Chain of Responsibility Design Pattern
Video Lecture
Overview
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Terminology
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Chain of Responsibility UML Diagram
Source Code
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./chain_of_responsibility/chain_of_responsibility_concept.py
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52 | # pylint: disable=too-few-public-methods
"The Chain Of Responsibility Pattern Concept"
import random
from abc import ABCMeta, abstractmethod
class IHandler(metaclass=ABCMeta):
"The Handler Interface that the Successors should implement"
@staticmethod
@abstractmethod
def handle(payload):
"A method to implement"
class Successor1(IHandler):
"A Concrete Handler"
@staticmethod
def handle(payload):
print(f"Successor1 payload = {payload}")
test = random.randint(1, 2)
if test == 1:
payload = payload + 1
payload = Successor1().handle(payload)
if test == 2:
payload = payload - 1
payload = Successor2().handle(payload)
return payload
class Successor2(IHandler):
"A Concrete Handler"
@staticmethod
def handle(payload):
print(f"Successor2 payload = {payload}")
test = random.randint(1, 3)
if test == 1:
payload = payload * 2
payload = Successor1().handle(payload)
if test == 2:
payload = payload / 2
payload = Successor2().handle(payload)
return payload
class Chain():
"A chain with a default first successor"
@staticmethod
def start(payload):
"Setting the first successor that will modify the payload"
return Successor1().handle(payload)
# The Client
CHAIN = Chain()
PAYLOAD = 1
OUT = CHAIN.start(PAYLOAD)
print(f"Finished result = {OUT}")
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Output
| python ./chain_of_responsibility/chain_of_responsibility_concept.py
Successor1 payload = 1
Successor2 payload = -1
Successor2 payload = -0.5
Successor2 payload = -0.25
Successor1 payload = -0.5
Successor1 payload = 0.5
Successor2 payload = -1.5
Finished result = -1.5
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Example Use Case
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Example UML Diagram
Source Code
./chain_of_responsibility/client.py
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12 | "An ATM Dispenser that dispenses denominations of notes"
import sys
from atm_dispenser_chain import ATMDispenserChain
ATM = ATMDispenserChain()
AMOUNT = int(input("Enter amount to withdrawal : "))
if AMOUNT < 10 or AMOUNT % 10 != 0:
print("Amount should be positive and in multiple of 10s.")
sys.exit()
# process the request
ATM.chain1.handle(AMOUNT)
print("Now go spoil yourself")
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./chain_of_responsibility/atm_dispenser_chain.py
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18 | "The ATM Dispenser Chain"
from dispenser10 import Dispenser10
from dispenser20 import Dispenser20
from dispenser50 import Dispenser50
class ATMDispenserChain: # pylint: disable=too-few-public-methods
"The Chain Client"
def __init__(self):
# initializing the successors chain
self.chain1 = Dispenser50()
self.chain2 = Dispenser20()
self.chain3 = Dispenser10()
# Setting a default successor chain that will process the 50s
# first, the 20s second and the 10s last. The successor chain
# will be recalculated dynamically at runtime.
self.chain1.next_successor(self.chain2)
self.chain2.next_successor(self.chain3)
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./chain_of_responsibility/interface_dispenser.py
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14 | "The ATM Notes Dispenser Interface"
from abc import ABCMeta, abstractmethod
class IDispenser(metaclass=ABCMeta):
"Methods to implement"
@staticmethod
@abstractmethod
def next_successor(successor):
"""Set the next handler in the chain"""
@staticmethod
@abstractmethod
def handle(amount):
"""Handle the event"""
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./chain_of_responsibility/dispenser10.py
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23 | "A dispenser of £10 notes"
from interface_dispenser import IDispenser
class Dispenser10(IDispenser):
"Dispenses £10s if applicable, otherwise continues to next successor"
def __init__(self):
self._successor = None
def next_successor(self, successor):
"Set the next successor"
self._successor = successor
def handle(self, amount):
"Handle the dispensing of notes"
if amount >= 10:
num = amount // 10
remainder = amount % 10
print(f"Dispensing {num} £10 note")
if remainder != 0:
self._successor.handle(remainder)
else:
self._successor.handle(amount)
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./chain_of_responsibility/dispenser20.py
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23 | "A dispenser of £20 notes"
from interface_dispenser import IDispenser
class Dispenser20(IDispenser):
"Dispenses £20s if applicable, otherwise continues to next successor"
def __init__(self):
self._successor = None
def next_successor(self, successor):
"Set the next successor"
self._successor = successor
def handle(self, amount):
"Handle the dispensing of notes"
if amount >= 20:
num = amount // 20
remainder = amount % 20
print(f"Dispensing {num} £20 note(s)")
if remainder != 0:
self._successor.handle(remainder)
else:
self._successor.handle(amount)
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./chain_of_responsibility/dispenser50.py
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23 | "A dispenser of £50 notes"
from interface_dispenser import IDispenser
class Dispenser50(IDispenser):
"Dispenses £50s if applicable, otherwise continues to next successor"
def __init__(self):
self._successor = None
def next_successor(self, successor):
"Set the next successor"
self._successor = successor
def handle(self, amount):
"Handle the dispensing of notes"
if amount >= 50:
num = amount // 50
remainder = amount % 50
print(f"Dispensing {num} £50 note(s)")
if remainder != 0:
self._successor.handle(remainder)
else:
self._successor.handle(amount)
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Output
| python ./chain_of_responsibility/client.py
Enter amount to withdrawal : 180
Dispensing 3 £50 note(s)
Dispensing 1 £20 note(s)
Dispensing 1 £10 note
Now go spoil yourself
|
New Coding Concepts
Floor Division
Normally division uses a single / character and will return a float even if the numbers are integers or exactly divisible with no remainder,
E.g.,
Python Version 3 also has an option to return an integer version (floor) of the number by using the double // characters instead.
See PEP-0238 : https://www.python.org/dev/peps/pep-0238/
In the file ./chain_of_responsibility/client.py above, there is a command input .
The input command allows your script to accept user input from the command prompt.
In the ATM example, when you start it, it will ask the user to enter a number.
Then when the user presses the enter key, the input is converted to an integer and the value tested if valid.
| AMOUNT = int(input("Enter amount to withdrawal : "))
if AMOUNT < 10 or AMOUNT % 10 != 0:
...continue
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Note that in Python 2.x, use the raw_input() command instead of input() .
See PEP-3111 : https://www.python.org/dev/peps/pep-3111/
Summary
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Paperback 
