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A simple C#-like implementation of the Observer design pattern.

Project description

nmevent v0.3.2 - C#-like implementation of the Observer pattern

This is a Python module nmevent, simple C#-like implementation of the Observer pattern ( It’s main purpose and goal is to allow developers to use events with C#-like syntax in their Python classes.

Usage example

The most straightfoward way to use nmevent is this:

>>> import nmevent
>>> class ExampleClass(object):
...    def __init__(self):
...       self.event = nmevent.Event()
...    def do_something(self):
...       self.event(self)
>>> def handler(sender, **keywords):
...    print "event occured"
>>> example = ExampleClass()
>>> example.event += handler
>>> example.do_something()
event occured

It should be noted, that event doesn’t necessarily need to be an object attribute. Event instance is basically just a callable object that works as a sort of “dispatch demultiplexer”.

This usage, however, isn’t very C#-like. In C#, events are declared in class scope and that’s why the Event class also supports the descriptor protocol (you can use the same way you use the built-in property object).

>>> from nmevent import Event
>>> class ExampleClass(object):
...    event = Event()
...    def _do_something(self):
...       self.event()
>>> def handler(sender, **keywords):
...    pass
>>> example = ExampleClass()
>>> example.event += handler

Perhaps this looks even more straightfoward than instantiating Event in object’s constructor, but there’s actually lot more going on under hood this time.

Finally, there is the Property descriptor and the associated nmproperty function decorator, which work very much like the built-in property object and decorator, except it can optionally call a callback function if the property’s value changes after calling the setter function. It can work in tandem with the with_events class decorator, which decorates the class with property change events and connects them to the instances of Property class. It also creates events for the built-in property objects, but you have to raise the events yourself in the setter function or elsewhere.

>>> @nmevent.with_events
... class ExampleClass(object):
...    @nmevent.nmproperty
...    def x(self):
...       return self._x
...    @x.setter
...    def x(self, value):
...       self._x = value
...    @property
...    def y(self):
...       return self._y
...    @y.setter
...    def y(self, value):
...       old_value, self._y = self._y, value
...       self.y_changed(old_value = old_value)
...    def __init__(self):
...       self._x = None
...       self._y = None
>>> def handler(sender, **keywords):
...    print "x changed"
>>> example = ExampleClass()
>>> example.x_changed += handler
>>> example.x = 10 # handler gets called
x changed


Copyright (c) 2010, Jan Milík.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope the it will be useful, but WITHOUT ANY WARRANTY; without event the implied warranty of MERCHANTIBILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this program. If not, see <>.



Added fairly useful functionality to the adapt function: it can now connect to “nested events”. See the functions’s documentation for more information. It can also disconnect the handlers as well.


Added docstring tests and fixed all the docstrings so that they would pass. As a result, another problem was found with the event binding. That problem was fixed by adding the InstanceEvent.bind method to be used mainly by the Property class when raising the “changed” events.


Fixed a major bug, which caused an unbound event not to be actually bound when called with an object instance as the first argument.

Added the with_properties class decorator, which automatically decorates a class with “private” attributes for each property and automatic getters and setters where either one of them is missing.


Rewritten some unit tests and added new ones. Improved documentation a little bit.


Rewritten most of the code. The Event class now works as a descriptor, which eliminated the need for a separate EventSlot class and simplified usage. Added CallbackStore to abstract the callback storage.


No changes in source code. Improved documentation and unit tests.


Initial release.

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