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Blinker Herald

The Herald Documentation Status Coverage

The Blinker Herald includes helpers to easily emit signals using the excelent blinker library.

Decorate a function or method with @blinker_herald.emit() and pre and post signals will be automatically emitted to all connected handlers.


  • All the features provided by blinker
  • + an easy decorator @emit() to magically emit signals when your functions are called and before it returns a result.
  • A signals namespace proxy to discover the signals in your project
  • Customizable for your needs


Let's say you have a class and wants to emit a signal for a specific method:

from blinker_herald import emit

class SomeClass(object):

    def do_something(self, arg1):
        # here is were magically the 'pre' signal will be sent
        return 'something done'
        # here is were magically the 'post' signal will be sent

using @emit decorator makes blinker_herald to emit a signal for that method and now you can connect handlers to capture that signals

You can capture pre signal to manipulate the object:

def handle_pre(sender, signal_emitter, **kwargs): = 'bar'

And you can also capture the post signal to log the results:
def handle_post(sender, signal_emitter, result, **kwargs):"The method {0} returned {1}".format(sender, result))


Post-signals are only called if there were no exceptions raised during the processing of their related function.

You can also use the namespace proxy blinker_herald.signals to connect handlers to signals, the signal name is the prefix pre or post followed by _ and the method name:

from blinker_herald import signals

def handle_pre(sender, signal_emitter, **kwargs):

If you have a lot of subclasses emitting signals with the same name and you need to capture only specific signals, you can specify that you want to listen to only one type of sender:

from blinker_herald import emit, signals, SENDER_CLASS
class BaseModel(object):
    def create(self, **kwargs):
        new_instance =, **kwargs)
        return new_instance

class One(BaseModel):

class Two(BaseModel):


By default the sender is always the instance but you can use SENDER_CLASS to force the sender to be the class another options are SENDER_CLASS_NAME, SENDER_MODULE, SENDER_NAME and you can also pass a string, an object or a lambda receiving the sender instance e.g: @emit(sender=lambda self: self.get_sender())

Using SENDER_CLASS you can now connect to specific signal:

from blinker_herald import signals

def handle_post_only_for_one(sender, signal_emitter, result, **kwargs):
    # sender is the class One (cls)
    # signal the instance of the class One (self)
    # result is the return of the method create

The above will handle the create method signal for the class One but not for the class Two

You can also be more specific about the signal you want to connect using the __ double underscore to provide method name:

from blinker_herald import signals

def handle_post(sender, signal_emitter, result, **kwargs):

The above will connect to the post signal emitted by module_name.ClassName.method_name


You don't have to use the pattern above if your project do not have a lot of method name collisions, using only the method name will be just fine for most cases.


This software was first created by SatelliteQE team to provide signals to Robottelo and Nailgun


The Blinker Herald includes helpers to easily emit signals using the excellent blinker library.




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