> For the complete documentation index, see [llms.txt](https://nining.website/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nining.website/python/think/dm/proxy.md).

# 代理模式

为其他对象提供一种代理以控制这个对象的访问

```python
from abc import ABCMeta, abstractmethod

# 抽象实体: 规定RealSubject, VirtualSubject, ProtectedProxy对外具有一致方法
class Subject(metaclass=ABCMeta):
    @abstractmethod
    def get_content(self):
        pass
        
    @abstractmethod
    def set_content(self, content):
        pass

# 实体
class RealSubject(Subject):
    def __init__(self, filename):
        self.filename = filename
        with open(filename, mode='r', encoding='utf-8') as f:
            self.content = f.read()

    def get_content(self):
        return self.content

    def set_content(self, content):
        with open(self.filename, mode='w', encoding='utf-8') as f:
            f.write(content)


subject = RealSubject('text.txt')   # 初始化就读取内容了，若文件较大会占用内存
subject.get_content()

# 代理
class VirtualSubject(Subject):
    def __init__(self, filename):
        self.filename = filename
        self.subject = None

    def get_content(self):
        if not self.subject:
            self.subject = RealSubject(self.filename)
        return self.subject.get_content()

    def set_content(self, content):
        if not self.subject:
            self.subject = RealSubject(self.filename)
        self.subject.set_content(content)

sub = VirtualSubject('text.txt')    # 没有读取内容
sub.get_content()                   # 需要的时候，才读取内容

class ProtectedProxy(Subject):
    def __init__(self, filename):
        self.subject = RealSubject(filename)

    def get_content(self):
        return self.subject.get_content()

    def set_content(self, content):
        raise PermissionError('无写入权限')
```

应用场景：

* 远程代理：为远程的对象提供代理
* 虚代理：根据需要创建很大的对象
* 保护代理：控制对原始对象的访问，用于对象有不同访问权限时

优点：

* 远程代理：可以隐藏对象位于远程地址空间的事实
* 虚代理：可以进行优化，例如根据要求创建对象
* 保护代理：允许在访问一个对象时有一些附加的内务处理