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
--
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