I am trying to build some UI panels for an Eclipse based tool. The API for the tool has a mechanism for event handling based on decorators, so for example, the following ties callbackOpen to the opening of a_panel_object:
#panelOpenHandler(a_panel_object)
def callbackOpen(event):
print "opening HERE!!"
This works fine, but I wanted to wrap all of my event handlers and actual data processing for the panel behind a class. Ideally I would like to do something like:
class Test(object):
def __init__(self):
# initialise some data here
#panelOpenHandler(a_panel_object)
def callbackOpen(self, event):
print "opening HERE!!"
But this doesn't work, I think probably because I am giving it a callback that takes both self and event, when the decorator is only supplying event when it calls the function internally (note: I have no access to source code on panelOpenHandler, and it is not very well documented...also, any error messages are getting swallowed by Eclipse / jython somewhere).
Is there any way that I can use a library decorator that provides one argument to the function being decorated on a function that takes more than one argument? Can I use lambdas in some way to bind the self argument and make it implicit?
I've tried to incorporate some variation of the approaches here and here, but I don't think that it's quite the same problem.
Your decorator apparently registers a function to be called later. As such, it's completely inappropriate for use on a class method, since it will have no idea of which instance of the class to invoke the method on.
The only way you'd be able to do this would be to manually register a bound method from a particular class instance - this cannot be done using the decorator syntax. For example, put this somewhere after the definition of your class:
panelOpenHandler(context.controls.PerformanceTuneDemoPanel)(Test().callbackOpen)
I found a work around for this problem. I'm not sure if there is a more elegant solution, but basically the problem boiled down to having to expose a callback function to global() scope, and then decorate it with the API decorator using f()(g) syntax.
Therefore, I wrote a base class (CallbackRegisterer), which offers the bindHandler() method to any derived classes - this method wraps a function and gives it a unique id per instance of CallbackRegisterer (I am opening a number of UI Panels at the same time):
class CallbackRegisterer(object):
__count = 0
#classmethod
def _instanceCounter(cls):
CallbackRegisterer.__count += 1
return CallbackRegisterer.__count
def __init__(self):
"""
Constructor
#param eq_instance 0=playback 1=record 2=sidetone.
"""
self._id = self._instanceCounter()
print "instantiating #%d instance of %s" % (self._id, self._getClassName())
def bindHandler(self, ui_element, callback, callback_args = [], handler_type = None,
initialize = False, forward_event_args = False, handler_id = None):
proxy = lambda *args: self._handlerProxy(callback, args, callback_args, forward_event_args)
handler_name = callback.__name__ + "_" + str(self._id)
if handler_id is not None:
handler_name += "_" + str(handler_id)
globals()[handler_name] = proxy
# print "handler_name: %s" % handler_name
handler_type(ui_element)(proxy)
if initialize:
proxy()
def _handlerProxy(self, callback, event_args, callback_args, forward_event_args):
try:
if forward_event_args:
new_args = [x for x in event_args]
new_args.extend(callback_args)
callback(*new_args)
else:
callback(*callback_args)
except:
print "exception in callback???"
self.log.exception('In event callback')
raise
def _getClassName(self):
return self.__class__.__name__
I can then derive a class from this and pass in my callback, which will be correctly decorated using the API decorator:
class Panel(CallbackRegisterer):
def __init__(self):
super(Panel, self).__init__()
# can bind from sub classes of Panel as well - different class name in handle_name
self.bindHandler(self.controls.test_button, self._testButtonCB, handler_type = valueChangeHandler)
# can bind multiple versions of same function for repeated ui elements, etc.
for idx in range(0, 10):
self.bindHandler(self.controls["check_box_"+str(idx)], self._testCheckBoxCB,
callback_args = [idx], handler_type = valueChangeHandler, handler_id = idx)
def _testCheckBoxCB(self, *args):
check_box_id = args[0]
print "in _testCheckBoxCB #%d" % check_box_id
def _testButtonCB(self):
"""
Handler for test button
"""
print "in _testButtonCB"
panel = Panel()
Note, that I can also derive further sub-classes from Panel, and any callbacks bound there will get their own unique handler_name, based on class name string.
I have a scenario where I'm dynamically running functions at run-time and need to keep track of a "localized" scope. In the example below, "startScope" and "endScope" would actually be creating levels of "nesting" (in reality, the stuff contained in this localized scope isn't print statements...it's function calls that send data elsewhere and the nesting is tracked there. startScope / endScope just set control flags that are used to start / end the current nesting depth).
This all works fine for tracking the nested data, however, exceptions are another matter. Ideally, an exception would result in "falling out" of the current localized scope and not end the entire function (myFunction in the example below).
def startScope():
#Increment our control object's (not included in this example) nesting depth
control.incrementNestingDepth()
def endScope():
#Decrement our control object's (not included in this example) nesting depth
control.decrementNestingDepth()
def myFunction():
print "A"
print "B"
startScope()
print "C"
raise Exception
print "D"
print "This print statement and the previous one won't get printed"
endScope()
print "E"
def main():
try:
myFunction()
except:
print "Error!"
Running this would (theoretically) output the following:
>>> main()
A
B
C
Error!
E
>>>
I'm quite certain this isn't possible as I've written it above - I just wanted to paint a picture of the sort of end-result I'm trying to achieve.
Is something like this possible in Python?
Edit: A more relevant (albeit lengthy) example of how this is actually being used:
class Log(object):
"""
Log class
"""
def __init__(self):
#DataModel is defined elsewhere and contains a bunch of data structures / handles nested data / etc...
self.model = DataModel()
def Warning(self, text):
self.model.put("warning", text)
def ToDo(self, text):
self.model.put("todo", text)
def Info(self, text):
self.model.put("info", text)
def StartAdvanced(self):
self.model.put("startadvanced")
def EndAdvanced(self):
self.model.put("endadvanced")
def AddDataPoint(self, data):
self.model.put("data", data)
def StartTest(self):
self.model.put("starttest")
def EndTest(self):
self.model.put("endtest")
def Error(self, text):
self.model.put("error", text)
#myScript.py
from Logger import Log
def test_alpha():
"""
Crazy contrived example
In this example, there are 2 levels of nesting...everything up to StartAdvanced(),
and after EndAdvanced() is included in the top level...everything between the two is
contained in a separate level.
"""
Log.Warning("Better be careful here!")
Log.AddDataPoint(fancyMath()[0])
data = getSerialData()
if data:
Log.Info("Got data, let's continue with an advanced test...")
Log.StartAdvanced()
#NOTE: If something breaks in one of the following methods, then GOTO (***)
operateOnData(data)
doSomethingCrazy(data)
Log.ToDo("Fill in some more stuff here later...")
Log.AddDataPoint(data)
Log.EndAdvanced()
#(***) Ideally, we would resume here if an exception is raised in the above localized scope
Log.Info("All done! Log some data and wrap everything up!")
Log.AddDataPoint({"data": "blah"})
#Done
#framework.py
import inspect
from Logger import Log
class Framework(object):
def __init__(self):
print "Framework init!"
self.tests = []
def loadTests(self, file):
"""
Simplifying this for the sake of clarity
"""
for test in file:
self.tests.append(test)
def runTests(self):
"""
Simplifying this for the sake of clarity
"""
#test_alpha() as well as any other user tests will be run here
for test in self.tests:
Log.StartTest()
try:
test()
except Exception,e :
Log.Error(str(e))
Log.EndTest()
#End
You can achieve a similar effect with a context manager using a with statement. Here I use the contextlib.contextmanager decorator:
#contextlib.contextmanager
def swallower():
try:
yield
except ZeroDivisionError:
print("We stopped zero division error")
def foo():
print("This error will be trapped")
with swallower():
print("Here comes error")
1/0
print("This will never be reached")
print("Merrily on our way")
with swallower():
print("This error will propagate")
nonexistentName
print("This won't be reached")
>>> foo()
This error will be trapped
Here comes error
We stopped zero division error
Merrily on our way
This error will propagate
Traceback (most recent call last):
File "<pyshell#4>", line 1, in <module>
foo()
File "<pyshell#3>", line 10, in foo
nonexistentName
NameError: global name 'nonexistentName' is not defined
It cannot be done with an ordinary function call as in your example. In your example, the function startScope returns before the rest of the body of myFunction executes, so startScope can't have any effect on it. To handle exceptions, you need some kind of explicit structure (either a with statement or a regular try/except) inside myFunction; there's no way to make a simple function call magically intercept exceptions that are raised in its caller.
You should read up on context managers as they seem to fit what you're trying to do. The __enter__ and __exit__ methods of the context manager would correspond to your startScope and endScope. Whether it will do exactly what you want depends on exactly what you want those "manager" functions to do, but you will probably have more luck doing it with a context manager than trying to do it with simple function calls.
So this is kind of a python design question + multiple heritance. I'm working on a program of mine and I've ran into an issue I can't figure out a decent way of solving.
To keep it simple. The software scans a log event file generated from another program. Initially it creates and stores each event in a representative event object. But I want to access them quickly and with a more robust language so I'm loading them into a SQL DB after doing a little processing on each event, so theres more data than previous. When I query the DB I'm wanting to recreate an object for each entry representative of the event so its easier to work with.
The problem I'm running into is that I want to avoid a lot of duplicate code and technically I should be able to just reuse some of the code in the original classes for each event. Example:
class AbstractEvent:
__init__(user, time)
getTime()
getUser()
class MessageEvent(AbstractEvent):
__init__(user,time,msg)
getMessage()
class VideoEvent(AbstractEvent):
pass
But, there is extra data after its gone into the DB so there needs to be new subclasses:
class AbstractEventDB(AbstractEvent):
__init__(user, time, time_epoch)
getTimeEpoch()
(static/classmethod) fromRowResult(row)
class MessageEventDB(AbstractEventDB, MessageEvent):
__init__(user, time, msg, time_epoch, tags)
getTags()
(static/classmethod) fromRowResult(row)
class VideoEventDB(AbstractEventDB, VideoEvent):
pass
This is a simpler version than whats happening, but it shows some of what does happen. I change long form time stamps from the log file into epoch timestamps when they go into the DB and various tags are added on message events but other events have nothing extra really beyond the timestamp change.
The above is ideally how I would like to format it, but the problem I've ran into is that the call signatures are completely different on the DB object side compared to the Simple Event side; so when I try to call super() I get an error about expected arguements missing.
I was hoping someone might be able to offer some advice on how to structure it and avoid duplicating code 10-20 times over, particularly in the fromRowResult (a factory method). Help much appreciated.
I thin what you are looking for is a Python implementation for the decorator design pattern.
http://en.wikipedia.org/wiki/Decorator_pattern
The main idea is to replace multiple inheritance with inheritance + composition:
class AbstractEvent(object):
def __init__(self, user, time):
self.user = user
self.time = time
class MessageEvent(AbstractEvent):
def __init__(self, user, time, msg):
super(MessageEvent, self).__init__(user, time)
self.msg = msg
class AbstractEventDBDecorator(object):
def __init__(self, event, time_epoch):
# event is a member of the class. Using dynamic typing, the event member will
# be a AbstractEvent or a MessageEvent at runtime.
self.event = event
self.time_epoch = time_epoch
#classmethod
def fromRowResult(cls, row):
abstract_event = AbstractEvent(row.user, row.time)
abstract_event_db = AbstractEventDBDecorator(abstract_event, row.time_epoch)
return abstract_event_db
class MessageEventDB(AbstractEventDBDecorator):
def __init__(self, message_event, time_epoch, tags):
super(MessageEventDB, self).__init__(message_event, time_epoch)
self.tags = tags
#classmethod
def fromRowResult(cls, row):
message_event = MessageEvent(row.user, row.time, row.msg)
message_event_db = MessageEventDB(message_event, row.time_epoch, row.tags)
return message_event_db
class Row:
def __init__(self, user, time, msg, time_epoch, tags):
self.user = user
self.time = time
self.msg = msg
self.time_epoch = time_epoch
self.tags = tags
if __name__ == "__main__":
me = MessageEvent("user", "time", "msg")
r = Row("user", "time", "Message", "time_epoch", "tags")
med = MessageEventDB.fromRowResult(r)
print med.event.msg
Is it possible to create new signals in Python GTK+ ?
I'd like a skeleton code example, please.
An excerpt:
Creating your own signals
The other thing you probably want to
use when subclassing GObject is define
custom signals. You can create your
own signals that can be emitted so
users of your class can connect to
them.
When a signal is emitted a set of
closures will be executed. A closure
is an abstraction of the callback
concept. A closure is the callback
itself (a function pointer), the user
data (it will be the last parameter to
the callback) and another function for
cleanup issues, which will not be
discussed in this document.
For the sake of this article you don't
really need to know the difference
between a callback and a closure so
both terms will be used. But be
advised that this is not totally
correct.
As we said before, when a signal is
emitted, a set of closures will be
executed. One of them is the same one
for all the instances of this class
and hence its name: the class closure,
and the other ones are custom user
callbacks. Note that not all the
signals need to have a class closure
because it is optional.
From, http://www.pygtk.org/articles/subclassing-gobject/sub-classing-gobject-in-python.htm, hope that helps. There is example code on the site and here , a snippet:
import pygtk
pygtk.require('2.0')
import gobject
class Car(gobject.GObject):
__gproperties__ = {
'fuel' : (gobject.TYPE_FLOAT, 'fuel of the car',
'amount of fuel that remains in the tank',
0, 60, 50, gobject.PARAM_READWRITE)
}
__gsignals__ = {
'engine-started' : (gobject.SIGNAL_RUN_LAST, gobject.TYPE_NONE,
(gobject.TYPE_FLOAT,))
}
def __init__(self):
gobject.GObject.__init__(self)
self.fuel = 50
def do_get_property(self, property):
if property.name == 'fuel':
return self.fuel
else:
raise AttributeError, 'unknown property %s' % property.name
def do_set_property(self, property, value):
if property.name == 'fuel':
self.fuel = value
else:
raise AttributeError, 'unknown property %s' % property.name
def do_engine_started(self, remaining_fuel):
print '***** Beginning of class closure *****'
print 'The engine is ready and we still have %f of fuel' % self.fuel
print '***** End of class closure *****'
def start(self):
self.emit('engine-started', self.get_property('fuel'))
gobject.type_register(Car)
The answers are correct of course, but using libraries is less prone to pains:
import gobject
from pygtkhelpers.utils import gsignal
class Car(gobject.GObject):
gsignal('engine-started', float) # yeah baby
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I am aware of pydispatcher, but there must be other event-related packages around for Python.
Which libraries are available?
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.
PyPI packages
As of October 2022, these are the event-related packages available on PyPI,
ordered by most recent release date.
PyDispatcher 2.0.6: Aug 2022
blinker 1.5: Jun 2022
pymitter 0.4.0: June 2022
python-dispatch 0.2.0: Apr 2022
pluggy 1.0.0: August 2021
Events 0.4: October 2020
zope.event 4.5.0: Sept 2020
RxPy3 1.0.1: June 2020
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
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 "advanced features that facilitate debugging and maintaining topics and messages".
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 "in a wide variety of contexts".
pypydispatcher is based on (you guessed it...) PyDispatcher and also works in PyPy.
django.dispatch is a rewritten PyDispatcher "with a more limited interface, but higher performance".
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.
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:
>>> def f(x):
... print 'f(%s)' % x
>>> def g(x):
... print 'g(%s)' % x
>>> e = Event()
>>> e()
>>> e.append(f)
>>> e(123)
f(123)
>>> e.remove(f)
>>> e()
>>> e += (f, g)
>>> e(10)
f(10)
g(10)
>>> del e[0]
>>> 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.
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
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.
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()
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')
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 "Hello {}".format(name)
def greetings(name):
print "Greetings {}".format(name)
EventManager.addEvent( salute = [greetings] )
EventManager.salute += hello
print "\nInitial salute"
EventManager.salute('Oscar')
print "\nNow remove greetings"
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("functions parameter has to be a list")
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):
"""
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 "Hello ",
def world(): print "World"
EventManager.addEvent( salute = [hello] )
EventManager.salute += world
EventManager.salute()
Output:
Hello World
"""
for key in kvargs.keys():
if type(kvargs[key]) is not list:
raise ValueError("value has to be a list")
else:
kvargs[key] = cls.Event(kvargs[key])
cls.__dict__.update(kvargs)
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")
# -> "handler1 called with foo"
ee.emit("myotherevent", "bar")
# -> "handler2 called with bar"
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.
>>> event = EventHook(id=int, data=dict)
>>> event += lambda id, data: print("%d %s" % (id, data))
>>> event(id=5, data={"foo": "bar"})
5 {'foo': 'bar'}
>>> event = EventHook(id=int)
>>> event += lambda wrong_name: None
Traceback (most recent call last):
...
ValueError: Listener must have these arguments: (id=int)
>>> event = EventHook(id=int)
>>> event += lambda id: None
>>> 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()
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
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!
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.
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
Another handy package is events. It encapsulates the core to event subscription and event firing and feels like a "natural" 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 "slot" 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 "something changed because %s" % 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.
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) -> 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
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