Django defines a single Signal base class in dispatcher.py and is a crucial part of the dispatching system. The responsibility of the Signal class is to serve as a base class for all signal types that may dispatched in the system. In the Django signal dispatching system, signal instances are dispatched to receivers. Signal instances can't just spontaneously decide to send themselves. There has to be some motivating party and in the Django signal dispatching system, this concept is referred to as the sender. Thus, the three core concepts of the Django signal dispatching system are signal, sender, and receiver. The relationship between the three concepts is illustrated below.
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Senders of signals may dispatch a signal to zero or more receivers. The only way that zero receivers receive a given signal is if zero receivers have been connected to that signal. Additionally, receivers, once connected to a given signal, have the option of only accepting signals from a specific sender.
So how does one wire the required connections between these signal concepts in the Django signal dispatching system? Receivers can connect to specific signal types by invoking the Signal.connect() method on the desired signal instance. The receiver that is being connected to the signal is passed to this method as a parameter. If this receiver is to only accept these signals from specific senders, the sender can also be specified as an parameter to this method. Once connected, the receiver will be activated once any of these signal types have been sent by a sender. A sender can send a signal by invoking the Signal.send() method. The sender itself is passed as a parameter to this method. This is a required parameter even though the receiver may not necessarily care who sent the signal. However, it is good practice to not take these chances. If, from a signal sending point of view, there is always a consistency in regards to who the sender is, there is a new lever of flexibility on the receiving end. Illustrated below is a sample interaction between a sender and a receiver using the Django signal dispatching system to send a signal.
![](http://3.bp.blogspot.com/_qOKUA2GftVM/SjAIHFpI1VI/AAAAAAAAALM/212osNnqXAc/s320/django_signal_dispatch_interaction.png)
The fact that the signal instances themselves are responsible for connecting receivers to signals as well as the actual sending of the signals may seem counter-intuitive at first. Especially if one is used to working with publish-subscribe style event systems. In these event systems, the publishing and subscribing mechanisms are independent from the publisher and subscriber entities. However, in the end, the same effect is achieved.