returning another class when instantiating a class in python

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?

AKX · Accepted Answer · 2012-03-26 20:11:19Z

4

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.

answered Mar 26, 2012 at 20:11

AKX

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1 Comment

John La Rooy Over a year ago

Usually a function of this type is called a Factory Function. In this case you could refer to it as a Handler Factory

kindall · Accepted Answer · 2012-03-26 20:22:51Z

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.

answered Mar 26, 2012 at 20:22

kindall

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4 Comments

Igor Serebryany Over a year ago

thanks for answering the question! this looks pretty elegant; are there any other reasons besides simplicity for recommending the function route?

John La Rooy Over a year ago

@Igor, how about the principle of least astonishment?

kindall Over a year ago

I'd agree. Returning an instance of some other class is a kind of surprising outcome for a class instantiation operation.

Lampe2020 Over a year ago

@kindall But isn't that exactly what type() does? It's a class but when instantiated returns the class that was used to instantiate the passed-in object.

Sid · Accepted Answer · 2012-03-26 20:11:38Z

1

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.

answered Mar 26, 2012 at 20:11

Sid

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1 Comment

Igor Serebryany Over a year ago

the singleton in this case is already the logging module itself; you set it up once with your handlers and settings, and every subsequent instantiation/import uses those.

Silas Ray · Accepted Answer · 2012-03-26 20:21:57Z

0

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.

answered Mar 26, 2012 at 20:21

Silas Ray

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