 13 1  June 2020  beta  Louie 2 0  Sept 2019 PyPubSub 4 0 3  Jan 2019 pyeventdispatcher 0 2 3a0  2018 buslane 0 0 5  2018 PyPyDispatcher 2 1 2  2017 axel 0 0 7  2016 blinker 1 4  2015 PyDispatcher 2 0 5  2015 dispatcher 1 0  2012 py-notify 0 3 1  2008  There s more That s a lot of libraries to choose from  using very different terminology  events  signals  handlers  method dispatch  hooks        I m trying to keep an overview of the above packages  plus the techniques mentioned in the answers here  First  some terminology    Observer pattern The most basic style of event system is the  bag of handler methods   which is a simple implementation of the Observer pattern  Basically  the handler methods  callables  are stored in an array and are each called when the event  fires   Publish-Subscribe The disadvantage of Observer event systems is that you can only register the handlers on the actual Event object  or handlers list    So at registration time the event already needs to exist  That s why the second style of event systems exists  the publish-subscribe pattern  Here  the handlers don t register on an event object  or handler list   but on a central dispatcher  Also the notifiers only talk to the dispatcher   What to listen for  or what to publish is determined by  signal   which is nothing more than a name  string   Mediator pattern Might be of interest as well  the Mediator pattern  Hooks A  hook  system is usally used in the context of application plugins   The application contains fixed integration points  hooks   and each plugin may connect to that hook and perform certain actions  Other  events  Note  threading Event is not an  event system  in the above sense   It s a thread synchronization system where one thread waits until another thread  signals  the Event object  Network messaging libraries often use the term  events  too  sometimes these are similar in concept  sometimes not  They can of course traverse thread-  process- and computer boundaries  See e g  pyzmq  pymq  Twisted  Tornado   gevent  eventlet  Weak references In Python  holding a reference to a method or object ensures that it won t get deleted by the garbage collector   This can be desirable  but it can also lead to memory leaks  the linked handlers are never cleaned up  Some event systems use weak references instead of regular ones to solve this  Some words about the various libraries Observer-style event systems   zope event shows the bare bones of how this works  see Lennart s answer   Note  this example does not even support handler arguments  LongPoke s  callable list  implementation shows that such an event system can be implemented very minimalistically by subclassing list  Felk s variation EventHook also ensures the signatures of callees and callers  spassig s EventHook  Michael Foord s Event Pattern  is a straightforward implementation  Josip s Valued Lessons Event class is basically the same  but uses a set instead of a list to store the bag  and implements   call   which are both reasonable additions  PyNotify is similar in concept and also provides additional concepts of variables and conditions   variable changed event     Homepage is not functional  axel is basically a bag-of-handlers with more features related to threading  error handling      python-dispatch requires the even source classes to derive from pydispatch Dispatcher  buslane is class-based  supports single- or multiple handlers and facilitates extensive type hints  Pithikos  Observer Event is a lightweight design   Publish-subscribe libraries   blinker has some nifty features such as automatic disconnection and filtering based on sender  PyPubSub is a stable package  and promises  quot advanced features that facilitate debugging and maintaining topics and messages quot   pymitter is a Python port of Node js EventEmitter2 and offers namespaces  wildcards and TTL  PyDispatcher seems to emphasize flexibility with regards to many-to-many publication etc   Supports weak references  louie is a reworked PyDispatcher and should work  quot in a wide variety of contexts quot   pypydispatcher is based on  you guessed it     PyDispatcher and also works in PyPy  django dispatch is a rewritten PyDispatcher  quot with a more limited interface  but higher performance quot   pyeventdispatcher is based on PHP s Symfony framework s event-dispatcher  dispatcher was extracted from django dispatch but is getting fairly old  Cristian Garcia s EventManger is a really short implementation   Others   pluggy contains a hook system which is used by pytest plugins  RxPy3 implements the Observable pattern and allows merging events  retry etc  Qt s Signals and Slots are available from PyQt or PySide2  They work as callback when used in the same thread  or as events  using an event loop  between two different threads   Signals and Slots have the limitation that they only work in objects of classes that derive from QObject

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Another handy package is events  It encapsulates the core to event subscription and event firing and feels like a  quot natural quot  part of the language  It seems similar to the C  language  which provides a handy way to declare  subscribe to and fire events  Technically  an event is a  quot slot quot  where callback functions  event handlers  can be attached to - a process referred to as subscribing to an event    Define a callback function def something changed reason       print  quot something changed because  s quot    reason    Use events module to create an event and register one or more callback functions from events import Events events   Events   events on change    something changed  When the event is fired  all attached event handlers are invoked in sequence  To fire the event  perform a call on the slot  events on change  it had to happen    This will output   something changed because it had to happen   More documentation can be found in the github repo or the documentation

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Here is a minimal design that should work fine  What you have to do is to simply inherit Observer in a class and afterwards use observe event name  callback fn  to listen for a specific event  Whenever that specific event is fired anywhere in the code  ie  Event  USB connected     the corresponding callback will fire   class Observer         observers          def   init   self           self  observers append self          self  observed events          def observe self  event name  callback fn           self  observed events append   event name    event name   callback fn    callback fn     class Event        def   init   self  event name   callback args           for observer in Observer  observers              for observable in observer  observed events                  if observable  event name      event name                      observable  callback fn    callback args    Example   class Room Observer       def   init   self           print  Room is ready            Observer   init   self    DON T FORGET THIS     def someone arrived self  who           print who     has arrived       Observe for specific event room   Room   room observe  someone arrived    room someone arrived     Fire some events Event  someone left       John   Event  someone arrived    Lenard     will output  Lenard has arrived   Event  someone Farted     Lenard

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I use zope event  It s the most bare bones you can imagine   -  In fact  here is the complete source code   subscribers       def notify event       for subscriber in subscribers          subscriber event    Note that you can t send messages between processes  for example  It s not a messaging system  just an event system  nothing more  nothing less

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I ve been doing it this way   class Event list          Event subscription       A list of callable objects  Calling an instance of this will cause a     call to each item in the list in ascending order by index       Example Usage       gt  gt  gt  def f x               print  f  s     x      gt  gt  gt  def g x               print  g  s     x      gt  gt  gt  e   Event        gt  gt  gt  e        gt  gt  gt  e append f       gt  gt  gt  e 123      f 123       gt  gt  gt  e remove f       gt  gt  gt  e        gt  gt  gt  e     f  g       gt  gt  gt  e 10      f 10      g 10       gt  gt  gt  del e 0       gt  gt  gt  e 2      g 2               def   call   self   args    kwargs           for f in self              f  args    kwargs       def   repr   self           return  Event  s     list   repr   self    However  like with everything else I ve seen  there is no auto generated pydoc for this  and no signatures  which really sucks

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If you need an eventbus that works across process or network boundaries you can try PyMQ  It currently supports pub sub  message queues and synchronous RPC  The default version works on top of a Redis backend  so you need a running Redis server  There is also an in-memory backend for testing  You can also write your own backend   import pymq    common code class MyEvent      pass    subscribe code  pymq subscriber def on event event  MyEvent       print  event received      publisher code pymq publish MyEvent       you can also customize channels pymq subscribe on event  channel  my channel   pymq publish MyEvent    channel  my channel     To initialize the system   from pymq provider redis import RedisConfig    starts a new thread with a Redis event loop pymq init RedisConfig       main application control loop  pymq shutdown     Disclaimer  I am the author of this library

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You can try buslane module    This library makes implementation of message-based system easier  It supports commands  single handler  and events  0 or multiple handlers  approach  Buslane uses Python type annotations to properly register handler   Simple example   from dataclasses import dataclass  from buslane commands import Command  CommandHandler  CommandBus    dataclass frozen True  class RegisterUserCommand Command       email  str     password  str   class RegisterUserCommandHandler CommandHandler RegisterUserCommand         def handle self  command  RegisterUserCommand  - gt  None          assert command    RegisterUserCommand              email  john lennon com               password  secret               command bus   CommandBus   command bus register handler RegisterUserCommandHandler    command bus execute command RegisterUserCommand      email  john lennon com       password  secret        To install buslane  simply use pip     pip install buslane

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If you wanted to do more complicated things like merging events or retry you can use the Observable pattern and a mature library that implements that  https   github com ReactiveX RxPY   Observables are very common in Javascript and Java and very convenient to use for some async tasks    from rx import Observable  Observer   def push five strings observer           observer on next  Alpha           observer on next  Beta           observer on next  Gamma           observer on next  Delta           observer on next  Epsilon           observer on completed     class PrintObserver Observer        def on next self  value           print  Received  0   format value        def on completed self           print  Done         def on error self  error           print  Error Occurred   0   format error    source   Observable create push five strings   source subscribe PrintObserver      OUTPUT   Received Alpha Received Beta Received Gamma Received Delta Received Epsilon Done

[python] Event system in Python

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