在python中实例化一个类时返回另一个类

Question

I am trying to use logging's memory handler to buffer log messages to my actual logging handler. You use the memory handler by instantiating it with some arguments, like this:

import logging
buffered_handler = logging.handlers.MemoryHandler(capacity=5,target=myActualHandler)

However, I don't want my application to have to instantiate a handler from logging directly, or pass any of the arguments; I'd like this code to look like this:

import myhandlers
buffered_handler = myhandlers.BufferedRemoteHandler()

The question is: how do I do this? I'd like an attempt to instantiate my BufferedRemoteHandler to actually return logging's MemoryHandler with some specific arguments. I thought of just making BufferedRemoteHandler a function which returns the handler I want; is that the correct way? Is there a way to create a class which, when you instantiate it, actually returns a totally different class?

Answer 1

Maybe point 3 of PEP 8 is the answer:

Simple is better than complex.

I'd go the function way. Maybe call it something like get_handler() (or get_buffered_remote_handler() ) though, so you realize it is something other than a real class at first glance.

Answer 2

Yes just make it a function. Something like getMemoryHandler(). THat's the best way to do it. Also, if only one instance of MemoryHandler is going to be around you may want to make it a singleton.

Answer 3

To answer your exact question, you may, if you like, override __new__() on your subclass to return an instance of a class other than yours.

import logging

class BufferedRemoteHandler(object):
   def __new__(cls):
       return logging.handlers.MemoryHandler(capacity=5,target=myActualHandler)

But I'd actually suggest the function.

Answer 4

Well, to answer you directly, yes, there's __new__() . However, I think what would probably be an easier solution would be to either hard code or allow special handler definitions to be registered to myhandlers. You'd define a member method for each handler you want in myhandlers that just instantiated the MemoryHandler as you want and returned it.