context.py
class ProcessError(Exception):
pass
class BufferTooShort(ProcessError):
pass
class TimeoutError(ProcessError):
pass
class AuthenticationError(ProcessError):
pass
ProcessError BufferTooShort TimeoutError 都是自定义异常类。ProcessError是自定义异常类的父类，继承自Python自带的异常类，Exception。
class BaseContext(object):
ProcessError = ProcessError
BufferTooShort = BufferTooShort
TimeoutError = TimeoutError
AuthenticationError = AuthenticationError
current_process = staticmethod(process.current_process)
active_children = staticmethod(process.active_children)
用内建函数staticmethod,直接把外部的函数变成这个类的一个静态方法。这个操作以前我没见过，我只会用@staticmethod
