Python Design Patterns Resources

Python Design Patterns - Chain of Responsibility Pattern

Quiz

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.

Example - How to implement the chain of responsibility pattern?

We will now see how to implement the chain of responsibility pattern.

main.py

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 −

*****Monthly report*****
Things are going
really, really well.

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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