1. Dmitry Vakhrushev
  2. GreenRocket



Green Rocket

Green Rocket is a simple and compact implementation of Observer (or Publish/Subscribe) design pattern via signals.

Create specific signal using base one:

>>> from greenrocket import Signal
>>> class MySignal(Signal):
...     pass

Subscribe handler:

>>> @MySignal.subscribe
... def handler(signal):
...     print('handler: ' + repr(signal))

Fire signal:

>>> MySignal().fire()
handler: MySignal()

If you are using asyncio, you can also use coroutines as handlers and fire signal asynchronously using await Signal.afire() or yield from Signal().afire(). Method afire() works well with synchronous handlers too.

Note, that signal propagates over inheritance, i.e. all subscribers of base signal will be called when child one is fired:

>>> @Signal.subscribe
... def base_handler(signal):
...     print('base_handler: ' + repr(signal))
>>> MySignal().fire()
handler: MySignal()
base_handler: MySignal()

Unsubscribe handler:

>>> MySignal.unsubscribe(handler)
>>> MySignal().fire()
base_handler: MySignal()

The handler is subscribed using weak reference. So if you create and subscribe a handler in local scope (for example inside a generator), it will be unsubscribed automatically.

>>> def gen():
...     @MySignal.subscribe
...     def local_handler(signal):
...         print('local_handler: ' + repr(signal))
...     yield 1
>>> for value in gen():
...     MySignal(value=value).fire()
local_handler: MySignal(value=1)
base_handler: MySignal(value=1)
>>> import gc                    # PyPy fails the following test without
>>> _ = gc.collect()             # explicit call of garbage collector.
>>> MySignal(value=2).fire()
base_handler: MySignal(value=2)
>>> Signal.unsubscribe(base_handler)

As you can see above, signal constructor accepts keyword arguments. These arguments are available as signal's attributes:

>>> s = MySignal(a=1, b=2)
>>> s.a
>>> s.b

Signal suppresses any exception which is raised on handler call. It uses logger named greenrocket from standard logging module to log errors and debug information.

The library also provides Watchman class as a convenient way for testing signals.

Create watchman for specific signal:

>>> from greenrocket import Watchman
>>> watchman = Watchman(MySignal)

Fire signal:

>>> MySignal(x=1).fire()

Test signal:

>>> watchman.assert_fired_with(x=1)
>>> watchman.assert_fired_with(x=2)          # DOCTEST: +ellipsis
Traceback (most recent call last):
AssertionError: Failed assertion on MySignal.x: 1 != 2
>>> watchman.assert_fired_with(x=1, y=2)     # DOCTEST: +ellipsis
Traceback (most recent call last):
AssertionError: MySignal has no attribute y

Watchman object saves each fired signal to its log:

>>> watchman.log
>>> MySignal(x=2).fire()
>>> watchman.log
[MySignal(x=1), MySignal(x=2)]

The method assert_fired_with tests the last signal from the log by default:

>>> watchman.assert_fired_with(x=2)

But you can specify which one to test:

>>> watchman.assert_fired_with(-2, x=1)