added chain of responsibility pattern

Author: Sean-Bradley

README.md

@@ -9,3 +9,4 @@ Common GOF Patterns implemented in Python
     * [Prototype](prototype)
 * Behavioural
     * [Command](command)
+    * [Chain of Responsibiltiy](chain_of_responsibility)

chain_of_responsibility/README.md

@@ -0,0 +1,18 @@
+# Chain of Responsibility Design Pattern
+
+Chain of responsibility pattern is a behavioral pattern used to achieve loose coupling
+in software design.
+Example, a request from a client is passed to a chain of objects to process them. 
+The objects in the chain will decide themselves how the request is handled and/or 
+passed to the next processor in the chain.
+
+![Chain of Responsibility UML Diagram in the context of an ATM](atm.png)
+
+```bash
+$ python atm.py
+Enter amount to withdrawal
+130
+Dispensing 2 £50 note
+Dispensing 1 £20 note
+Dispensing 1 £10 note
+```

chain_of_responsibility/atm.png

@@ -0,0 +1,103 @@
+"""Chain of Responsibility Pattern
+Chain of responsibility pattern is a behavioral pattern used to achieve loose coupling
+in software design.
+Example, a request from a client is passed to a chain of objects to process them. 
+The objects in the chain will decide themselves how the request is handled and/or 
+passed to the next processor in the chain.
+"""
+
+from abc import ABCMeta, abstractstaticmethod
+
+
+class IDispenseChain(metaclass=ABCMeta):
+
+    @abstractstaticmethod
+    def set_next_chain(next_chain):
+        """Set the next processor in the chain"""
+
+    @abstractstaticmethod
+    def dispense(currency):
+        """Dispense the amount"""
+
+
+class Dispenser50(IDispenseChain):
+
+    def __init__(self):
+        self.chain = None
+
+    def set_next_chain(self, next_chain):
+        self.chain = next_chain
+
+    def dispense(self, amount):
+        if amount >= 50:
+            num = amount // 50
+            remainder = amount % 50
+            print(f"Dispensing {num} £50 note")
+            if remainder != 0:
+                self.chain.dispense(remainder)
+        else:
+            self.chain.dispense(amount)
+
+
+class Dispenser20(IDispenseChain):
+
+    def __init__(self):
+        self.chain = None
+
+    def set_next_chain(self, next_chain):
+        self.chain = next_chain
+
+    def dispense(self, amount):
+        if amount >= 20:
+            num = amount // 20
+            remainder = amount % 20
+            print(f"Dispensing {num} £20 note")
+            if remainder != 0:
+                self.chain.dispense(remainder)
+        else:
+            self.chain.dispense(amount)
+
+
+class Dispenser10(IDispenseChain):
+
+    def __init__(self):
+        self.chain = None
+
+    def set_next_chain(self, next_chain):
+        self.chain = next_chain
+
+    def dispense(self, amount):
+        if amount >= 10:
+            num = amount // 10
+            remainder = amount % 10
+            print(f"Dispensing {num} £10 note")
+            if remainder != 0:
+                self.chain.dispense(remainder)
+        else:
+            self.chain.dispense(amount)
+
+
+class ATMDispenserChain:
+
+    def __init__(self):
+        # initialize the chain
+
+        self.chain1 = Dispenser50()
+        self.chain2 = Dispenser20()
+        self.chain3 = Dispenser10()
+
+        # set the chain of responsibility
+        self.chain1.set_next_chain(self.chain2)
+        self.chain2.set_next_chain(self.chain3)
+
+
+if __name__ == "__main__":
+
+    ATM = ATMDispenserChain()
+    
+    print("Enter amount to withdrawal")
+    AMOUNT = int(input())
+    if AMOUNT % 10 != 0:
+        print("Amount should be in multiple of 10s.")
+    # process the request
+    ATM.chain1.dispense(AMOUNT)

chain_of_responsibility/atm.py

@@ -0,0 +1,19 @@
+digraph "classes_atm" {
+charset="utf-8"
+rankdir=BT
+
+"Dispenser50" [label="{Dispenser50|chain : NoneType\l|dispense()\lset_next_chain()\l}", shape="record"];
+"Dispenser10" [label="{Dispenser10|chain : NoneType\l|dispense()\lset_next_chain()\l}", shape="record"];
+"Dispenser20" [label="{Dispenser20|chain : NoneType\l|dispense()\lset_next_chain()\l}", shape="record"];
+"IDispenseChain" [label="{IDispenseChain|\l|dispense()\lset_next_chain()\l}", shape="record"];
+
+"ATMDispenserChain" [label="{ATMDispenserChain|chain1\lchain2\lchain3\l|}", shape="record"];
+
+{ "Dispenser10" "Dispenser20" "Dispenser50" } -> "IDispenseChain" [arrowhead="empty", arrowtail="none"];
+
+"Dispenser10" -> "ATMDispenserChain" [arrowhead="diamond", arrowtail="none", fontcolor="green", label="chain3", style="solid"];
+"Dispenser20" -> "ATMDispenserChain" [arrowhead="diamond", arrowtail="none", fontcolor="green", label="chain2", style="solid"];
+"Dispenser50" -> "ATMDispenserChain" [arrowhead="diamond", arrowtail="none", fontcolor="green", label="chain1", style="solid"];
+
+
+}