In this design pattern, there are some sorts of executors of a command tied in a chain - at different levels. Depending upon the complexity of the task, the command is handled at different levels. Each receiver is well aware of if he can handle the command. If he can't he simply passes the buck to the next person in the chain. Once the command reaches the highest level of the chain, it must be handled there as there is no more successor after the highest authority.
The idea is that the command is always delivered at the lowest level of the chain and then it moves forward - one step at a time till it is handled at one such level.
Here is the class diagram of the Chain of Responsibility design pattern.
Here goes the source code of the chain of responsibility implemented in Python...
from abc import ABC, abstractmethod class ICommand(ABC): def __init__(self, level): self.level = level class CommandReceiver(ABC): def __init__(self, successor): self.successor = successor @abstractmethod def handleCommandImpl(self, command): pass def handleCommand(self, command): retType = self.handleCommandImpl(command) if self.successor != None and retType == False: self.successor.handleCommand(command) else: return class ReceiverLevel1(CommandReceiver): def __init__(self, successor): self.successor = successor def handleCommandImpl(self, command): if command.level == 1: print("The Command is handled at level 1") return True else: print("Forwarding the command from level 1 to level 2") return False class ReceiverLevel2(CommandReceiver): def __init__(self, successor): self.successor = successor def handleCommandImpl(self, command): if command.level == 2: print("The Command is handled at level 2") return True else: print("Forwarding the command from level 2 to level 3") return False class ReceiverLevel3(CommandReceiver): def __init__(self, successor): self.successor = successor def handleCommandImpl(self, command): if command.level == 3: print("The Command is handled at level 3") return True else: print("Last level... The Command has to be managed here...") return False # Press the green button in the gutter to run the script. if __name__ == '__main__': commandLevel1 = ICommand(1) commandLevel2 = ICommand(2) commandLevel3 = ICommand(3) receiverLevel3 = ReceiverLevel3(None) receiverLevel2 = ReceiverLevel2(receiverLevel3) receiverLevel1 = ReceiverLevel1(receiverLevel2) receiverLevel1.handleCommand(commandLevel3) 
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