Strategy example:

# Strategies common interface, Strategies:

class PaymentInterface:
    def pay(self, amount):
        pass

class CardPayment:
    def pay(self, amount):
        print(f"Paying {amount} with card.")

class PayPalPayment:
    def pay(self, amount):
        print(f"Paying {amount} via PayPal.")

class BankTransferPayment:
    def pay(self, amount):
        print(f"Paying {amount} via bank transfer.")

# Context
class PaymentProcessor:
    def __init__(self, strategy):
        self.strategy = strategy

    def set_strategy(self, strategy):
        self.strategy = strategy

    def process_payment(self, amount):
        self.strategy.pay(amount)

# Usage
processor = PaymentProcessor(CardPayment())
processor.process_payment(100)

processor.set_strategy(PayPalPayment())
processor.process_payment(200)