Bringing the elegance of C# EventHandler to Python
The concept of events is heavily used in GUI libraries and is the foundation for most implementations of the MVC (Model, View, Controller) design pattern. Another prominent use of events is in communication protocol stacks, where lower protocol layers need to inform upper layers of incoming data and the like. Here is a handy class that encapsulates the core to event subscription and event firing and feels like a “natural” part of the language.
The package has been tested under Python 2.6, 2.7, 3.3 and 3.4.
The C# language 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. To subscribe to an event:
>>> def something_changed(reason): ... print "something changed because %s" % reason ... >>> from events import Events >>> events = Events() >>> events.on_change += something_changed
Multiple callback functions can subscribe to the same event. 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') something changed because it had to happen
Usually, instances of
Events will not hang around loosely like
above, but will typically be embedded in model objects, like here:
class MyModel(object): def __init__(self): self.events = Events() ...
Similarly, view and controller objects will be the prime event subscribers:
class MyModelView(SomeWidget): def __init__(self, model): ... self.model = model model.events.on_change += self.display_value ... def display_value(self): ...
_EventSlot classes provide
some introspection support. This is usefull for example for automatic event
subscription based on method name patterns.
>>> from events import Events >>> events = Events() >>> print events <events.events.Events object at 0x104e5d5f0> >>> def changed(): ... print "something changed" ... >>> def another_one(): ... print "something changed here too" ... >>> def deleted(): ... print "something got deleted!" ... >>> events.on_change += changed >>> events.on_change += another_one >>> events.on_delete += deleted >>> print len(events) 2 >>> for event in events: ... print event.__name__ ... on_change on_delete >>> event = events.on_change >>> print event event 'on_change' >>> print len(event) 2 >>> for handler in event: ... print handler.__name__ ... changed another_one >>> print event <function changed at 0x104e5c230> >>> print event.__name__ changed >>> print len(events.on_delete) 1 >>> events.on_change() something changed somethind changed here too >>> events.on_delete() something got deleted!
Note that by default
Events does not check if an event that is
being subscribed to can actually be fired, unless the class attribute
__events__ is defined. This can cause a problem if an event name is
slightly misspelled. If this is an issue, subclass
list the possible events, like:
class MyEvents(Events): __events__ = ('on_this', 'on_that', ) events = MyEvents() # this will raise an EventsException as `on_change` is unknown to MyEvents: events.on_change += changed
You can also predefine events for a single
Events instance by
passing an iterator to the constructor.
events = Events(('on_this', 'on_that')) # this will raise an EventsException as `on_change` is unknown to MyEvents: events.on_change += changed
It is recommended to use the constructor method for one time use cases. For more
complicated use cases, it is recommended to subclass
You can also leverage both the constructor method and the
attribute to restrict events for specific instances:
DatabaseEvents(Events): __events__ = ('insert', 'update', 'delete', 'select') audit_events = ('select') AppDatabaseEvents = DatabaseEvents() # only knows the 'select' event from DatabaseEvents AuditDatabaseEvents = DatabaseEvents(audit_events)
Events is on PyPI so all you need to do is:
pip install events
python setup.py test
The package has been tested under Python 2.6, Python 2.7 and Python 3.3.