Re: Distinction between “class” and “type”

[Terry Reedy]

On 5/13/2016 1:07 AM, Ben Finney wrote:
> Howdy all,
>
> Ever since Python's much-celebrated Grand Unification of classes and
> types, I have used those terms interchangeably: every class is a type,
> and every type is a class.
>
> That may be an unwise conflation. With the recent rise of optional type
> annotation in Python 3, more people are speaking about the important
> distinction between a class and a type.
>
> This recent message from GvR, discussing a relevant PEP, advocates
> keeping them separate:
>
>     PEP 484 […] tries to make a clear terminological between classes
>     (the things you have at runtime) and types (the things that type
>     checkers care about).
>
>     There's a big overlap because most classes are also types -- but not
>     the other way around! E.g. Any is a type but not a class (you can
>     neither inherit from Any nor instantiate it), and the same is true
>     for unions and type variables. […]
>
>     <URL:https://mail.python.org/pipermail/python-ideas/2016-May/040237.html>
>
> As a Bear of Little Brain, this leaves me clueless. What is the
> distinction Guido alludes to, and how are Python classes not also types?

I suspect that one could produce a class that is not a type, in Guido's 
meaning, with a metaclass that is not a subclass of the type class.  I 
don't otherwise know what Guido might have meant.

-- 
Terry Jan Reedy