Design Patterns
Behavioral Patterns
State

State

The State design pattern is a behavioral design pattern that allows an object to change its behavior when its internal state changes. The object will appear to change its class.

from abc import ABC, abstractmethod

class State(ABC):
    @abstractmethod
    def insert_coin(self, context):
        pass

    @abstractmethod
    def eject_coin(self, context):
        pass

    @abstractmethod
    def dispense(self, context):
        pass

class NoCoinState(State):
    def insert_coin(self, context):
        print("Coin inserted.")
        context.state = HasCoinState()

    def eject_coin(self, context):
        print("No coin to eject.")

    def dispense(self, context):
        print("Insert coin first.")

class HasCoinState(State):
    def insert_coin(self, context):
        print("Coin already inserted.")

    def eject_coin(self, context):
        print("Coin ejected.")
        context.state = NoCoinState()

    def dispense(self, context):
        print("Item dispensed.")
        context.state = NoCoinState()

class VendingMachine:
    def __init__(self):
        self.state = NoCoinState()

    def insert_coin(self):
        self.state.insert_coin(self)

    def eject_coin(self):
        self.state.eject_coin(self)

    def dispense(self):
        self.state.dispense(self)

# Client code
if __name__ == "__main__":
    machine = VendingMachine()
    
    machine.insert_coin()
    machine.dispense()
    machine.eject_coin()
    
    machine.insert_coin()
    machine.eject_coin()
    machine.dispense()

The State pattern allows the objects (ex: VendingMachine) to change their behavior dynamically based on their current state without using complex conditionals.

ObserverStrategy