On Friday, May 13, 2016 at 10:37:34 AM UTC+5:30, 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? > > And why is this distinction important, and who moved my cheesecake?
I cannot speak for Guido. However there is a well-known confusion here, most recently spelt out in Bob Harper's epochal book: http://www.cs.cmu.edu/~rwh/pfpl.html If types are compile time things then they are part of the *syntax* of the language If types are runtime things they are some kind of tags attached to objects *at runtime* When a language like python with a clear historical orientation towards the second starts embracing the first, it is natural that people start getting confused -- which I suspect includes Guido. -- https://mail.python.org/mailman/listinfo/python-list
