[Python-Dev] type(obj) vs. obj.__class__

[David Mertz]

This recipe looks like a bad design to me to start with.  It's
too-clever-by-half, IMO.

If I were to implement RingBuffer, I wouldn't futz around with the
__class__ attribute to change it into another thing when it was full.  A
much more obvious API for users would be simply to implement a
RingBuffer.isfull() method, perhaps supported by an underlying
RingBuffer._full boolean attribute.  That's much friendlier than expecting
people to introspect the type of the thing for a question that only
occasionally matters; and when it does matter, the question is always
conceived exactly as "Is it full?" not "What class is this currently?"

So I think I'm still waiting for a compelling example where type(x) !=
x.__class__ would be worthwhile (yes, of course it's *possible*)

On Sat, Oct 17, 2015 at 10:55 PM, Steven D'Aprano <steve at pearwood.info>
wrote:

> On Sat, Oct 17, 2015 at 03:45:19PM -0600, Eric Snow wrote:
> > In a recent tracker issue about OrderedDict [1] we've had some
> > discussion about the use of type(od) as a replacement for
> > od.__class__.
> [...]
> > The more general question of when we use type(obj) vs. obj.__class__
> > applies to both the language and to all the stdlib as I expect
> > consistency there would result in fewer surprises.  I realize that
> > there are some places where using obj.__class__ makes more sense (e.g.
> > for some proxy support).  There are other places where using type(obj)
> > is the way to go (e.g. special method lookup).  However, the
> > difference is muddled enough that usage is inconsistent in the stdlib.
> > For example, C-implemented types use Py_TYPE() almost exclusively.
> >
> > So, would it make sense to establish some concrete guidelines about
> > when to use type(obj) vs. obj.__class__?  If so, what would those be?
> > It may also be helpful to enumerate use cases for "type(obj) is not
> > obj.__class__".
>
> I for one would like to see a definitive explanation for when they are
> different, and when you should use one or the other. The only
> obvious example I've seen is the RingBuffer from the Python Cookbook:
>
> http://code.activestate.com/recipes/68429-ring-buffer/
>
>
>
> --
> Steve
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