Welcome back! We are continuing our journey through Behavioral Patterns in software design. In previous lessons, we explored the Command and Observer patterns, focusing on object communication and state changes. Now, we will learn how to implement the Strategy Pattern in Python. We will break down the pattern into manageable parts and illustrate its practical application through a clear example.

Consider a scenario where you have a ShoppingCart class that can handle payments through different methods, such as credit cards or PayPal. Using the Strategy Pattern, we can encapsulate these payment methods into separate classes and have the ShoppingCart class use any of these strategies interchangeably.

First, we need to define an abstract base class that all payment strategies will inherit from. This ensures that all payment methods follow a common interface and can be used interchangeably.

Python
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1from abc import ABC, abstractmethod
2
3class PaymentStrategy(ABC):
4    @abstractmethod
5    def pay(self, amount):
6        pass

In this snippet, we create an abstract class PaymentStrategy with an abstract method pay. Any class that inherits from PaymentStrategy must implement the pay method.

Next, we implement concrete strategies that encapsulate different payment methods. Here, we define two strategies: CreditCardStrategy and PayPalStrategy.

Python
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1class CreditCardStrategy(PaymentStrategy):
2    def __init__(self, card_number):
3        self.card_number = card_number
4
5    def pay(self, amount):
6        print(f"Paid {amount} using Credit Card: {self.card_number}")

In the CreditCardStrategy class, we implement the pay method to handle credit card transactions. This class requires a card number upon initialization.

Python
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1class PayPalStrategy(PaymentStrategy):
2    def __init__(self, email):
3        self.email = email
4
5    def pay(self, amount):
6        print(f"Paid {amount} using PayPal: {self.email}")

Similarly, the PayPalStrategy class implements the pay method for PayPal transactions. It requires an email address to initialize.

The ShoppingCart class is our context class that will use any given payment strategy. This class keeps a reference to a PaymentStrategy object and can switch strategies at runtime.

Python
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1class ShoppingCart:
2    def __init__(self):
3        self.strategy = None
4
5    def set_payment_strategy(self, strategy):
6        self.strategy = strategy
7
8    def checkout(self, amount):
9        if self.strategy:
10            self.strategy.pay(amount)
11        else:
12            print("No payment strategy set.")

In the ShoppingCart class, the set_payment_strategy method allows us to set the payment strategy, and the checkout method uses the selected strategy to make a payment.

Below is the complete code for our example, integrating all the parts we've discussed:

Python
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1from abc import ABC, abstractmethod
2
3class PaymentStrategy(ABC):
4    @abstractmethod
5    def pay(self, amount):
6        pass
7
8class CreditCardStrategy(PaymentStrategy):
9    def __init__(self, card_number):
10        self.card_number = card_number
11
12    def pay(self, amount):
13        print(f"Paid {amount} using Credit Card: {self.card_number}")
14
15class PayPalStrategy(PaymentStrategy):
16    def __init__(self, email):
17        self.email = email
18
19    def pay(self, amount):
20        print(f"Paid {amount} using PayPal: {self.email}")
21
22class ShoppingCart:
23    def __init__(self):
24        self.strategy = None
25
26    def set_payment_strategy(self, strategy):
27        self.strategy = strategy
28
29    def checkout(self, amount):
30        if self.strategy:
31            self.strategy.pay(amount)
32        else:
33            print("No payment strategy set.")
34
35if __name__ == "__main__":
36    cart = ShoppingCart()
37
38    credit_card = CreditCardStrategy("1234-5678-9876-5432")
39    paypal = PayPalStrategy("user@example.com")
40
41    cart.set_payment_strategy(credit_card)
42    cart.checkout(100) # Outputs: Paid 100 using Credit Card: 1234-5678-9876-5432
43
44    cart.set_payment_strategy(paypal)
45    cart.checkout(200) # Outputs: Paid 200 using PayPal: user@example.com

Understanding the Strategy Pattern is crucial because it promotes flexibility and reusability in your code. Instead of hardcoding multiple algorithms within a class, you can encapsulate them into separate strategy classes. This makes your code more maintainable and scalable. Consider a real-world example: an e-commerce platform. One customer might prefer to pay using a credit card, while another might choose PayPal. With the Strategy Pattern, you can easily switch payment methods without altering the underlying business logic of the shopping cart. By mastering the Strategy Pattern, you'll be equipped to build systems that can adapt to varying requirements with minimal changes to the codebase.