Proxy Pattern

Introduction

The Proxy Pattern is a structural design pattern that lets you provide a substitute or "proxy" for another object. A proxy controls access to the original object, allowing you to perform something either before or after the request gets through to the original object.

Why use a Proxy?

Imagine you have a massive object that consumes a vast amount of system resources. You only need it from time to time, not always. You could implement lazy initialization: create this object only when it's actually needed.

The problem is that all of the object's clients would need to execute the initialization code. The Proxy pattern solves this by creating a new proxy class with the same interface as the original service object.

Real-World Example: Credit Card

A physical credit card is a proxy for a bank account. Both the credit card and the bank account implement the same interface (they both can be used to make a payment). A client (the shopper) can use the credit card in place of cash. The credit card controls access to the bank account and performs security checks before deducting funds.

Code Implementation (Python)

from abc import ABC, abstractmethod

# The Interface
class Subject(ABC):
    @abstractmethod
    def request(self):
        pass

# The Real Object
class RealSubject(Subject):
    def request(self):
        print("RealSubject: Handling request.")

# The Proxy
class Proxy(Subject):
    def __init__(self, real_subject: RealSubject):
        self._real_subject = real_subject

    def check_access(self) -> bool:
        print("Proxy: Checking access prior to firing a real request.")
        return True

    def request(self):
        if self.check_access():
            self._real_subject.request()
            print("Proxy: Logging the time of request.")

# Usage
real_subject = RealSubject()
proxy = Proxy(real_subject)
proxy.request()

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