[Python-Dev] type(obj) vs. obj.class (original) (raw)

Steven D'Aprano steve at pearwood.info
Sun Oct 18 01:55:00 EDT 2015

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 PyTYPE() 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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