Editing Software Design Patterns (section)

== <span style="color: #FFFFFF;">Applying</span> ==
'''Modeling 'The Strategy Pattern' (Selecting behavior at runtime):'''
<syntaxhighlight lang="python">
class PaymentStrategy:
    def pay(self, amount): pass

class CreditCardPay(PaymentStrategy):
    def pay(self, amount): print(f"Paid ${amount} via Credit Card.")

class BitcoinPay(PaymentStrategy):
    def pay(self, amount): print(f"Paid ${amount} via Bitcoin.")

class ShoppingCart:
    def __init__(self, strategy):
        self.strategy = strategy
        
    def checkout(self, total):
        self.strategy.pay(total)

# The user can choose the payment method at the last second:
cart1 = ShoppingCart(CreditCardPay())
cart1.checkout(100)

cart2 = ShoppingCart(BitcoinPay())
cart2.checkout(100)
</syntaxhighlight>

; Design Landmarks
: '''Design Patterns (1994)''' → The "Bible" of software engineering by Gamma, Helm, Johnson, and Vlissides, which introduced the 23 core patterns still used today.
: '''Dependency Injection''' → A modern structural pattern used in almost every major framework (like Spring or Angular) to manage how objects are created and shared.
: '''The 'Observer' in the Real World''' → How your phone works: When a message arrives, the "Subject" (the system) notifies all "Observers" (the screen, the vibration motor, the speaker).
: '''Head First Design Patterns''' → A famous, visual book that taught an entire generation of programmers how to think in patterns rather than just syntax.
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