minimalist event system for Python





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pyobservable is a minimalist event system for python. It provides you an easy-to-use interface to trigger arbitrary functions when specific events occur.

from observable import Observable

obs = Observable()

def error_handler(message):
    # do some fancy error handling
    logging.error(f"An error occured: {message}")


def do_time_travel():
    # do some time traveling
    if year != 1291:
        obs.trigger("error", "Time travel to 1291 didn't work")

An ObservableProperty type is included as well, which makes observing object properties a breeze.

Note: We are Python 3 only! Only Python Versions >= 3.5 are supported. Use v0.3.2 for older Python Versions.

How to use

Use a pip to install it from PyPI:

pip install observable

After completion you can start using observable:

from observable import Observable

obs = Observable()

Usage of observable.Observable

on: Register event handler with on

There are two ways to register a function to an event.
The first way is to register the event with a decorator like this:

def error_func(message):
    print("Error: %s" % message)

The second way is to register it with a method call:

def error_func(message):
    print("Error: %s" % message)
obs.on("error", error_func)

once: Register event handler with once

once works like on, but once the event handler is triggered it will be removed and cannot be triggered again.

trigger: trigger event

You can trigger a registered event with the trigger method:

obs.trigger("error", "This is my error message")

If no handler for the event error could be found an Observable.NoHandlerFound-Exception will be raised.

off: remove handler and events

Remove a handler from a specified event:

obs.off("error", error_func)
obs.off("error", [error_func, second_error_func])

Remove all handlers from a specified event:


Clear all events:


get_all_handlers, get_handlers and is_registered: Check which handlers are registered

Imagine you registered the following handlers:

def success_func():

def error_func(message):
    print("Error: %s" % message)

Then you can do the following to inspect the registered handlers:

>>> obs.get_all_handlers()
{'success': [<function success_func at 0x7f7f32d0a1e0>], 'error': [<function error_func at 0x7f7f32d0a268>]}
>>> obs.get_handlers("success")
[<function success_func at 0x7f7f32d0a1e0>]
>>> obs.get_handlers("other_event")

Usage of observable.property.ObservableProperty

A property that can be observed easily by listening for some special, auto-generated events. Its API is identical to that of the build-in property.

The ObservableProperty needs an Observable object for triggering events. If the property is an attribute of an object of type Observable, that one will be used automatically. To specify a different Observable object, pass it as the observable keyword argument when initializing the ObservableProperty. You may also pass a string for observable, which looks for an attribute of that name in the containing object. This is useful to specify the name of an attribute which will only be created at object initialization and thus isn't there when defining the property.

The following events, of which "after_set_<name>" is probably the one used most, are triggered:

  • "before_get_<name>"() and "after_get_<name>"(value)
  • "before_set_<name>"(value) and "after_set_<name>"(value)
  • "before_del_<name>"() and "after_del_<name>"()

<name> has to be replaced with the property's name. Note that names are taken from the individual functions supplied as getter, setter and deleter, so please name those functions like the property itself. Alternatively, the name to use can be specified with the name keyword argument when initializing the ObservableProperty.

The convenience helper ObservableProperty.create_with() can be used as a decorator for creating ObservableProperty objects with custom event and/or observable. It returns a functools.partial() with the chosen attributes pre-set.

Here's an example for using the event and observable keyword arguments:

>>> from observable import Observable
>>> from observable.property import ObservableProperty
>>> class MyObject:
...     def __init__(self):
...         self.events = Observable()
...         self._value = 10
...     @ObservableProperty.create_with(event="prop", observable="events")
...     def some_obscure_name(self):
...         return self._value
...     @some_obscure_name.setter
...     def some_obscure_name(self, value):
...         self._value += value
>>> obj = MyObject()
>>> obj.obs.on("after_get_prop", lambda v: print("got", v))
>>> obj.obs.on("before_set_prop",
...            lambda v: print("setting", obs.some_obscure_name, v))
>>> obj.obs.on("after_set_prop", lambda v: print("set", v))
>>> obj.some_obscure_name = 32
got 10
setting 10 32
set 32
>>> obj.some_obscure_name
got 42

This project is published under MIT.
A Timo Furrer project.
- 🎉 -

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