一个系统会随着演进而变得非常复杂，最终形成大量的（并且有时是令人迷惑的）类和交互，这种情况并不少见。
但许多情况下，我们并不想把这种复杂性暴露给客户端。而外观设计模式有助于隐藏系统的内部复杂性，并通过一个简化的接口向客户端暴露必要的部分。本质上，外观模式是在已有的系统上实现的一个抽象层。
这里以一个简单的操作系统示例来说明外观模式：
from enum import Enum
from abc import ABCMeta, abstractmethod
State = Enum('State', ['new', 'running', 'sleeping', 'restart', 'zombie'])
class Server(metaclass=ABCMeta):
@abstractmethod
def __init__(self):
pass
def __str__(self):
return self.name
@abstractmethod
def boot(self):
pass
@abstractmethod
def kill(self, restart=True):
pass
class FileServer(Server):
def __init__(self):
'''初始化文件服务进程要求的操作'''
self.name = 'FileServer'
self.state = State.new
def boot(self):
print('booting the {}'.format(self))
'''启动文件服务进程要求的操作'''
self.state = State.running.value
def kill(self, restart=True):
print('Killing {}'.format(self))
'''终止文件服务进程要求的操作'''
self.state = State.restart if restart else State.zombie
def create_file(self, user, name, permissions):
'''检查访问权限的有效性和用户权限等'''
print(f"trying to create the file '{name}'"
f" for user '{user}' with permissions {permissions}")
class ProcessServer(Server):
def __init__(self):
'''初始化进程服务进程要求的操作'''
self.name = 'ProcessServer'
self.state = State.new
def boot(self):
print('booting the {}'.format(self))
'''启动进程服务进程要求的操作'''
self.state = State.running
def kill(self, restart=True):
print('Killing {}'.format(self))
'''终止进程服务进程要求的操作'''
self.state = State.restart if restart else State.zombie
def create_process(self, user, name):
'''检查用户权限和生成PID等'''
print(f"trying to create the process '{name}' for user '{user}'")
class OperatingSystem:
def __init__(self):
self.fs = FileServer()
self.ps = ProcessServer()
def start(self):
[item.boot() for item in (self.fs, self.ps)]
def create_file(self, user, name, permissions):
self.fs.create_file(user, name, permissions)
def create_process(self, user, name):
self.ps.create_process(user, name)
def main():
os = OperatingSystem()
os.start()
os.create_file('foo', 'hello', '-rw-r-r')
os.create_process('bar', 'ls /tmp')
if __name__ == '__main__':
main()