Assigning to __class__ : bad form?

[The Pythonista]

I've been wondering for a while about whether assigning to __class__ is
bad form or not. Specifically, I mean doing so when some other method of
implementing the functionality you're after is available (i.e. using an
adapter, or something like the strategy pattern).

To give an example and a non-example of what I'm talking about, consider
the following recipes from the online Python Cookbook:

Ring Buffer: http://aspn.activestate.com/ASPN/Coo...n/Recipe/68429

In this case, I think the assignment to __class__ just obfuscates things,
and the example would be better coded as a single class.

On the other hand,

Fast copy of an object having a slow __init__ : http://
aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66507

This seems like a reasonable use case for assigning to __class__ (except
that it's already implemented in the 'new' module, but, read the Martelli-
bot's comments near the end of the recipe). I consider this a reasonable
use case for assigning to __class__, because, short of the 'new' module,
I don't see any other way to accomplish it.

So, what is the opinion of the broader Python community? Is code that
assigns to __class__ just clever trickiness to be avoided, or is it
sometimes a good thing?

--
code.py: A blog about life, the universe, and Python

http://pythonista.wordpress.com
** Posted from http://www.teranews.com **

[bruno.desthuilliers]

On 6 juin, 19:51, The Pythonista <[e-mail]:

I've been wondering for a while about whether assigning to __class__ is
bad form or not. Specifically, I mean doing so when some other method of
implementing the functionality you're after is available (i.e. using an
adapter, or something like the strategy pattern).

To give an example and a non-example of what I'm talking about, consider
the following recipes from the online Python Cookbook:

Ring Buffer:http://aspn.activestate.com/ASPN/Coo...n/Recipe/68429

In this case, I think the assignment to __class__ just obfuscates things,
and the example would be better coded as a single class.

On the other hand,

Fast copy of an object having a slow __init__ : http://
aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66507

This seems like a reasonable use case for assigning to __class__ (except
that it's already implemented in the 'new' module, but, read the Martelli-
bot's comments near the end of the recipe). I consider this a reasonable
use case for assigning to __class__, because, short of the 'new' module,
I don't see any other way to accomplish it.

So, what is the opinion of the broader Python community?

My first opinion is that your formulation, ie "assigning *to*
__class__" is perhaps a bit misleading. What you're talking about is
rebinding the __class__ attribute, while, from your subject line, I
thought you were talking about reassigning to (rebinding) a class
attribute from the instance, ie : self.__class__.attrib = value.

Now to the point:

Is code that
assigns to __class__ just clever trickiness to be avoided, or is it
sometimes a good thing?

Both, definitively !-)

Like most of Python's "advanced" features, it's nice to have it
because it can easily solve problems that would otherwise be at best a
PITA, but it's not something you use on a daily basis - nor without
thinking twice. And obviously, there's no clear rule here, except good
taste and common sense. Anyway, the mere fact that you're asking
yourself if it's a good idea in such or such case is a probably a good
indication that you'll find out by yourself the day you'll be tempted
to use this trick whether it's a good or bad idea in this particular
context.