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The chain of responsibility pattern is used to achieve loose coupling in software where a specified request from the client is passed through a chain of objects included in it. It helps in building a chain of objects. The request enters from one end and moves from one object to another.
This pattern allows an object to send a command without knowing which object will handle the request.
How to implement the chain of responsibility pattern?
We will now see how to implement the chain of responsibility pattern.
class ReportFormat(object):
PDF = 0
TEXT = 1
class Report(object):
def __init__(self, format_):
self.title = ''Monthly report''
self.text = [''Things are going'', ''really, really well.'']
self.format_ = format_
class Handler(object):
def __init__(self):
self.nextHandler = None
def handle(self, request):
self.nextHandler.handle(request)
class PDFHandler(Handler):
def handle(self, request):
if request.format_ == ReportFormat.PDF:
self.output_report(request.title, request.text)
else:
super(PDFHandler, self).handle(request)
def output_report(self, title, text):
print ''<html>''
print '' <head>''
print '' <title>%s</title>'' % title
print '' </head>''
print '' <body>''
for line in text:
print '' <p>%s'' % line
print '' </body>''
print ''</html>''
class TextHandler(Handler):
def handle(self, request):
if request.format_ == ReportFormat.TEXT:
self.output_report(request.title, request.text)
else:
super(TextHandler, self).handle(request)
def output_report(self, title, text):
print 5*''*'' + title + 5*''*''
for line in text:
print line
class ErrorHandler(Handler):
def handle(self, request):
print "Invalid request"
if __name__ == ''__main__'':
report = Report(ReportFormat.TEXT)
pdf_handler = PDFHandler()
text_handler = TextHandler()
pdf_handler.nextHandler = text_handler
text_handler.nextHandler = ErrorHandler()
pdf_handler.handle(report)
Output
The above program generates the following output −
Explanation
The above code creates a report for monthly tasks where it sends commands through each function. It takes two handlers – for PDF and for text. It prints the output once the required object executes each function.
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