Updated April 13, 2023
Introduction to Python Event Handler
An Event Handler is a class, part of events, which simply is responsible for managing all callbacks which are to be executed when invoked. An Event is simply an action, like clicking a button, which then leads to another event, which the developer already assigns. The core of Python Event Handler is to manage these events and organize and make them work as intended in an intended manner. It is a known fact that programs that follow proper event handling are flexible and easy to manage. Debugging such programs is easy. Another advantage is the ability to change the program at any given point, according to needs.
One of the examples for python is the Python Interface of Tkinter. Tkinter is a GUI tool for python, which is always waiting for an event to happen. It provides an easy mechanism for a developer to efficiently manage the events. To manage the link between the event of click and the next action’s event is the core process of an event handler in python.
How Does Event Handler Work in Python?
Event Handler class in python works similarly as in other programming languages. We have understood that the working of events is basically like raising a flag for an event. And a class responsible for handling and monitoring these events is known as an event handler. A very simple example to understand the working of events and event handlers is a user form. After the user is done filling out the information, the user has to click on the submit button, sending the details and processing it. In Python, where we mentioned events, there are two ends.
One is the class that raises the event, which is called a publisher, and the other class responsible for receiving the raised events is known as subscribers. Similar to other programming languages, Events in Python can call for multiple numbers of subscribers, and at the same time, these subscribers can call for multiple numbers of publishers. this is one of the great features related to event handling. We have also understood that multiple functions with callback can be used for a single same event. So when the even is fired, every other event handler, which is attached to the event, is invoked in a sequential manner.
Examples to Implement Python Event Handler
Below are the examples of Python Event Handler:
Example #1
Code:
class Eventsample(object):
def __init__(self):
self.__eventhandlersample = []
def __iadd__(self, Ehandler):
self.__eventhandlersample.append(Ehandler)
return self
def __isub__(self, Ehandler):
self.__eventhandlersample.remove(Ehandler)
return self
def __call__(self, *args, **keywargs):
for eventhandlersample in self.__eventhandlersample:
eventhandlersample(*args, **keywargs)
class MessToDisplay(object):
def __init__(self, Mess):
self._tele = Mess
def PrintM(self):
print "Simple Message for an Event..."
class sampleclass(object) :
def __init__(self) :
self.OnEH = Eventsample()
def Ehnew(self) :
self.OnEH()
def anotherevent(self,objMeth):
self.OnEH += objMeth
def nexteven(self,objMeth) :
self.OnEH -= objMeth
def Simulation() :
newsample = sampleclass()
displayamess = MessToDisplay(100)
newsample.anotherevent(displayamess.PrintM)
newsample.EHnew()
if __name__ == "__main__":
Simulation()
Output:
Explanation: For our example, we have simply tried to create a chain of events. Few classes and methods, in the way of event, handled by an event handler. Started with a simple class named Eventsample, with all the necessary methods defined. self.__eventhandlersample = [] is our first instance of the event. Then we have the same functions but with different operations. Our first instance simply prints the message, while the second appends and the third is used to remove.
Then we have our second class, which is primary for us as it holds the actual message that will be printed. We have two functions defined inside this class. The print message is of importance here. In case if you practice, you can add multiple such classes with multiple print lines.
Then we have our last class, the sample class, which has multiple functions. For these multiple functions, various objects are created. Then we have our final function, which works in sequence for the proper execution of these events. When executed, our code will print this: “This is a message for the event”.
As explained, the program has been executed without any error. Though it looks like a single line printed, there are many events running in the program and making this happen. There are many real-time applications that implement event handling. For example, a simple home alarm system is quite based on events. To extend, we can say, when the system detects unauthorized activity around or inside the house, it raises a flag. The flag is then sent to the system, which tells the system to send out alarm messages for the owner of the alarm company, Police, etc. This is a sequence of events, which triggers the next event on the basis of last.
Example #2
Here we will implement classes and functions to execute the events in order. Code is as follows:
Code:
class EventSample(object):
callbacks = None
def on(self, EHname, callback):
if self.callbacks is None:
self.callbacks = {}
if EHname not in self.callbacks:
self.callbacks[EHname] = [callback]
else:
self.callbacks[EHname].append(callback)
def trigger(self, EHname):
if self.callbacks is not None and EHname in self.callbacks:
for callback in self.callbacks[EHname]:
callback(self)
class MClass(EventSample):
def __init__(self, mess):
self.mess = mess
def __str__(self):
return "Message from other class: " + repr(self.mess)
def echo(text):
print text
MC = MClass("Sample text")
MC.on("sample_event", echo)
MC.trigger("sample_event")
Output:
Explanation: Similar to our first example, we have multiple classes and functions, which all lead to multiple events. For our second example, we have used the callback function; it works with events. We have our first class defined, with a callback function, then our multiple definitions with if. We have thoroughly implemented a callback function. Finally, we have our MClass, which consists of the message we intend to print. Then a simple function to echo, print our text. Lastly, three simple lines of code for printing the events.
Event-driven programming approaches are largely implemented with User Interface programs.
With UI programs, we have different types of components and actions waiting for an event to happen. And when the event happens, it is followed by the intended event. This is a whole process of event and is managed and monitored by the event handler in python. There is a list of packages available for python programmers to choose from in order to use events.
Conclusion
To handle every single action within a program is the primary operation of the Python Event Handler. There are subscribers and publishers as part of events in Python. Both these subscribers and publishes can be multiple and furthermore. The execution of these events in an intended sequential manner is important for the program execution.
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