"""
Proxy Design Pattern
Intent: Provide a surrogate or placeholder for another object to control access
to the original object or to add other responsibilities.
"""
from abc import ABC, abstractmethod
class Subject(ABC):
"""
The Subject interface declares common operations for both RealSubject and
the Proxy. As long as the client works with RealSubject using this
interface, you'll be able to pass it a proxy instead of a real subject.
"""
@abstractmethod
def request(self) -> None:
pass
class RealSubject(Subject):
"""
The RealSubject contains some core business logic. Usually, RealSubjects are
capable of doing some useful work which may also be very slow or sensitive -
e.g. correcting input data. A Proxy can solve these issues without any
changes to the RealSubject's code.
"""
def request(self) -> None:
print("RealSubject: Handling request.")
class Proxy(Subject):
"""
The Proxy has an interface identical to the RealSubject.
"""
def __init__(self, real_subject: RealSubject) -> None:
self._real_subject = real_subject
def request(self) -> None:
"""
The most common applications of the Proxy pattern are lazy loading,
caching, controlling the access, logging, etc. A Proxy can perform one
of these things and then, depending on the result, pass the execution to
the same method in a linked RealSubject object.
"""
if self.check_access():
self._real_subject.request()
self.log_access()
def check_access(self) -> bool:
print("Proxy: Checking access prior to firing a real request.")
return True
def log_access(self) -> None:
print("Proxy: Logging the time of request.", end="")
def client_code(subject: Subject) -> None:
"""
The client code is supposed to work with all objects (both subjects and
proxies) via the Subject interface in order to support both real subjects
and proxies. In real life, however, clients mostly work with their real
subjects directly. In this case, to implement the pattern more easily, you
can extend your proxy from the real subject's class.
"""
subject.request()
if __name__ == "__main__":
print("Client: Executing the client code with a real subject:")
real_subject = RealSubject()
client_code(real_subject)
print("")
print("Client: Executing the same client code with a proxy:")
proxy = Proxy(real_subject)
client_code(proxy)